Special Issue The National Conference on Emerging Foodborne Pathogens: Implications and Control, March 24-26m 1997, Alexandria, Virginia, USA Infectious Disease as an Evolutionary Paradigm Joshua Lederberg Sackler Foundation Scholar, Rockefeller University, New York, New York, USA --------------------------------------------------------------------------- The basic principles of genetics and evolution apply equally to human hosts and to emerging infections, in which foodborne outbreaks play an important and growing role. However, we are dealing with a very complicated coevolutionary process in which infectious agent outcomes range from mutual annihilation to mutual integration and resynthesis of a new species. In our race against microbial evolution, new molecular biology tools will help us study the past; education and a global public health perspective will help us deal better with the future. Life expectancy in the United States from 1900 to the present (Figure 1) shows an overall steady rise, reflecting improved health conditions in general, the result of advances in medical science, hygiene, personal care, health technologies, and public health administrations. The rise decelerates asymptotically to a near plateau from the 1950s to the 1970s, reflecting an epidemic of coronary disease, which we do not yet fully understand. Improvements in medical care, attention to life style, or indiscriminate use of aspirin may all be responsible for the subsequent decrease in deaths from coronary disease. Up to the 1940s, the rising curve is jagged, reflecting sporadic infectious disease outbreaks, especially the Spanish influenza outbreak of 1918. Whether the life expectancy curve continues to rise smoothly or whether it has some jagged declines depends on what we do about transmission of infectious disease, including foodborne disease. When plotted another way (Figure 2), both the absolute number of deaths from infectious disease and the proportion of total deaths attributable to infectious disease also show steady amelioration from 1900 almost to the present. [Figure Not Available in ASCII] Figure 1. Life expectancy in the United States, at birth, 20th century. [Figure Not Available in ASCII] Figure 2. Trends in infectious diseases mortality, 1900-1992. Source: CDC, unpub. data. The 1918 Spanish influenza pandemic may be a prototype for future emerging infections. Although minimized as not much more than a bad cold, influenza took a terrible toll in 1918, especially on young people (Figure 3). [Figure Not Available in ASCII] Figure 3. Pneumonia and influenza mortality, by age, in certain epidemic years. (Reprinted with permission of W. Paul Glezen and Epidemiologic Reviews. Emerging Infections: Pandemic Influenza. Epi Rev 1996;18:66). Somewhat older persons may have been protected by immunity from prior exposure to related strains of influenza. The disease, with rapid onset of fulminating pneumonic symptoms, killed 20 to 25 million persons worldwide. The infectious agent was not available for study at that time. However, very recently the Armed Forces Institute of Pathology recovered with PCR technology genetic fragments of the 1918 influenza virus (1). Less than 10% of the entire genome has been recovered to date, but recovery of complete sequences is likely. Although the target genes have not yet provided a clue as to why the 1918 influenza was so devastating, they demonstrate the enormous potential of today's molecular biology tools. These tools will enable us to better study paleovirology and paleomicrobiology. We are accustomed to stereotyping historical disease outbreaks as if we really knew what they were, but we really know very little detail about their genetic features. For example, we talk about the great historic plagues as if they indeed were Yersinia or cholera or malaria. We should look forward to finding out about the 14th century black death, if it was indeed Yersinia pestis. Although clinically unmistakable, that is not to say it was caused by the identical genotype of present Yersinia strains. We need to look ahead as well as back. In this century, emerging and reemerging infections have stimulated flurries of interest, but in general we have been complacent about infectious diseases ever since the introduction of antibiotics. The effect of antibiotics on acute infections and tuberculosis as well as the effect of polio vaccination led to a national, almost worldwide, redirection of attention to chronic and constitutional diseases. However, the HIV pandemic in the early 1980s caught us off guard, reminding us that there are many more infectious agents in the world. It is fortuitous that retroviruses had already been studied from the perspective of cancer etiology; otherwise, we would have had no scientific platform whatsoever for coping with HIV and AIDS. The Committee on International Science Engineering and Technology provided an interagency review setting out a policy framework for the United States' global response to infectious disease (Table 1). The policy provides a worldwide mantle for surveillance and monitoring, remedial measures, development of new drugs, vaccines, and treatment modalities. The global outlook is necessary, even if for purely selfish reasons, because to infectious agents the world is indivisible, with no national boundaries. Our thinking has been impoverished in terms of budget allocations for dealing with health on an international basis. Table 1. Examples of pathogenic microbes and infectious diseases recognized since 1973 (2) --------------------------------------------------------------------------- Year Microbe Type Disease --------------------------------------------------------------------------- 1973 Rotavirus Virus Major cause of infantile diarrhea worldwide 1975 Parvovirus B19 Virus Aplastic crisis in chronic hemolytic anemia 1976 Cryptosporidium Parasite Acute and chronic diarrhea parvum 1977 Ebola virus Virus Ebola hemorrhagic fever 1977 Legionella Bacteria Legionnaires' disease pneumophila 1977 Hantaan virus Virus Hemorrhagic fever with renal syndrome (HRFS) 1977 Campylobacter jejuni Bacteria Enteric pathogens distributed globally 1980 Human T-lymphotropic virus I Virus T-cell lymphoma-leukemia (HTLV-1) 1981 Toxic producing strains of Bacteria Toxic shock syndrome(tampon Staphylococcus aureus use) 1982 Escherichia coli O157:H7 Bacteria Hemorrhagic colitis; hemolytic uremic syndrome 1982 HTLV-II Virus Hairy cell leukemia 1982 Borrelia burgdorferi Bacteria Lyme disease 1983 Human immunodeficiency Virus Acquired immunodeficiency virus(HIV) syndrome (AIDS) 1983 Helicobacter pylori Bacteria Peptic ulcer disease 1985 Enterocytozoon bieneusi Parasite Persistent diarrhea 1986 Cyclospora cayetanensis Parasite Persistent diarrhea 1988 Human herpes-virus-6 (HHV-6) Virus Roseola subitum 1988 Hepatitis E Virus Enterically transmitted non-A, non-B hepatitis 1989 Ehrlichia chafeensis Bacteria Human ehrlichiosis 1989 Hepatitis C Virus Parenterally transmitted non-A, non-B liver infection 1991 Guanarito virus Virus Venezuelan hemorrhagic fever 1991 Encephalitozoon hellem Parasite Conjunctivitis, disseminated disease 1991 New species of Babesia Parasite Atypical babesiosis 1992 Vibrio cholerae O139 Bacteria New strain associated with epidemic cholera 1992 Bartonella henselae Bacteria Cat-scratch disease; bacillary angiomatosis 1993 Sin Nombre virus Virus Adult respiratory distress syndrome 1993 Encephalitozoon cuniculi Parasite Disseminated disease 1994 Sabia virus Virus Brazilian hemorrhagic fever 1995 HHV-8 Virus Associated with Kaposi sarcoma in AIDS patients --------------------------------------------------------------------------- We are engaged in a type of race, enmeshing our ecologic circumstances with evolutionary changes in our predatory competitors. To our advantage, we have wonderful new technology; we have rising life expectancy curves. To our disadvantage, we have crowding; we have social, political, economic, and hygienic stratification. We have crowded together a hotbed of opportunity for infectious agents to spread over a significant part of the population. Affluent and mobile people are ready, willing, and able to carry afflictions all over the world within 24 hours' notice. This condensation, stratification, and mobility is unique, defining us as a very different species from what we were 100 years ago. We are enabled by a different set of technologies. But despite many potential defenses-vaccines, antibiotics, diagnostic tools-we are intrinsically more vulnerable than before, at least in terms of pandemic and communicable diseases. We could imaginably adapt in a Darwinian fashion, but the odds are stacked against us. We cannot compete with microorganisms whose populations are measured in exponents of 1012, 1014, 1016 over periods of days. Darwinian natural selection has led to the evolution of our species but at a terrible cost. If we were to rely strictly on biologic selection to respond to the selective factors of infectious disease, the population would fluctuate from billions down to perhaps millions before slowly rising again. Therefore, our evolutionary capability may be dismissed as almost totally inconsequential. In the race against microbial genes, our best weapon is our wits, not natural selection on our genes. New mechanisms of genetic plasticity of one microbe species or another are uncovered almost daily. Spontaneous mutation is just the beginning. We are also dealing with very large populations, living in a sea of mutagenic influences (e.g., sunlight). Haploid microbes can immediately express their genetic variations. They have a wide range of repair mechanisms, themselves subject to genetic control. Some strains are highly mutable by not repairing their DNA; others are relatively more stable. They are extraordinarily flexible in responding to environmental stresses (e.g., pathogens' responses to antibodies, saprophytes' responses to new environments). Mechanisms proliferate whereby bacteria and viruses exchange genetic material quite promiscuously. Plasmids now spread throughout the microbial world (3). They can cross the boundaries of yeast and bacteria. Lateral transfer is very important in the evolution of microorganisms. Their pathogenicity, their toxicity, their antibiotic resistance do not rely exclusively on evolution within a single clonal proliferation. We have a very powerful theoretical basis whereby the application of selective pressure (e.g., antibiotics in food animals) will result in drug resistance carried by plasmids, or pathogens attacking humans. It is not easy to get direct and immediate epidemiologic evidence, but the foundations for these phenomena exist and must be taken into account in the development of policies. We have barely begun to study the responses of microorganisms under stress, although we have examples where root mechanisms of adaptive mutability are themselves responses to stress. In recent experiments, bacterial restriction systems are more permissive of the introduction of foreign DNA, possibly letting down their guard in response to "mutate or die" circumstances. This does not reflect bacterial intelligence-that they know exactly what mutations they should undergo in response to environmental situations. Their intrinsic mutability and capacity to exchange genetic information without knowing what it is going to be is not a constant; it is certainly under genetic control and in some circumstances varies with the stress under which the microbes are placed. Evolution is more or less proportionate to the degree of genetic divergence among the different branches of the three-tiered tree of life, with the archaeal branch, the eubacterial branch, and the eukaryotes (Figure 4). The tree illustrates the small territory occupied by humans in the overall world of biodiversity. It shows mitochondria right next to Escherichia coli. Bacterial invasion of a primitive eukaryote 2-1/2 to 3 billion years ago, synchronized with the development of primitive green oxygen-generating plants, conferred a selective advantage to complexes that could use oxygen in respiration. Our ancestors were once invaded by an oxidative-capable bacterium that we now call a mitochondrium and that is present in every cell of every body and almost every species of eukaryote. We did not evolve in a monotonous treelike development; we are also the resynthesis of components of genetic development that diverged as far as the bacteria and were reincorporated into the mitochondrial part of our overall genome. Another example of lateral transfer is the symbiosis that resulted from chloroplast invasion of green plants. [Figure Not Available in ASCII] Figure 4. The three-domain tree of life based on small-subunit rRNA sequences. Reprinted with permission of Norman R. Pace and ASM News. ASM News 1996;62 (9):464. The outcome of encounters between mutually antagonistic organisms is intrinsically unpredictable. The 1918 influenza outbreak killed half percent of the human population; but because the consequences were to either kill the host or leave the host immune, the virus died out totally, leaving no trace in our genomes, as far as we know. Historic serology on survivors has found memory cells and antibodies against H1N1, the serotype of the resurrected 1918 virus. Unlike the influenza virus, which left no known genetic imprint, 400 to 500 retroviruses are integrated into our human genome. The full phylogeny of these encounters is unknown, but many of these viruses may precede the separation of homo sapiens from the rest of the hominid line. Infectious agent outcomes range from mutual annihilation to mutual integration and resynthesis of a new species. Much has been made of the fact that zoonoses are often more lethal to humans than to their original host, but this phenomenon cannot necessarily be generalized. Most zoonoses do not affect humans adversely. Some are equally capable in a new host. We tend to pay most attention, however, to those, such as yellow fever, for which we have not genetically or serologically adapted and which cause severe disease. Canine distemper provides an example of a quasihereditary adaptation. In the Serengeti, the disease migrated from village dogs to jackals, which shared prey and had contact with lions. About one-fourth of the preserve's 4,000 lions died of canine distemper (4) but the survivors are immune and will pass immunoglobulin, to their offspring. The cubs' maternal immunity will likely mitigate infection and permit a new equilibrium, not because of genetic adaptation but because of the preimmunized host. This is also the most plausible explanation for how savage the polio virus has been as a paralytic infection of young people. It may also apply to hepatitis, where cleaner is not always better if it means we do not have the "street smarts" to respond to new infectious challenges. These nongenetic adaptations between parasite and host complicate our outcome expectations. Short-term shifts in equilibrium can give ferocious but temporary advantages to a virus. Long-term outcomes are most stable when they involve some degree of mutual accommodation, with both surviving longer. New short-term deviants, however, can disrupt this equilibrium. The final outcome of the HIV pandemic cannot be predicted. More strains with longer latency may be taking over, mitigating the disease. However, deviant strains could counteract this effect by overcoming immunity and rapidly proliferating, with earlier and more lethal consequences. We should also consider somatic evolution, a Darwinian process that occurs with every infection. In the clonal selection model of immunogenesis (5), an apparently random production of immunoglobulin variants, both by reassortment of parts and by localized mutagenesis, gives rise to candidate antibodies, which then proliferate in response to matching epitopes. We do not understand the details of how a given epitope enhances stepwise improvements in affinity and productivity of antibodies at various stages. The process may be more complicated than we realize; so may Darwinian evolution. Despite the prior arguments against relying on host or genotype evolution as a response to infection, historically we have done so and now have "scars of experience." A notable example is malaria, wherein the Duffy mutation against Plasmodium vivax is the only host defense with no deleterious consequences. The thalassemias, G6PD deficiency, and hemoglobin S are all hemopoietic modifications that thwart the plasmodia; but in homozygotes, they themselves cause disease. In the evolution of our species, for every child spared an early death because a hemoglobin S mutation impeded Plasmodium development, another will succumb to sickle cell disease unless we can intervene. Specific remedies do not exist. Although somatic gene therapy is an interesting possibility, one that will probably progress in the next 20 years, it is paradoxical that we know more about hemoglobin S than any other molecular disease. The entire concept of genetic determination of protein structure has been based on these early observations, yet we are still searching with limited success for ways to put it to therapeutic use. Biotechnology may enable other forms of genetic intervention through which homo sapiens could conceivably bypass natural selection and random variation. In the absence of alternatives, we might speculate about these kinds of "aversive therapies" as a last resort to save our species. The ultimate origin of life is still the subject of many theories, as is the origin of viruses (Table 2). Each virus is different. We know nothing of virus phylogenies and cannot even substantiate the distinctions of the several hundred categories. We do not know their origin, only that they interact with host genomes in many ways. Particles could come out of any genome, become free-living (i.e., independent, autonomously replicating units in host cells), reenter a host genome as retroviruses and possibly others do, and repeat the cycle dozens of times. But no one can give a single example or claim to have significant knowledge of how any particular virus evolved, thus presenting a scientific challenge for the next 20 or 30 years. Table 2. The origin of viruses --------------------------------------------------------------------------- Viruses are genomic fragments that can replicate only in the context of an intact living cell. They cannot therefore be primitive antecedents of cells. Within a given species, viruses may have emerged as genetic fragments or reduced versions from chromosomes, plasmids, or RNA of 1) the host or related species 2) distant species 3) larger parasites of the same or different hosts 4) further evolution and genetic interchange among existing viruses Once established, they may then cycle back into the genome of the host as an integrated episome; there they may have genetic functions or in principle might reemerge as new viruses. These cycles have some substantiation in the world of bacterial viruses; but we have no clear data on the provenience of plant or animal viruses. --------------------------------------------------------------------------- We are dealing with more than just predation and competition. We are dealing with a very complicated coevolutionary process, involving merger, union, bifurcation, and reemergence of new species (Table 3). Divergent phenomena can occur in any binary association, with unpredictable outcomes. We have hundreds of retroviruses in our genome and no knowledge of how they got there. As to HIV, we have no evidence as yet that it has ever entered anyone's germ line genome: we really do not know whether it ever enters germ cells. The outcomes of even that interaction could be much more complicated than the purely parasite/host relationships we are accustomed to. Table 3. Genetic evolution ------------------------------------------------------------------------- Microbes (bacteria, viruses, fungi, protozoa) Rapid and incessant Huge population sizes 1014+ and generation times in minutes vs. years Intraclonal process DNA replicationmay be error-pronein sea of mutagens sunlight; unshielded chemicals, incl. natural products RNA replicationintrinsically unedited, >10-3 swarm species Haploid: immediate manifestation, but partial recessives not accumulated contra multicopy plasmids Amplification Site-directed inversions and transpositions: phase variation ?? Other specifically evolved mechanisms: genome quadrant duplication; silencing Interclonal process Promiscuous recombinationnot all mechanisms are known Conjugationdozens of species Viral transduction and lysogenic integration: universal Classical: phage-borne toxins in C. diphtheriae Plasmid interchange (by any of above) and integration Toxins of B. anthracis Pasteur: heat attenuation: plasmid loss; chemically induced RNA viral reassortment; ?? and recombination? Transgressive-across all boundaries Artificial gene splicing Bacteria and viruses have picked up host genes (antigenic masking?) Interkingdom: P. tumefaciens and plants, E. coli and yeast Vegetable and mineral! oligonucleotides and yeast. Host-parasite coevolution Coadaptation to mutualism or accentuation of virulence? Jury is still out (May and Anderson). Many zoonotic convergences. Probably divergent phenomena, with short-term flareups and Pyrrhic victories, atop long-term trend to coadaptation. ------------------------------------------------------------------------- Innovative technologies for dealing with microbial threats have the potential for fascinating therapeutic opportunities (Table 4). Some, like bacteriophage, have been set aside as laboratory curiosities. Nothing is more exciting than unraveling the details of pathogenesis. Having the full genomes of half a dozen parasitic organisms opens up new opportunities for therapeutic invention in ways that we could not have dreamed of even 5 years ago, which will lead to many more technologies. In food microbiology, we should keep in mind the probiotic as well as the adversarial and pathogenetic opportunities in our alimentary tracts. Table 4. Technologies to address microbial threats --------------------------------------------------------------------------- Antibacterial chemotherapy Potentially unlimited capability; bacterial metabolism and genetic structure notably different from human genome sequencing pointing to bacterial vulnerabilities Economic-structural factors public expectation for unachievable bargains in safety assurance, cost of development, and ultimate pricing Dilemmas of regulation of (ab)use Resurgent interest in bacteriophage and other biologically oriented approaches Antiviral chemotherapy Much more difficult program, inherently Gross underinvestment New approaches: antisense, ribozymes, targeted D/RNA cleavers Problematics of sequence-selective targets Vaccines Gross underinvestment; other structural problems as above Liability/indemnification Vaccination as service to the herd New approaches: hot biotechnology is coming along especially live attenuated: but testing dilemmas Safety issues about use of human cells lines; adjuvants Immunoglobulins and their progeny Phage display and diversification: biosynthetic antibody Passive immunization for therapy Biologic response modifiers New world of interleukins, cell growth factors so far just scratching surface Interaction with pathogenesis Intersection with somatic gene therapy Technologies for diagnosis and monitoring Etiologic agents and control Host polymorphisms and sensitivities Homely technologies needed Simple, effective face-masks Palatable water-disinfectants Home-use diagnostics of contamination ------------------------------------------------------------------------- The Committee on International Science Engineering and Technology report (2) provides some recommendations (Table 5). We need a global perspective. We need to invest in public health, especially food microbiology, not just medical care, in dealing with disease. It is important to prevent foodborne disease through sensible monitoring, standards of cleanliness, and consumer and food-handler education and not just care for its victims. Table 5. CISET* recommendations for addressing global infectious disease threats --------------------------------------------------------------------------- 1. Concerted global and domestic surveillance and diagnosis of disease outbreaks and endemic occurrence. This must entail the installation of sophisticated laboratory capabilities at many centers now lacking them. 2. Vector management and monitoring and enforcement of safe water and food supplies; and personal hygiene (e.g., Operation Clean Hands). 3. Public and professional education. 4. Scientific research on causes of disease, pathogenic mechanisms, bodily defenses, vaccines, and antibiotics. 5. Cultivation of the technical fruits of such research, with the full involvement of the pharmaceutical industry and a public understanding of the regulatory and incentive structures needed to optimize the outcomes. --------------------------------------------------------------------------- *Committee on International Science, Engineering and Technology Policy of the National Science and Technology Council. Today we emphasize individual rights over community needs more than we did 50 to 75 years ago. Restraining the rights and freedoms of individuals is a far greater sin than allowing the infection of others. The restraints placed on Typhoid Mary might not be acceptable today, when some would prefer to give her unlimited rein to infect others, with litigation their only recourse. In the triumph of individual rights, the public health perspective has had an uphill struggle in recent pandemics. Education, however, is a universally accepted countermeasure, especially important in foodborne diseases. Food safety programs should more specifically target food handlers, examining their hands to determine if they are carriers, to ensure they are complying with basic sanitation. We typically do this only after an outbreak. Perhaps we should have further debate on the social context for constraints and persuasion to contain the spread of infectious agents. Address for correspondence: Joshua Lederberg, Rockefeller University, 1230 York Avenue, New York, NY 10021-6399, USA; fax: 212-327-8651; e-mail: Lederberg@mail.rockefeller.edu. References 1. Taubenberger JK, Reid AH, Frafft AE, Bijwaard KE, Fanning TG. Initial genetic characterization of the 1918 "Spanish" influenza virus. Science 1997;275:1793-6. 2. NSTC-CISET Working Group on Emerging and Reemerging Infectious Diseases. Infectious disease-a global health threat. Washington (DC): The Group;1996. 3. Lederberg J. Plasmid (1952-1997). Plasmid. In press 1997. 4. Roelke-Parker ME, Munson L, Packer C, Kock R, Cleaveland S, Carpenter M, et al. A canine distemper virus epidemic in Serengeti lions (Panthera leo). Nature 1996;379:441-5. 5. Lederberg J. The ontogeny of the clonal selection theory of antibody formation: reflections on Darwin and Ehrlich. Ann N Y Acad Sci 1988;546:175-87. --------------------------------------------------------------------------- Emerging Infectious Diseases National Center for Infectious Diseases Centers for Disease Control and Prevention Atlanta, GA URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/lederber.txt Emerging Foodborne Diseases: An Evolving Public Health Challenge Robert V. Tauxe Centers for Disease Control and Prevention, Atlanta, Georgia, USA --------------------------------------------------------------------------- The epidemiology of foodborne disease is changing. New pathogens have emerged, and some have spread worldwide. Many, including Salmonella, Escherichia coli O157:H7, Campylobacter, and Yersinia enterocolitica, have reservoirs in healthy food animals, from which they spread to an increasing variety of foods. These pathogens cause millions of cases of sporadic illness and chronic complications, as well as large and challenging outbreaks over many states and nations. Improved surveillance that combines rapid subtyping methods, cluster identification, and collaborative epidemiologic investigation can identify and halt large, dispersed outbreaks. Outbreak investigations and case-control studies of sporadic cases can identify sources of infection and guide the development of specific prevention strategies. Better understanding of how pathogens persist in animal reservoirs is also critical to successful long-term prevention. In the past, the central challenge of foodborne disease lay in preventing the contamination of human food with sewage or animal manure. In the future, prevention of foodborne disease will increasingly depend on controlling contamination of feed and water consumed by the animals themselves. Every year, in the United States foodborne infections cause millions of illnesses and thousands of deaths; most infections go undiagnosed and unreported. As the epidemiology of foodborne infections evolves, old scenarios and solutions need to be updated. This article reviews main trends in the evolution of foodborne disease epidemiology and their effect on surveillance and prevention activities. Preventing foodborne disease is a multifaceted process, without simple and universal solutions. For most foodborne pathogens, no vaccines are available. Consumer education about basic principles of food safety, an important component of prevention, by itself is insufficient. Food reaches the consumer through long chains of industrial production, in which many opportunities for contamination exist. The general strategy of prevention is to understand the mechanisms by which contamination and disease transmission can occur well enough to interrupt them. An outbreak investigation or epidemiologic study should go beyond identifying a suspected food and pulling it from the shelf to defining the chain of events that allowed contamination with an organism in large enough numbers to cause illness. We learn from the investigation what went wrong, in order to devise strategies to prevent similar events in the future. Although outbreaks make the news, most foodborne infections occur as individual or sporadic cases. Therefore, the sources of sporadic cases must also be investigated and understood. Emerging Foodborne Pathogens Substantial progress has been made in preventing foodborne diseases. For example, typhoid fever, extremely common at the beginning of the 20th century, is now almost forgotten in the United States. It was conquered in the preantibiotic era by disinfection of drinking water, sewage treatment, milk sanitation and pasteurization, and shellfish bed sanitation (Figure 1). Similarly, cholera, bovine tuberculosis, and trichinosis have also been controlled in the United States. However, new foodborne pathogens have emerged. Among the first of these were infections caused by nontyphoid strains of Salmonella, which have increased decade by decade since World War II (Figure 1). In the last 20 years, other infectious agents have been either newly described or newly associated with foodborne transmission (Table 1). Vibrio vulnificus, Escherichia coli O157:H7, and Cyclospora cayetanensis are examples of newly described pathogens that often are foodborne. V. vulnificus was identified in the bloodstream of persons with underlying liver disease who had fulminant infections after eating raw oysters or being exposed to seawater; this organism lives in the sea and can be a natural summertime commensal organism in shellfish (1). E. coli O157:H7 was first identified as a pathogen in 1982 in an outbreak of bloody diarrhea traced to hamburgers from a fast-food chain (2); it was subsequently shown to have a reservoir in healthy cattle (3). Cyclospora, known previously as a cyanobacterialike organism, received its current taxonomic designation in 1992 and emerged as a foodborne pathogen in outbreaks traced to imported Guatemalan raspberries in 1996 (4,5). The similarity of Cyclospora to Eimeria coccidian pathogens of birds suggests an avian reservoir (4,5). [Figure Not Available in ASCII] Figure 1. Reported incidence of typhoid fever and nontyphoidal salmonellosis in the United States, 1920-1995. ----------------------------------------------------- Table 1. New pathogens that are foodborne and pathogens newly recognized as predominatly foodborne in the United States in the last 20 years ----------------------------------------------------- Campylobacter jejuni Campylobacter fetus ssp. fetus Cryptosporidium cayetanensis Escherichia coli O157:H7 and related E. coli (e.g., O111:NM, O104:H21) Listeria monocytogenes Norwalk-like viruses Nitzschia pungens (cause of amnesic shellfish poisoning) Salmonella Enteritidis Salmonella Typhimurium DT 104 Vibrio cholerae 01 Vibrio vulnificus Vibrio parahaemolyticus Yersinia enterocolitica ----------------------------------------------------- Some known pathogens have only recently been shown to be predominantly foodborne. For example, Listeria monocytogenes was long known as a cause of meningitis and other invasive infections in immunocompromised hosts. How these hosts became infected remained unknown until a series of investigations identified food as the most common source (6). Similarly, Campylobacter jejuni was known as a rare opportunistic bloodstream infection until veterinary diagnostic methods used on specimens from humans showed it was a common cause of diarrheal illness (7). Subsequent epidemiologic investigations implicated poultry and raw milk as the most common sources of sporadic cases and outbreaks, respectively (8). Yersinia enterocolitica, rare in the United States but a common cause of diarrheal illness and pseudoappendicitis in northern Europe and elsewhere, is now known to be most frequently associated with undercooked pork (9). These foodborne pathogens share a number of characteristics. Virtually all have an animal reservoir from which they spread to humans; that is, they are foodborne zoonoses. In marked contrast to many established zoonoses, these new zoonoses do not often cause illness in the infected host animal. The chicken with lifelong ovarian infection with Salmonella serotype Enteritidis, the calf carrying E. coli O157:H7, and the oyster carrying Norwalk virus or V. vulnificus appear healthy; therefore, public health concerns must now include apparently healthy animals. Limited existing research on how animals acquire and transmit emerging pathogens among themselves often implicates contaminated fodder and water; therefore, public health concerns must now include the safety of what food animals themselves eat and drink. For reasons that remain unclear, these pathogens can rapidly spread globally. For example, Y. enterocolitica spread globally among pigs in the 1970s (10); Salmonella serotype Enteritidis appeared simultaneously around the world in the 1980s (11); and Salmonella Typhimurium Definitive Type (DT) 104 is now appearing in North America, Europe, and perhaps elsewhere (12); therefore, public health concerns must now include events happening around the world, as harbingers of what may appear here. Many emerging zoonotic pathogens are becoming increasingly resistant to antimicrobial agents, largely because of the widespread use of antibiotics in the animal reservoir. For example, Campylobacter isolated from human patients in Europe is now increasingly resistant to fluoroquinolones, after these agents were introduced for use in animals (13). Salmonellae have become increasing resistant to a variety of antimicrobial agents in the United States (14); therefore, public health concerns must include the patterns of antimicrobial use in agriculture as well as in human medicine. The foods contaminated with emerging pathogens usually look, smell, and taste normal, and the pathogen often survives traditional preparation techniques: E. coli O157:H7 in meat can survive the gentle heating that a rare hamburger gets (15); Salmonella Enteritidis in eggs survives in an omelette (16); and Norwalk virus in oysters survives gentle steaming (17). Following standard and traditional recipes can cause illness and outbreaks. Contamination with the new foodborne zoonoses eludes traditional food inspection, which relies on visual identification of foodborne hazards. These pathogens demand new control strategies, which would minimize the likelihood of contamination in the first place. The rate at which new pathogens have been identified suggests that many more remain to be discovered. Many of the foodborne infections of the future are likely to arise from the animal reservoirs from which we draw our food supply. Once a new foodborne disease is identified, a number of critical questions need to be answered to develop a rational approach to prevention: What is the nature of the disease? What is the nature of the pathogen? What are simple ways to easily identify the pathogen and diagnose the disease? What is the incidence of the infection? How can the disease be treated? Which foods transmit the infection? How does the pathogen get into the food, and how well does it persist there? Is there is an animal reservoir? How do the animals themselves become infected? How can the disease be prevented? Does the prevention strategy work? The answers to these questions do not come rapidly. Knowledge accumulates gradually, as a result of detailed scientific investigations, often conducted during outbreaks (18). After 15 years of research, we know a great deal about infections with E. coli O157:H7, but we still do not know how best to treat the infection, nor how the cattle (the principal source of infection for humans) themselves become infected. Better slaughter procedures and pasteurization of milk are useful control strategies for this pathogen in meat and milk, as irradiation of meat may be in the future. More needs to be learned: for example, it remains unclear how best to prevent this organism from contaminating lettuce or apple juice. For more recently identified agents, even less is known. New Food Vehicles of Transmission Along with new pathogens, an array of new food vehicles of transmission have been implicated in recent years. Traditionally, the food implicated in a foodborne outbreak was undercooked meat, poultry or seafood, or unpasteurized milk. Now, additional foods previously thought safe are considered hazardous. For example, for centuries, the internal contents of an egg were presumed safe to eat raw. However, epidemic Salmonella Enteritidis infection among egg-laying flocks indicates that intact eggs may have internal contamination with this Salmonella serotype. Many outbreaks are caused by contaminated shell eggs, including eggs used in such traditional recipes as eggnog and Caesar salad, lightly cooked eggs in omelettes and French toast, and even foods one would presume thoroughly cooked, such as lasagna and meringue pie (19,20). E. coli O157:H7 has caused illness through an ever-broadening spectrum of foods, beyond the beef and raw milk that are directly related to the bovine reservoir. In 1992, an outbreak caused by apple cider showed that this organism could be transmitted through a food with a pH level of less than 4.0, possibly after contact of fresh produce with manure (21). A recent outbreak traced to venison jerky suggests a wild deer reservoir, so both cattle and feral deer manure are of concern (22). Imported raspberries contaminated with Cyclospora caused an epidemic in the United States in 1996, possibly because contaminated surface water was used to spray the berries with fungicide before harvest (5). Norwalklike viruses, which appear to have a human reservoir, have contaminated oysters harvested from pristine waters by oyster catchers who did not use toilets with holding tanks on their boats and were themselves the likely source of the virus (23). The new food vehicles of disease share several features. Contamination typically occurs early in the production process, rather than just before consumption. Because of consumer demand and the global food market, ingredients from many countries may be combined in a single dish, which makes the specific source of contamination difficult to trace. These foods have fewer barriers to microbial growth, such as salt, sugar, or preservatives; therefore, simple transgressions can make the food unsafe. Because the food has a short shelf life, it may often be gone by the time the outbreak is recognized; therefore, efforts to prevent contamination at the source are very important. An increasing, though still limited, proportion of reported foodborne outbreaks are being traced to fresh produce (24). A series of outbreaks recently investigated by the Centers for Disease Control and Prevention (CDC) has linked a variety of pathogens to fresh fruits and vegetables harvested in the United States and elsewhere (Table 2). The investigations have often been triggered by detection of more cases than expected of a rare serotype of Salmonella or Shigella or by diagnosis of a rare infection like cyclosporiasis. Outbreaks caused by common serotypes are more likely to be missed. Various possible points of contamination have been identified during these investigations, including contamination during production and harvest, initial processing and packing, distribution, and final processing (Table 3). For example, fresh or inadequately composted manure is used sometimes, although E. coli O157:H7 has been shown to survive for up to 70 days in bovine feces (25). Untreated or contaminated water seems to be a particularly likely source of contamination. Water used for spraying, washing, and maintaining the appearance of produce must be microbiologically safe. After two large outbreaks of salmonellosis were traced to imported cantaloupe, the melon industry considered a "Melon Safety Plan," focusing particularly on the chlorination of water used to wash melons and to make ice for shipping them. Although the extent to which the plan was implemented is unknown, no further large outbreaks have occurred. After two large outbreaks of salmonellosis were traced to a single tomato packer in the Southeast, an automated chlorination system was developed for the packing plant wash tank. Because tomatoes absorb water (and associated bacteria) if washed in water colder than they are, particular attention was also focused on the temperature of the water bath (26,27). No further outbreaks have been linked to southeastern tomatoes. Similar attention is warranted for water used to rinse lettuce heads in packing sheds and to crisp them in grocery stores as well as for water used in processing other fresh produce. Table 2. Foodborne outbreaks traced to fresh produce 1990-1996 ---------------------------------------------------------------- Yr. Pathogen Vehicle Cases States Source (No.) (No.) --------------------------------------------------------------- '90 S. Chester Cantaloupe 245 30 C.A. (superscript a) '90 S. Javiana Tomatoes 174 4 U.S. (superscript b) '90 Hepatitis A Strawberries 18 2 U.S. '91 S. Poona Cantaloupe >400 23 U.S./C.A '93 E. coli O157:H7 Apple cider 23 1 U.S. '93 S. Montevideo Tomatoes 84 3 U.S. '94 Shigella flexneri Scallions 72 2 C.A. '95 S. Stanley Alfalfa sprouts 242 17 N.K. (superscript c) '95 S. Hartford Orange juice 63 21 U.S. '95 E. coli O157:H7 Leaf lettuce 70 1 U.S. '96 E. coli O157:H7 Leaf lettuce 49 2 U.S. '96 Cyclospora Raspberries 978 20 C.A. '96 E. coli O157:H7 Apple juice 71 3 U.S. --------------------------------------------------------------------------- (superscript a) Central America (superscript b) United States (superscript c) Source not known Table 3. Events and potential contamination sources during produce processing --------------------------------------------------------- Event Contamination sources -------------------------------------------------------- Production and harvest Growing, picking, Irrigation water, manure, bundling lack of field sanitation Initial processing Washing, waxing, Wash water, handling sorting, boxing Distribution Trucking Ice, dirty trucks Final processing Slicing, squeezing, Wash water, handling, shredding, peeling cross-contamination --------------------------------------------------------- A New Outbreak Scenario Because of changes in the way food is produced and distributed, a new kind of outbreak has appeared. The traditional foodborne outbreak scenario often follows a church supper, family picnic, wedding reception, or other social event. This scenario involves an acute and highly local outbreak, with a high inoculum dose and a high attack rate. The outbreak is typically immediately apparent to those in the local group, who promptly involve medical and public health authorities. The investigation identifies a food-handling error in a small kitchen that occurs shortly before consumption. The solution is also local. Such outbreaks still occur, and handling them remains an important function of a local health department. However, diffuse and widespread outbreaks, involving many counties, states, and even nations (28), are identified more frequently and follow an entirely different scenario. The new scenario is the result of low-level contamination of a widely distributed commercial food product. In most jurisdictions, the increase in cases may be inapparent against the background illness. The outbreak is detected only because of a fortuitous concentration of cases in one location, because the pathogen causing the outbreak is unusual, or because laboratory-based subtyping of strains collected over a wide area identifies a diffuse surge in one subtype. In such outbreaks, investigation can require coordinated efforts of a large team to clarify the extent of the outbreak, implicate a specific food, and determine the source of contamination. Often, no obvious terminal food-handling error is found. Instead, contamination is the result of an event in the industrial chain of food production. Investigating, controlling, and preventing such outbreaks can have industrywide implications. These diffuse outbreaks can be caused by a variety of foods. Because fresh produce is usually widely distributed, most of the produce-related outbreaks listed in Table 2 were multistate events. Some of the largest outbreaks affected most states at once. For example, a recent outbreak of Salmonella Enteritidis infections caused by a nationally distributed brand of ice cream affected the entire nation (29). Although it caused an estimated 250,000 illnesses, it was detected only when vigorous routine surveillance identified a surge in reported infections with S. Enteritidis in one area of southern Minnesota. The consumers affected did not make food-handling errors with their ice cream, so food safety instruction could not have prevented this outbreak. The ice cream premix was transported after pasteurization to the ice cream factory in tanker trucks that had been used to haul raw eggs. The huge epidemic was the result of a basic failure on an industrial scale to separate the raw from the cooked. S. Enteritidis infections also illustrate why surveillance and investigation of sporadic cases are needed. A diffuse increase in sporadic cases can occur well before a local or large outbreak focuses attention on the emergence of a pathogen. The isolation rate for S. Enteritidis began to increase sharply in the New England region in 1978 (Figure 2); all cases were sporadic. In 1982, an outbreak in a New England nursing home was traced to eggs from a local supplier. However, the egg connection was not really appreciated until 1986, when a large multistate outbreak of S. Enteritidis infections was traced to stuffed pasta made with raw eggs and labeled "fully cooked." This outbreak, affecting an estimated 3,000 persons in seven states, led to the documentation that S. Enteritidis was present on egg-laying farms and to the subsequent demonstration that both outbreaks and sporadic cases of infections were associated with shell eggs (19,30). Since then, Enteritidis has become the most common serotype of Salmonella isolated in the United States, accounting for 25% of all Salmonella reported in the country and causing outbreaks coast to coast. Eggs remain the dominant source of these infections, causing large outbreaks when they are pooled and undercooked and individual sporadic cases among consumers who eat individual eggs (20,31). Perhaps focused investigation and control measures taken when the localized increase in sporadic Salmonella cases was just beginning might have prevented the subsequent spread. [Figure Not Available in Ascii] Figure 2. Salmonella Enteritidis isolation rates from humans by region, United States, 1970-1996. Changing Surveillance Strategies In the United States, surveillance for diseases of major public health importance has been conducted for many years. The legal framework for surveillance resides in the state public health epidemiology offices, which share data with CDC. The first surveillance systems depended on physician or coroner notification of specific diseases and conditions, with reports going first to the local health department, then to state and federal offices. Now electronic, this form of surveillance is still used for many specific conditions (32). In 1962, a second channel was developed specifically for Salmonella, to take advantage of the added public health information provided by subtyping the strains of bacteria (33). Clinical laboratories that isolated Salmonella from humans were requested or required to send the strains to the state public health laboratory for serotyping. Although knowing the serotype is usually of little benefit to the individual patient, it has been critical to protecting and improving the health of the public at large. Serotyping allows cases that might otherwise appear unrelated to be included in an investigation because they are of the same serotype. Moreover, infections that are close in time and space to an outbreak but are caused by nonoutbreak serotypes and are probably unrelated can be discounted. Results of serotyping are now sent electronically from public health laboratories and can be rapidly analyzed and summarized. Salmonella serotyping was the first subtype-based surveillance system and is a model for similar systems (34). Yet another source of surveillance data involves summary reports of foodborne disease outbreak investigations from local and state health departments (35). About 400 such outbreaks are reported annually, by a system that remains paper-based, labor-intensive, and slow. Existing surveillance systems provide a limited and relatively inexpensive net for tracing large-scale trends in foodborne diseases under surveillance and for detecting outbreaks of established pathogens in the United States. However, they are less sensitive to diffuse outbreaks of common pathogens, provide little detail on sporadic cases, and are not easy to extend to emerging pathogens. In the future, changes in health delivery may impinge on the way that diagnoses are made and reported, leading to artifactual changes in reported disease incidence. Therefore, CDC, in collaboration with state health departments and federal food regulatory agencies, is enhancing national surveillance for foodborne diseases in several ways. First, the role of subtyping in public health laboratories is being expanded to encompass new molecular subtyping methods. Beginning in 1997, a national subtyping network for E. coli O157:H7 of participating state public health department laboratories and CDC will use a single standardized laboratory protocol to subtype strains of this important pathogen. The standard method, pulsed-field gel electrophoresis, can be easily adapted to other bacterial pathogens. In this network, each participating laboratory will be able to routinely compare the genetic gel patterns of strains of E. coli O157:H7 with the patterns in a national pattern bank. This will enable rapid detection of clusters of related cases within the state and will focus investigative resources on the cases most likely to be linked. It will also enable related cases scattered across several states to be linked so that a common source can be sought. Another surveillance strategy, now implemented, is active surveillance in sentinel populations. Since January 1996, at five U.S. sentinel sites, additional surveillance resources make it possible to contact laboratories directly for regular reporting of bacterial infections likely to be foodborne (36; Figure 3). In addition, surveys of the population, physicians, and laboratories measure the proportion of diarrheal diseases that are undiagnosed and unreported so that the true disease incidence can be estimated. This surveillance, known as FoodNet, is the platform on which more detailed investigations, including case-control studies of sporadic cases of common foodborne infections, are being conducted. [Figure Not Available in ASCII] Figure 3. Incidence of three infections in FoodNet surveillance areas, 1996. Yet another new surveillance initiative is the routine monitoring of antimicrobial resistance among a sample of Salmonella and E. coli O157:H7 bacteria isolated from humans (37). A new cluster detection algorithm is being applied routinely to surveillance data for Salmonella at the national level, making it possible to detect and flag possible outbreaks as soon as the data are reported (38). Implementation of such algorithms for other infections and at the state level will further increase the usefulness of routine surveillance. Further enhancements are possible as active surveillance through FoodNet is extended to a wider spectrum of infections, including foodborne parasitic and viral infections. In 1997, active surveillance for Cyclospora began in FoodNet, which quickly resulted in the detection of a diffuse outbreak among persons who had been on a Caribbean cruise ship that made stops in Mexico and Central America (CDC, unpub. data). Application of standardized molecular subtyping methods to other foodborne pathogens will provide a more sensitive warning system for diffuse outbreaks of a variety of pathogens. To handle outbreaks in areas not covered by FoodNet, standard surveillance and investigative capacities in state health department epidemiology offices and laboratories should be strengthened. In addition, enhanced international consultation will be critical to better detect and investigate international or global outbreaks (28). Implications of the New Outbreak Scenario for Public Health Activities Our public health infrastructure is tiered, both in surveillance responsibilities and in response to emergency situations (39). At the local level, the county or city health department, first developed in response to epidemic cholera and other challenges in the 19th century, is responsible for most basic surveillance, investigation, and prevention activities. At the state level, epidemiologists, public health laboratorians, sanitarians, and educators conduct statewide surveillance and prevention activities and consult with and support local authorities. At the national level, CDC is the primary risk-assessment agency for public health hazards and conducts the primary national surveillance as well as epidemic response in support of state health departments. The Food and Drug Administration, Department of Agriculture, and Environmental Protection Agency are the primary regulatory agencies, charged with specific responsibilities regarding the nation's food and water supplies that interlock and are not always predictable. The Food and Drug Administration regulates low-acid canned foods, imported foods, pasteurized milk, many seafoods, rabbits raised for meat, and food and water provided on aircraft and trains. The Department of Agriculture regulates meat and poultry, including primary slaughter and further processing, and pasteurized eggs; investigates animal and plant diseases; and maintains the county extension outreach program. Shell eggs do not have a clear regulatory home, as the Department of Agriculture regulates the grading of shell eggs for quality, but the Food and Drug Administration, since 1995, has responsibility for the microbiologic safety of shell eggs. The new outbreak scenario has several implications for the practice of public health, starting at the local level. One is that when diffuse outbreaks are detected, a local health department may need to investigate a few cases that are part of a larger outbreak despite their apparently small local impact. Second, an apparently local outbreak may herald the first recognized manifestation of a national or even international event. When a diffuse outbreak of a potentially foodborne pathogen is detected, rapid investigation is needed to determine whether the outbreak is foodborne, and if possible, identify a specific food vehicle. These investigations, which typically include case-control studies, may need to be conducted in several locations at once. While all cases or all affected states may not need to be included in such an investigation, combining cases from several locations in one investigation and repeating the investigation in more than one location can be helpful. For example, in a recent international outbreak of Salmonella Stanley infections traced to alfalfa sprouts, concentrations of cases in Arizona, Michigan, and Finland led to case-control studies in each location, each of which linked illness to eating sprouts grown from the same batch of alfalfa seeds. This proved that the seeds were contaminated at the source (40). Parallel investigations can also lead to new twists. In the large West Coast outbreak of E. coli O157:H7 infections in 1993, a parallel investigation conducted in Nevada identified a type of hamburger other than the one implicated in the initial case-control investigation in Washington, leading to a broader recall and a more complete investigation of the circumstances of contamination (15,41). Because well-conducted investigations may lead to major product recalls, industrial review, and overhaul, and even international embargoes, it is essential that they be of the highest scientific quality. Foodborne outbreaks are investigated for two main reasons. The first is to identify and control an ongoing source by emergency action: product recall, restaurant closure, or other temporary but definitive solutions. The second reason is to learn how to prevent future similar outbreaks from occurring. In the long run this second purpose will have an even greater impact on public health than simply identifying and halting the outbreaks. Because all the answers are not available and existing regulations may not be sufficient to prevent outbreaks, the scientific investigation often requires a careful evaluation of the chain of production. This traceback is an integral part of the outbreak investigation. It is not a search for regulatory violations, but rather an effort to determine where and how contamination occurred. Often, the contamination scenario reveals that a critical point has been lost. Therefore, epidemiologists must participate in traceback investigations. Intervention during outbreaks often depends on having enough good epidemiologic data to act with confidence, without waiting for a definitive laboratory test, particularly if potentially lethal illnesses are involved. For example, if five persons with classic clinical botulism ate at the same restaurant the preceding day (but have nothing else apparent in common), prudence dictates closing the restaurant quickly while the outbreak is sorted out-that is, before a specific food is identified or confirmatory cultures are made, which may take several days or even weeks. Good epidemiologic data, including evidence of a clear statistical association with a specific exposure, biologic plausibility of the illness syndrome, the potential hazard of that food, and the logical consistency of distribution of the suspect food and cases are essential. The role of the regulatory agency laboratory is also affected by the new scenario. Because of the short shelf life and broad distribution of many of the new foods responsible for infection, by the time the outbreak is recognized and investigated the relevant food may no longer be available for culture. Because contamination may be restricted to a single production lot, blind sampling of similar foods that does not include the implicated lot can give a false sense of security. Good epidemiologic information pointing to contamination of a specific food or production lot should guide the microbiologic sampling and the interpretation of the results. Available methods may be insufficient to detect low-level contamination, even of well-established pathogens. New Approaches to the Prevention of Foodborne Disease Meeting the complex challenge of foodborne disease prevention will require the collaboration of regulatory agencies and industry to make food safely and keep it safe throughout the industrial chain of production. Prevention can be "built in" to the industry by identifying and controlling the key points-from field, farm, or fishing ground to the dinner table-at which contamination can either occur or be eliminated. The general strategy known as Hazard Analysis and Critical Control Points (HACCP) replaces the strategy of final product inspection. Some simple control strategies are self-evident, once the reality of microbial contamination is recognized. For example, shipping fruit from Central America with clean ice or in closed refrigerator trucks, rather than with ice made from untreated river water, is common sense. Similarly, requiring oyster harvesters to use toilets with holding tanks on their oyster boats is an obvious way to reduce fecal contamination of shallow oyster beds. Pasteurization provides the extra barrier that will prevent E. coli O157:H7 and other pathogens from contaminating a large batch of freshly squeezed juice. For many foodborne diseases, multiple choices for prevention are available, and the best answer may be to apply several steps simultaneously. For E. coli O157:H7 infections related to the cattle reservoir, pasteurizing milk and cooking meat thoroughly provide an important measure of protection but are insufficient by themselves. Options for better control include continued improvements in slaughter plant hygiene and control measures under HACCP, developing additives to cattle feed that alter the microbial growth either in the feed or in the bovine rumen to make cows less hospitable hosts for E. coli O157, immunizing or otherwise protecting the cows so that they do not become infected in the first place, and irradiating beef after slaughter. For C. jejuni infections related to the poultry reservoir, future control options may include modification of the slaughter process to reduce contamination of chicken carcasses by bile or by water baths, freezing chicken carcasses to reduce Campylobacter counts, chlorinating the water that chickens drink to prevent them from getting infected, vaccinating chickens, and irradiating poultry carcasses after slaughter. Outbreaks are often fertile sources of new research questions. Translating these questions into research agendas is an important part of the overall prevention effort. Applied research is needed to improve strategies of subtyping and surveillance. Veterinary and agricultural research on the farm is needed to answer the questions about whether and how a pathogen such as E. coli O157:H7 persists in the bovine reservoir, to establish the size and dynamics of a reservoir for this organism in wild deer, and to look at potential routes of contamination connecting animal manure and lettuce fields. More research is needed regarding foods defined as sources in large outbreaks to develop better control strategies and better barriers to contamination and microbial growth and to understand the behavior of new pathogens in specific foods. Research is also needed to improve the diagnosis, clinical management, and treatment of severe foodborne infections and to improve our understanding of the pathogenesis of new and emerging pathogens. To assess and evaluate potential prevention strategies, applied research is needed into the costs and potential benefits of each or of combinations. To prepare for the 21st century, we will enhance our public health food safety infrastructure by adding new surveillance and subtyping strategies and strengthening the ability of public health practitioners to investigate and respond quickly. We need to encourage the prudent use of antibiotics in animal and human medicine to limit antimicrobial resistance. We need to continue basic and applied research into the microbes that cause foodborne disease and into the mechanisms by which they contaminate our foods and cause outbreaks and sporadic cases. Better understanding of foodborne pathogens is the foundation for new approaches to disease prevention and control. Address for correspondence: Robert V. Tauxe, Centers for Disease Control and Prevention, 1600 Clifton Road, MS A38, Atlanta, GA 30333 USA; fax: 404-639-2205. References 1. Blake PA, Merson MH, Weaver RE, Hollis DG, Heublein PC. Disease caused by a marine vibrio: clinical characteristics and epidemiology. N Engl J Med 1979;300:1-5. 2. Riley LW, Remis RS, Helgerson SD, McGee HB, Wells JG, Davis BR, et al. Hemorrhagic colitis associated with a rare Escherichia coli serotype. N Engl J Med 1983;308:681. 3. Martin ML, Shipman LD, Wells JG, Potter ME, Hedberg K, Wachsmuth IK, et al. Isolation of Escherichia coli O157:H7 from dairy cattle associated with two cases of hemolytic uremic syndrome. Lancet 1986;2:1043. 4. Ortega YR, Sterling CR, Gilman RH, Cama VA, Diaz F. Cyclospora species-a new protozoan pathogen of humans. N Engl J Med 1993;328:1308-12. 5. Herwaldt BL, Ackers M-L, the Cyclospora Working Group. International outbreak of cyclosporiasis associated with imported raspberries. N Engl J Med. In press 1997. 6. Jackson LA, Wenger JD. Listeriosis: a foodborne disease. Infections in Medicine 1993;10:61-6. 7. Dekeyser PJ, Gossin-Detrain M, Butzler JP, Sternon J. Acute enteritis due to related Vibrio; first positive stool cultures. J Infect Dis 1972;125:390-2. 8. Tauxe RV. Epidemiology of Campylobacter jejuni infections in the United States and other industrialized nations. In: Nachamkin I, Blaser MJ, Tompkins L, editors. Campylobacter jejuni: current status and future trends, eds. Washington (DC): American Society of Microbiology, 1992. pp 9-19. 9. Tauxe RV, Vandepitte J, Wauters G, Martin SM, Goosens V, DeMol P, et al. Yersinia enterocolitica infections and pork: the missing link. Lancet 1987;1:1129-32. 10. World Health Organization. Worldwide spread of infections with Yersinia enterocolitica. WHO Chronicle 1976;30:494-6. 11. Rodrigue DC, Tauxe RV, Rowe B. International increase in Salmonella enteritidis: a new pandemic? Epidemiol Infect 1990;105:21-7. 12. Centers for Disease Control and Prevention. Multidrug-resistant Salmonella serotype Typhimurium-United States, 1996. MMWR Morb Mortal Wkly Rep 1997;46:308-10. 13. Endt HP, Ruijs GJ, van Klingeren B, Jansen WH, van der Reyden T, Mouton RP. Quinolone resistance in Campylobacter isolated from man and poultry following the introduction of fluoroquinolones in veterinary medicine. J Antimicrob Chemother 1991;27:199-208. 14. Lee LA, Puhr ND, Maloney K, Bean NH, Tauxe RV. Increase in antimicrobial-resistant Salmonella infections in the United States, 1989-1990. J Infect Dis 1994;170:128-34. 15. Cieslak PR, Noble SJ, Maxson DJ, Empey LC, Ravenholt O, Legarza G, et al. Hamburger-associated Escherichia coli O157:H7 in Las Vegas: a hidden epidemic. Am J Public Health 1997;87:176-80. 16. Humphrey TJ, Greenwood M, Gilbert RJ, Rowe B, Chapman PA. The survival of salmonellas in shell eggs cooked under simulated domestic conditions. Epidemiol Infect 1989;103:35-45. 17. Kirkland KB, Meriwether RA, Leiss JK, MacKenzie WR. Steaming oysters does not prevent Norwalk-like gastroenteritis. Public Health Rep 1996;111:527-30. 18. Holmberg SD, Feldman RA. New and newer enteric pathogens: stages in our knowledge. Am J Public Health 1984;74:205-7. 19. St. Louis ME, Morse DL, Potter ME, DeMelfi TM, Guzewich JJ, Tauxe RV, et al. The emergence of Grade A eggs as a major source of Salmonella enteritidis infections: implications for the control of salmonellosis. JAMA 1988;259:2103-7. 20. Mishu B, J Koehler, Lee LA, Rodrigue D, Brenner FH, Blake P, Tauxe RV. Outbreaks of Salmonella enteritidis infections in the United States, 1985-1991. J Infect Dis 1994;169:547-52. 21. Besser RE, Lett SM, Weber JT, Doyle MP, Barrett TJ, Wells JG, Griffin PM. An outbreak of diarrhea and hemolytic uremic syndrome from Escherichia coli O157:H7 in fresh-pressed apple cider. JAMA 1993;269:2217-20. 22. Keene WE, Sazie E, Kok J, Rice DH, Hancock DD, Balan VK, et al. An outbreak of Escherichia coli O157:H7 infections traced to jerky made from deer meat. JAMA 1997;277:1229-31. 23. Kohn MA, Farley TA, Ando T, Curtis M, Wilson SA, Jin Q, et al. An outbreak of Norwalk virus gastroenteritis associated with eating raw oysters: implications for maintaining safe oyster beds. JAMA 1995;273:466-71. 24. Tauxe R, Kruse H, Hedberg C, Potter M, Madden J, Wachsmuth K. Microbial hazards and emerging issues associated with produce; a preliminary report to the National Advisory Committee on Microbiologic Criteria for Foods. Journal of Food Protection. In press 1997. 25. Wang G, Zhao T, Doyle MP. Fate of enterohemorrhagic Escherichia coli O157:H7 in bovine feces. Appl Environ Microbiol 1996;62:2567-70. 26. Zhuang R-Y, Beuchat LR, Angulo FJ. Fate of Salmonella montevideo on and in raw tomatoes as affected by temperature and treatment with chlorine. Appl Environ Microbiol 1995;61:2127-31. 27. Rushing JW, Angulo FJ, Beuchat LR. Implementation of a HACCP program in a commercial fresh-market tomato packinghouse: a model for the industry. Dairy, Food and Environmental Sanitation 1996;16:549-53. 28. Tauxe RV, Hughes JM. International investigations of outbreaks of foodborne disease: public health responds to the globalization of food. BMJ 1996;313:1093-4. 29. Hennessey TW, Hedberg CW, Slutsker L, White KE, Besser-Wiek JM, Moen ME, et al. A national outbreak of Salmonella enteritidis infections from ice cream. N Engl J Med 1996;334:1281-6. 30. Passarro DJ, Reporter R, Mascola L, Kilman L, Malcolm GB, Rolka H, et al. Epidemic Salmonella Enteritidis infection in Los Angeles County, California: the predominance of phage type 4. West J Med 1996;165:126-30. 31. Centers for Disease Control and Prevention. Outbreaks of Salmonella serotype Enteritidis infection associated with consumption of raw shell eggsUnited States, 1994-1995. MMWR Morb Mortal Wkly Rep 1996;45:737-42. 32. Centers for Disease Control and Prevention. Summary of notifiable diseases, United States, 1995. MMWR Morb Mortal Wkly Rep 1995;44(53). 33. Centers for Disease Control and Prevention. Proceedings of a national conference on salmonellosis, March 11-13, 1964. U.S. Public Health Service Publication No 1262. Washington (DC): U.S. Government Printing Office; 1965. 34. Bean NH, Morris SM, Bradford H. PHLIS: an electronic system for reporting public health data from remote sites. Am J Public Health 1992;82:1273-6. 35. Bean NH, Goulding JS, Lao C, Angulo FJ. Surveillance for foodborne-disease outbreaksUnited States, 1988-1992. CDC Surveillance Summaries, October 25, 1996. MMWR Morb Mortal Wkly Rep 1996;45(SS-5). 36. Centers for Disease Control and Prevention. Foodborne Diseases Active Surveillance Network, 1996. MMWR Morb Mortal Wkly Rep 1997;46:258-61. 37. Centers for Disease Control and Prevention. Establishment of a national surveillance program for antimicrobial resistance in Salmonella. MMWR Morb Mortal Wkly Rep 1996;45:110-1. 38. Hutwagner LC, Maloney EK, Bean NH, Slutsker L, Martin SM. Using laboratory-based surveillance data for prevention: an algorithm for detecting Salmonella outbreaks. Emerg Infect Dis 1997;3:395-400. 39. Meriwether RA. Blueprint for a national public health surveillance system for the 21st Century. Journal of Public Health Management and Practice 1996;2:16-23. 40. Mahon BE, Pönkä A, Hall WN, Komatsu K, Dietrich SE, Siitonen A, et al. An international outbreak of Salmonella infections caused by alfalfa sprouts grown from contaminated seed. J Infect Dis 1997;175:876-82. 41. Bell BP, Goldoft M, Griffin PM, Davis MA, Gordon DC, Tarr PI, et al. A multistate outbreak of Escherichia coli O157:H7-associated bloody diarrhea and hemolytic uremic syndrome from hamburgers: the Washington experience. JAMA 1994;272:1349-53. --------------------------------------------------------------------------- Emerging Infectious Diseases National Center for Infectious Diseases Centers for Disease Control and Prevention Atlanta, GA URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/tauxe.txt Emergence of New Pathogens as a Function of Changes in Host Susceptibility J. Glenn Morris, Jr.* and Morris Potter† *University of Maryland School of Medicine, Baltimore, Maryland, USA; and †Centers for Disease Control and Prevention, Atlanta, Georgia, USA --------------------------------------------------------------------------- A pathogen may emerge as an important public health problem because of changes in itself or its transmission pathways. Alternatively, a microorganism may emerge as a pathogen or acquire new public health importance because of changes in host susceptibility to infection. Factors influencing host susceptibility within the population as a whole include increases in the number of immunocompromised patients; increased use of immunosuppressive agents, particularly among persons receiving cancer chemotherapy or undergoing organ transplantation; aging of the population; and malnutrition. In considering the emergence of foodborne pathogens and designing interventions to limit their spread, the susceptibility of these population subgroups to specific infections should be taken into account. Occurrence of disease is a function of several major variables: the virulence of the microorganism (i.e., its possession of factors that allow it to cause illness), its mode of transmission (i.e., how it gets to the host), and host susceptibility (i.e., how well the host can defend itself against the microorganism). Increased susceptibility to infection may be measured in terms of infectious dose (the number of microorganisms it takes to cause illness) and of the ability of the host to limit spread of the microorganism (e.g., the ability to limit spread of microorganisms from the intestinal tract to the bloodstream). These same variables apply to emerging pathogens. Microorganisms may emerge as pathogens because they have developed new virulence genes or resistance to standard therapeutic methods. Emergence may be related to changes in transmission pathways, which permit a known pathogen to move into new, previously unexposed populations. Finally, increases in the number of persons susceptible to a specific microorganism may result in its emergence as an important public health problem; at the same time, when attention is focused on populations with increased susceptibility to infection, organisms that might not otherwise be recognized as pathogens may be identified. Many factors influence the susceptibility of populations to infection, including increases in diseases that cause immunosuppression, increased use of immunosuppressive agents, aging of the population, and malnutrition. In considering these categories, it should be recognized that "host susceptibility" is not a single entity. Changes in host susceptibility may be due to various mechanisms, with each mechanism having a greater or lesser impact on the ability of the host to defend itself against infection with specific pathogens or classes of pathogens. These are very complex biologic systems; nonetheless, the general categories outlined below may be of value in identifying groups or populations for further study. Increases in Diseases That Cause Immunosuppression Hereditary diseases associated with immunosuppression are present in a small but relatively constant proportion of the population. The most common of these diseases, a selective immunoglobulin A deficiency, has been found in as many as 0.3% of some blood donor populations (1) and may be associated with recurrent diarrhea; infections with giardia, in particular, have been noted to be more common among such immunocompromised patients. In contrast to patients with hereditary immunodeficiencies, the population of patients with acquired immunodeficiencies is rapidly increasing. As of June 1996 in the United States, an estimated 223,000 persons > 13 years of age had AIDS (Figure 1); this represents increases of 10% and 65% since mid-1995 and January 1993, respectively. During 1995, human immunodeficiency virus (HIV) infection remained the leading cause of death among persons 25 to 44 years of age (Figure 2), accounting for 19% of deaths from all causes in this age group (2). [Figure Not Available in ASCII] Figure 1. Cases among persons aged > 13 years, adjusted for delays in reporting, by quarter year, United States, 1988-June 1996. (Points represent quarterly prevalence; the line represents "smoothed" prevalence. Estimates are not adjusted for incomplete reporting of diagnosed AIDS cases or AIDS deaths. From the Centers for Disease Control and Prevention. Update: Trends in AIDS incidence, deaths, and prevalence-United States, 1996. MMWR Morb Mortal Wkly Rep 1997;46:165-73). [Figure Not Available in ASCII] Figure 2. Death rates per 100,000 population for leading causes of death among persons ages 25 to 44 years, by year, United States, 1982-1995. (Based on underlying causes of death reported on death certificates, using final data for 1982-1994 and preliminary data for 1995. From the Centers for Disease Control and Prevention. Update: Trends in AIDS incidence, deaths, and prevalence, United States, 1996. MMWR Morb Mortal Wkly Rep 1997;46:165-73). Persons with AIDS show a clear increase in susceptibility to infection with Salmonella species. Data suggest that risk for nontyphi Salmonella infections is increased 20- to 100-fold among AIDS patients (3-7). Among persons infected with Salmonella, AIDS results in a several-fold increase in the risk for septicemia (3,5,6); AIDS also results in increases in infections at other extraintestinal sites, compatible with an overall increase in risk for dissemination of the organism. This increase in risk is reflected in increases in the percentage of total Salmonella isolated from blood. For example, for persons ages 25 to 49 years in states with high AIDS incidence, the percentage of Salmonella isolates from blood increased from 2.3% in 1978 to 17.8% in 1987 among men and from 3.1% to 8.1% among women; in contrast, no changes in blood-isolate percentages occurred for either sex in states with low AIDS incidence (8). These latter studies further suggest that serotype is an important risk determinant, with increases in bacteremia in states with high AIDS incidence associated primarily with infections due to Salmonella Enteritidis and Salmonella Typhimurium. While these data are for nontyphoidal Salmonella, studies outside the United States suggest that AIDS patients have a similar increase in risk for infection with Salmonella typhi in areas endemic for typhoid fever. For example, in Lima, Peru, the risk for typhoid increased 25-fold in HIV-infected persons 15 to 35 years of age (9); HIV infection also appeared to influence the clinical presentation of typhoid fever, with severe diarrhea and gastrointestinal symptoms seen more often than expected. While the above data suggest a continuing climb in salmonellosis and, in particular, Salmonella bacteremia in conjunction with an increasing AIDS prevalence, anecdotal data from AIDS clinicians do not support this view. The standard of care for AIDS patients now includes routine prophylactic therapy with trimethoprim/sulfamethoxazole to prevent Pneumocystis carinii pneumonia. Trimethoprim/sulfamethoxazole is also one of the first-line therapies for salmonellosis; widespread prophylactic use of this drug in the AIDS population may have reduced the incidence of serious Salmonella infections, although this protective effect could be diminished in the face of increasing resistance to this antimicrobial agent among clinical isolates of Salmonella (10). Fewer data are available on susceptibility of AIDS patients to other acute bacterial foodborne infections. While the initial impression was that the risk for Campylobacter jejuni infections was not higher among AIDS patients, a 35-fold increase in the Campylobacter case rate among persons with AIDS was noted in one study from Los Angeles (11). Other data indicate that HIV-positive patients can contract persistent C. jejuni infections, with chronic diarrhea, fever, and fecal leukocytes (12). In studies in the San Francisco area, AIDS patients were estimated to have a 280-fold increase in incidence of listeriosis, as compared with the general population (13). Of 98 nonpregnant adults with invasive Listeria infection identified between November 1988 and December 1990 in selected counties in California, Tennessee, Georgia, and Oklahoma, 20% were HIV-positive (13). Before AIDS, Toxoplasma gondii was of concern primarily because of the risk for congenital infection in infants of mothers who had acute illness during pregnancy. T. gondii is now the leading cause of space-occupying cranial lesions in persons with AIDS (14,15); data from the 1980s suggest that 5% to 10% of AIDS patients get toxoplasmic encephalitis (16). In an estimated 50% of cases, Toxoplasma is transmitted by food (17). In this context, Toxoplasma must be regarded as an important emerging pathogen in this patient population. The AIDS epidemic has also drawn attention to microorganisms not previously recognized as pathogens. Perhaps the most important of these is Cryptosporidium. In early investigations of AIDS-associated diarrhea, it rapidly became apparent that most patients were not infected with traditional enteric pathogens. Many of these patients were infected with Cryptosporidium; an estimated 10% to 20% of cases of AIDS-associated diarrhea are due to this microorganism (18). In subsequent years, Cryptosporidium was also recognized as the cause of intestinal infections in healthy hosts (19); and, most recently, it has been recognized as a major waterborne pathogen (20). Isospora belli has also been implicated as a cause of diarrhea in AIDS patients, as have the microsporidia (18). More recently, enteroaggregative Escherichia coli and nonpathogenic bioserovars of Yersinia enterocolitica were associated with diarrheal disease in AIDS cases (21,22). Further work will determine if these latter agents are indeed emerging pathogens in this patient population. Increased Use of Immunosuppressive Agents Advances in medical treatment have resulted in increasing numbers of immunosuppressed patients (including patients undergoing organ transplantation or cancer chemotherapy) and patients with serious underlying chronic diseases; these patients, too, may be at increased risk for infection with microorganisms that might otherwise not be recognized or associated with serious illness. The number of new cancer cases has steadily increased over the past 20 years. For white males, cancers at all sites have increased from an estimated 364 new cases/100,000 population in 1973 to 462 new cases/100,000 population in 1994; white females show an increase from 295 new cases/100,000 population in 1973 to 347 new cases/100,000 population in 1994 (23). Patients are also surviving longer. For white males, the 5-year relative cancer survival rate was 56.6% in 1986 to 1993, compared with 42% in 1974 to 1976; for white females, the 1986 to 1993 rate was 62.3%, compared with 57.6% in 1974 to 1976. While specific data are lacking, these increases appear to have been accompanied by increased use of chemotherapeutic regimens and regimens that may have more toxicity than those used in the past. The number of solid organ transplants has also increased substantively (Figure 3). In particular, more complex procedures such as liver, heart, and lung transplants have increased. This, in turn, has resulted in larger numbers of chronically immunocompromised persons in the general population. [Figure Not Available in ASCII] Figure 3. Number of organ transplants, by year and by site, 1988-1996 (data obtained from the United Network for Organ Sharing). Note: Heart-lung transplants too few to be visible. Aside from the direct immunosuppressive effect of the agents administered to these patients, other associated factors may contribute to susceptibility to infection. Many, if not most, patients receiving chemotherapy or immunosuppressive agents are also treated with antimicrobial drugs, which can have profound effects on the bacterial flora of the intestinal tract. These disturbances of gut microbial ecology may predispose to colonization and infection with other microorganisms, some of which may have increased virulence. Many chemotherapeutic agents have direct toxicity for the gut mucosa; the resulting mucositis increases the susceptibility of these patients to bloodstream infections with whatever microorganisms are present in gut. The concentration of these highly susceptible patients on certain wards or units in a hospital may also increase the risk for nosocomial transfer of specific microorganisms. For example, vancomycin-resistant enterococci (VRE) (which, at least in Europe, have been associated with food [24]) have emerged as a substantive problem in cancer centers and transplant units (25-27). Persons who are immunosuppressed or have serious underlying illness are much more susceptible to colonization and infection with the organism. In some oncology and transplant units, more than 10% of patients are colonized or infected with VRE (26,27), providing well-documented opportunities for transfer of the organism to other immunosuppressed hosts in the same unit. Cancer patients who have just undergone chemotherapy are often profoundly neutropenic. In this setting, especially in conjunction with the mucositis mentioned above, virtually any microorganism in the intestinal tract can enter the bloodstream and cause potentially fatal illness. For example, it has been found that raw produce in salads may be an important route by which patients acquire Pseudomonas (28); as a result, severely neutropenic patients are generally restricted to cooked food. Salmonella infections are reported among cancer patients, although the relative risk for infection in this population is not well characterized. Patients with neoplastic disease do appear to have a substantively increased risk for Salmonella septicemia, with 35% patients in one series having septicemia (29), versus fewer than 1% in healthy hosts. Cancer patients appear to be at increased risk for invasive Listeria infections (13). Toxoplasmosis tends to be of particular concern among transplant patients (30,31). Aging of the Population The absolute number of the elderly in the United States is rapidly increasing, as is the proportion of the U.S. population they comprise. In 1950, 3.8 million persons (2.6% of the population) were over the age of 74, as opposed to 14.7 million (5.6% of the population) in 1995 (23; Figures 4 and 5). The elderly appear to be at a clearly increased risk for death from foodborne and diarrheal disease. Between 1979 and 1987, 28,538 persons in the United State had diarrhea as an immediate or underlying cause of death; 51% of these persons were more than 74 years of age, 27% were adults age 55 to 74, and 11% were children under the age of 5 (32). [Figure Not Available in ASCII] Figure 4. Number of persons >74 years of age, U.S. population, for selected years, 1950-1990. From the National Center for Health Statistics. Health, United States, 1996-97 and Injury Chartbook. Hyattsville, Maryland, 1997. [Figure Not Available in ASCII] Figure 5. Percentage of U.S. population >74 years of age, for selected years, 1950-1990. From National Center for Health Statistics. Health, United States, 1996-97 and Injury Chartbook. Hyattsville, Maryland, 1997. The increased susceptibility of the elderly to infection and death may be due to a number of factors. Aging results in senescence of the gut-associated lymphoid tissue, increasing susceptibility to infection. Aging may also result in a decrease in gastric acid secretion: in one study, stimulated acid output was reduced approximately 30% in persons aged 65 to 98, with a 40% reduction in pepsin output (33). As a low pH of the stomach is a major barrier to entry of enteric pathogens, reductions in gastric acidity can clearly increase the susceptibility to infection. Social factors may also influence susceptibility. For the elderly, residence in a long-term care facility was a major independent risk factor for diarrheal death (32). While a number of factors contribute to the increased risk, the communal living environment, combined with problems of fecal incontinence, create an environment in which enteric and foodborne pathogens are easily spread (32,34). Incidence of salmonellosis and Campylobacter diarrhea appears to be higher among the elderly (35,36). More striking, however, is the increase in frequency of Salmonella bacteremia as compared with isolations from other sites: Salmonella infections in the elderly are more likely to cause bacteremia (37; Figure 6), which, in turn, substantively increases the risk for death. For example, in a recent nursing home outbreak in Maryland, 50 (35%) of 141 residents became ill, seven had bacteremia, nine were hospitalized (with a median length of hospitalization of 22 days), and four died (38). E. coli O157:H7 is also a common pathogen in nursing homes and among the elderly. In one reported nursing home outbreak, 55 of 169 residents were affected; overall, 19 (35%) of the affected residents died (39). [Figure Not Available in ASCII] Figure 6. Ratio of blood isolates to total isolates of Salmonella by age group of the person from whom the isolate was obtained as reported to the Centers for Disease Control, Atlanta, GA, in 1968-79. From Blaser MJ, Feldman RA. Salmonella bacteremia: reports to the Centers for Disease Control and Prevention, 1968-1979. J Infect Dis 1981;143:743-6. Malnutrition The above discussions have focused on issues most relevant to the United States and other industrialized countries. However, on a global scale, probably the leading cause of increased host susceptibility to infection is malnutrition. While accurate data on the prevalence of malnutrition are difficult to obtain, problems are accentuated in developing countries, in areas of political unrest, and among marginalized populations in the United States and other affluent nations. In Mexico, according to a probabilistic survey in 1990, 42.3% of children under 5 years of age had some degree of malnutrition (40). Malnutrition increases host susceptibility through a number of mechanisms. It weakens epithelial integrity and may have a profound effect on cell-mediated immunity, with functional deficiencies in immunoglobulins and defects in phagocytosis. Malnutrition also may initiate a "vicious cycle" of infection predisposing to malnutrition and growth faltering, which in turn may lead to an increased risk for further infection (40,41). In studies in Bangladesh, malnourished and well-nourished children had the same number of infections with diarrheal pathogens such as enterotoxigenic E. coli; however, diarrhea in malnourished children was of longer duration and had greater potential long-term nutritional consequences (42). Overall, malnutrition appears to result in a 30-fold increase in the risk for diarrhea-associated death (40). Conclusions Host susceptibility (and changes in the susceptibility to infection of groups within the general population) is a critical variable in assessing the public health effects and understanding the emergence and spread of pathogenic microorganisms. Surveillance within populations with increased susceptibility to infection may allow identification of new pathogens before they are recognized within the general population. Studies designed to identify the reasons for the increased susceptibility of a specific population to a specific agent may reveal how a microorganism is able (or not able) to breach normal host defense mechanisms. Finally, from a public health standpoint, risk management strategies for emergent foodborne pathogens must clearly identify and focus on populations with increased susceptibility to infection. Address for correspondence: Glenn Morris, 934 MSTF, University of Maryland School of Medicine, 10 S. Pine St., Baltimore, MD, 21201, USA; fax: 410-706-4581; e-mail: jmorris@umppa1.ab.umd.edu. References 1. Buckley RH. Primary immunodeficiency diseases. In: Bennett JC, Plum F, editors. Cecil textbook of medicine. Philadelphia: W.B. Saunders Company; 1996 .p.1401-8. 2. Centers for Disease Control and Prevention. Update: trends in AIDS incidence, deaths, and prevalenceUnited States, 1996. MMWR Morb Mortal Wkly Rep 1997;46:165-73. 3. Celum CL, Chaisson RE, Rutherford GW, Barnhart JL, Echenberg DF. Incidence of Salmonellosis in patients with AIDS. J Infect Dis 1987;156:996-1002. 4. Smith PD, Macher AM, Bookman MA, Boccia RV, Steis RG, Gill V, et al. Salmonella typhimurium enteritis and bacteremia in the acquired immunodeficiency syndrome. Ann Intern Med 1985;102:207-9. 5. Profeta S, Forrester C, Eng RHK, Liu R, Johnson E, Palinkas R, Smith SM. Salmonella infections in patients with acquired immunodeficiency syndrome. Arch Intern Med 1985;145:670-2. 6. Gruenewald R, Blum S, Chan J. Relationship between human immunodeficiency virus infection and Salmonellosis in 20- to 59-year-old residents of New York City. Clin Infect Dis 1994;18:358-63. 7. Angulo FJ, Swerdlow DL. Bacterial enteric infections in persons infected with human immunodeficiency virus. Clin Infect Dis 1995;21:S8493. 8. Levine WC, Buehler JW, Bean NH, Tauxe RV. Epidemiology of nontyphoidal Salmonella bacteremia during the human immunodeficiency virus epidemic. J Infect Dis 1991;164:81-7. 9. Gotuzzo E, Frisancho O, Sanchez J, Liendo G, Carrillo C, Black RE, Morris JG Jr. Association between the acquired immunodeficiency syndrome and infection with Salmonella typhi or Salmonella paratyphi in an endemic typhoid area. Arch Intern Med 1991;151:381-2. 10. Lee LA, Puhr ND, Maloney EK, Bean NH, Tauxe RV. Increase in antimicrobial-resistant Salmonella infections in the United States, 19891990. J Infect Dis 1994;170:12834. 11. Sorvillo FJ, Lieb LE, Waterman SH. Incidence of campylobacteriosis among patients with AIDS in Los Angeles County. J Acquir Immune Defic Syndr Hum Retrovirol 1991;4:598-602. 12. Perlman DM, Ampel NM, Schifman RB, Cohn DL, Patton CM, Aguirre ML, et al. Persistent Campylobacter jejuni infections in patients infected with human immunodeficiency virus (HIV). Ann Intern Med 1988;108:540-6. 13. Schuchat A, Deaver KA, Wenger JD, Plikaytis BD, Mascola L, Pinner RW, et al. Role of foods in sporadic listeriosis. 1. Case-control study of dietary risk factors. JAMA 1992;267:2041-50. 14. Luft BJ, Brooks RG, Conley FK, McCabe RE, Remington JS. Toxoplasmid encephalitis in patients with acquired immune deficiency syndrome. JAMA 1984;252:913-7. 15. Porter SB, Sande MA. Toxoplasmosis of the central nervous system in the acquired immunodeficiency syndrome. N Engl J Med 1992;327:1643-8. 16. McCabe RE, Remington JS. Toxoplasma gondii. In: Mandell GL, Douglas RG, Bennett JE, editors. Principles and practice of infectious diseases. 3rd ed. New York: Churchill Livingston; 1990. p. 2090-103. 17. United States Department of Agriculture. Pathogen reduction; hazard analysis and critical control point (HACCP) systems; proposed rule. Washington (DC): Federal Register 1995;60:6774-889. 18. Smith PD, Quinn TC, Strober W, Janoff EN, Masur H. Gastrointestinal infections in AIDS. Ann Intern Med 1992;116:63-77. 19. Wolfson JS, Richter JM, Waldron MA, Weber DJ, McCarthy DM, Hopkins CC. Cryptosporidiosis in immunocompetent patients. N Engl J Med 1985;312:1278-82. 20. Centers for Disease Control and Prevention. Assessing the public health threat associated with waterborne cryptosporidiosis: report of a workshop. MMWR Morb Mortal Wkly Rep 1995;44(RR-6):19. 21. Mayer HB, Acheson D, Wanke CA. Enteroaggregative Escherichia coli are a potential cause of persistent diarrhea in adult HIV patients in the United States. In: Program and abstracts of the 31st US-Japan Cholera and Related Diarrheal Diseases Conference; 1995 Dec 1-3; Kiawah Island, South Carolina. 22. Saillour M, de Truchis P, Risbourg M, Nordman P, Nauciel C, Perronne C. Yersinia enterocolitica gastroenteritis in HIV infected patients. In: Abstracts of the 35th Interscience Conference on Antimicrobial Agents and Chemotherapy; 1995 Sept 17-20; San Francisco, California. 23. National Center for Health Statistics. Health, United States, 1996-97 and Injury Chartbook. Hyattsville (MD): The Center; 1997. 24. Bates J, Jordens JZ, Griffiths DT. Farm animals as a putative reservoir for vancomycin-resistant enterococcal infection in man. J Antimicrob Chemother 1994;34:507-16. 25. Morris JG Jr, Shay DK, Hebden JN, McCarter RJ, Perdue BE, Jarvis W, et al. Enterococci resistant to multiple antimicrobial agents, including vancomycin: establishment of endemicity in a university medical center. Ann Intern Med 1995;123:250-9. 26. Papanicolaou GA, Meyers BR, Meyers J, Mendelson MH, Lou W, Emre S, et al. Nosocomial infections with vancomycin-resistant Enterococcus faecium in liver transplant recipients: risk factors for acquisition and mortality. Clin Infect Dis 1996;23:760-6. 27. Montecalvo MA, Horowitz H, Gedris C, Carbonaro C, Tenover FC, Issah A, et al. Outbreak of vancomycin-, ampicillin-, and aminoglycoside-resistant Enterococcus faecium bacteremia in an adult oncology unit. Antimicrob Agents Chemother 1994;38:1363-7. 28. Remington JS, Schimpff SC. Please don't eat the salads. N Engl J Med 1981;304:433-5. 29. Wolfe MS, Armstrong D, Louria DB, Blevins A. Salmonellosis in patients with neoplastic disease. Arch Intern Med 1971;128:546-54. 30. Luft BJ, Naot Y, Araujo FG, Stinson EB, Remington JS. Primary and reactivated toxoplasma infection in patients with cardiac transplants. Ann Intern Med 1983;99:27-31. 31. Hofflin JM, Potasman I, Baldwin JC, Oyer PE, Stinson EB, Remington JS. Infectious complications in heart transplant recipients receiving cyclosporine and corticosteroids. Ann Intern Med 1987;106:209-16. 32. Lew JF, Glass RI, Gangarosa RE, Cohen IP, Bern C, Moe CL. Diarrheal deaths in the United States, 1979-87. JAMA 1991;265:3280-4. 33. Feldman M, Cryer B, McArthur KE, Huet BA, Lee E. Effects of aging and gastritis on gastric acid and pepsin secretion in humans: a prospective study. Gastroenterology 1996;110:1043-52. 34. Bennett RG. Diarrhea among residents of long-term care facilities. Infect Control Hosp Epidemiol 1993;14:397-404. 35. Hargrett-Bean N, Pavia AT, Tauxe RV. Salmonella isolates from humans in the United States, 1982-1986. MMWR Morb Mortal Wkly Rep 1988;30(SS-2):25-31. 36. Tauxe RV, Hargrett-Bean N, Patton CM, Wachsmuth K. Campylobacter isolates in the United States, 1982-1986. MMWR Morb Mortal Wkly Rep 1988;37(SS-2):1-13. 37. Blaser MJ, Feldman RA. Salmonella bacteremia: reports to the Centers for Disease Control, 1968-1979. J Infect Dis 1981;143:743-6. 38. Taylor JL, Dwyer DM, Groves C, Bailowitz A, Tilghman D, Kim V, et al. Simultaneous outbreak of Salmonella enteritidis and Salmonella schwarzengrund in a nursing home: association of S. enteritidis with bacteremia and hospitalization. J Infect Dis 1993;167:781-2. 39. Carter AO, Borczyk AA, Carlson JAK, Harvey B, Hogkin JC, Karmali MA, et al. A severe outbreak of Escherichia coli O157:H7-associated hemorrhagic colitis in a nursing home. N Engl J Med 1987;317:1496-500. 40. Santos JI. Nutrition, infection, and immunocompetence. Inf Dis Clin North Am 1994;8:243-67. 41. Black RE, Brown KH, Becker S. Effects of diarrhea associated with specific enteropathogens on the growth of children in rural Bangladesh. Pediatrics 1984;73:799-805. 42. Black RE, Brown KH, Becker S. Malnutrition is a determining factor in diarrheal duration, but not incidence, among young children in a longitudinal study in rural Bangladesh. Am J Clin Nutr 1984;37:87-94. --------------------------------------------------------------------------- Emerging Infectious Diseases National Center for Infectious Diseases Centers for Disease Control and Prevention Atlanta, GA URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/morris.txt Chronic Sequelae of Foodborne Disease James A. Lindsay University of Florida, Gainesville, Florida, USA --------------------------------------------------------------------------- In the past decade, the complexity of foodborne pathogens, as well as their adaptability and ability to cause acute illness, and in some cases chronic (secondary) complications, have been newly appreciated. This overview examines long-term consequences of foodborne infections and intoxications to emphasize the need for more research and education. The term foodborne disease encompasses a variety of clinical and etiologic conditions and describes a subset of enteric disease (1-4), which in the United States ranks second in prevalence to respiratory disease (2). In foodborne disease, the food may act as a vehicle for the transmission of actively growing microorganisms or products of metabolism (toxins), or it may have a passive role as a vehicle for the transmission of nonreplicating bacteria, viruses or protozoa, or stable biologic toxins. In most cases, the clinical conditions usually associated with foodborne disease are acute: diarrhea, vomiting, or other gastrointestinal manifestations such as dysentery. However, other pathophysiologic responses may occur independently or accompany acute-phase responses (1-4). A number of chronic sequelae may result from foodborne infections, including ankylosing spondylitis, arthropathies, renal disease, cardiac and neurologic disorders, and nutritional and other malabsorptive disorders (incapacitating diarrhea). The evidence that microorganisms or their products are either directly or indirectly associated with these long-term sequelae ranges from convincing to circumstantial (1-4). The reason for this disparity is that, except in rare circumstances, chronic complications are unlikely to be identified or epidemiologically linked to a foodborne cause because these data are not systematically collected. Moreover, host symptoms induced by a specific pathogen or product of a pathogen are often wide-ranging and overlapping and therefore difficult to link temporally to a specific incident. These impediments manifest themselves because the problems associated with chronic disease can result from an infection without overt illness. Alternatively, the chronic sequelae may be unrelated to the acute illness and may occur even if the immune system successfully eliminates the primary infection; therefore, activation of the immune system may initiate the chronic condition as a result of an autoimmune response (2-4). The variability of the human response-from overt illness to chronic carrier status-is perhaps the most confounding issue. Cost of Chronic Sequelae As the incidence of foodborne disease increases, the incidence of chronic sequelae may also rise. Several authors have estimated that chronic sequelae may occur in 2% to 3% of foodborne disease cases and suggest that the long-term consequences to human health and the economy may be more detrimental than the acute disease. An estimated 80 million cases of foodborne disease occur annually in the United States, which suggests significant morbidity figures and costs to society in the billions of dollars per year (2,4). Infection: The Microbe/Toxin versus the Host Several microbial pathogens are highly adapted to parasitization, exhibiting environmentally responsive and adaptive traits that allow attachment, invasion, and replication in the host (2,4). Microbial pathogenicity can be viewed solely from the perspective of the microbe; however, this would be not only unidimensional, but also wrong (2,4). A major selective force that regulates the phenotype of an infecting microbial pathogen population is the host's immune system, which is also highly adaptive, especially in discriminating self and nonself antigens (2,4). When the host-parasite relationship is examined holistically, mechanisms that successful pathogens have apparently evolved to elude the immune system include antigenic heterogeneity or variation; sequestration, either intracellularly or in certain specific host sites; molecular mimicry, through either imitation (cross-reaction) or adsorption of host protein; and direct immune stimulation and/or suppression (2-5). Rheumatoid Disease Several bacteria, including salmonellae, induce septic arthritis by hematogenous spread to the synovial space, causing inflammation. Viable organisms are recoverable from synovial fluid, and treatment usually involves antibiotic therapy. Prognosis depends on host factors and virulence of the organism; either complete resolution or permanent joint damage can occur (1-4,6,7). Yersinia enterocolitica, Y. pseudotuberculosis, Shigella flexneri, Sh. dysenteriae, Salmonella spp., Campylobacter jejuni, and Escherichia coli initiate aseptic or reactive arthritis, an acute, nonpurulent joint inflammation following infection elsewhere in the body, for example the bowel. Klebsiella pneumoniae has been implicated, although it appears now that the bacterium is connected with fecal carriage by ankylosing spondylitis probands (4). Although a distinct clinical disease, reactive arthritis also occurs in the Reiter syndrome triad with conjunctivitis and uveitis. A subset of patients with symptoms of reactive arthritis and Reiter syndrome get ankylosing spondylitis, a rheumatoid inflammation of synovial joints and entheses within and distal to the spine (8). The relative risk of developing these seronegative spondyloarthropathies after a gram-negative enterobacterial infection is high for persons positive for the major histocompatibility class (MHC) antigen B27 and the cross-reacting MHC B7 group. Indeed, persons who are human leukocyte antigen (HLA)-B27 positive have an 18-fold greater risk for reactive arthritis, a 37-fold greater risk for Reiter syndrome, and up to a 126-fold greater risk for ankylosing spondylitis than persons who are HLA-B27 negative and have the same enteric infections. Other genes that may be related or act in concert appear to determine which disease is acquired (2,5,7,9). These chronic complications are related to a genetically determined host risk factor in combination with an environmental trigger. No cause-and-effect relationship of enteric pathogens in ankylosing spondylitis has been established (4); however, a low but consistent incidence (0.2% to 2.4%) of reactive arthritis occurs after outbreaks of S. Typhimurium, Sh. flexneri, and C. jejuni. Biotypes and phage types of Y. enterocolitica O:3 and O:9, endemic to Scandinavia, are either highly arthritogenic or affect a more genetically predisposed population with persistent and debilitating symptoms that may last for years (2,3). The sharing of antigenic determinants by a microbe and its host is a frequent natural occurrence, and bacterial antigens from the pathogens that directly cross-react with MHC B27 have been demonstrated (6,9). Additionally, the plasmid-mediated synthesis of bacterial B27 "modifying factor," a protein that binds to and subsequently alters the conformation of B27, has been reported (10). In both of these models, immune recognition of the foreign antigen leads to an autoimmune anti-B27 response. Alternatively, B27 may act nonimmunologically because dissemination of bacterial antigens to infected joints stimulates a local T-cell inflammatory response. Here, B27 may act as a receptor for bacteria or antigens thereof, facilitating invasiveness from mucosal surfaces in the gut (9). Indeed, transfected B27 on the surface of mouse L cells reportedly can alter bacterial invasion capability (11). Despite the strong familial association related to the MHC B27 gene, B27-negative persons are known to become ill, albeit less often, but with apparently equal severity, as shown by an epidemiologic investigation of rheumatoid arthritis following the 1985 S. Typhimurium gastroenteritis outbreak due to contaminated milk (4). Autoimmune Thyroid Disease Graves disease is an autoimmune disease mediated by autoantibodies to the thyrotropin receptor (12,13). The first indication that the disease may be linked to infection was finding antibody titers to Y. enterocolitica serotype O:3 suggestive of molecular mimicry in a majority of patients with Graves disease. Several studies have shown that two low molecular weight envelope proteins of Yersinia contain epitopes cross-reactive with the thyrotropin receptor. These proteins are chromosomally encoded, exposed to the surface of the bacterium, and produced by both virulent and avirulent strains of Yersinia (Y. pestis, Y. pseudotuberculosis, Y. enterocolitica VW+ and WV-). In addition to autoantibody, a suppressor cell dysfunction may be involved in Graves disease (12,13). Severe hypothyroidism may also result from chronic intestinal giardiasis due to infection by Giardia lamblia; treatment with metronidazole can result in complete elimination of the parasite and recovery of regular intestinal thyroid hormone absorption (14). Inflammatory Bowel Disease Inflammatory bowel disease is the collective term for Crohn disease and ulcerative colitis. While both infections are chronic inflammatory diseases with histologic infiltrates of macrophages and lymphocytes and a prolonged clinical course, the primary clinical and pathologic effects are gastrointestinal. The infections can be difficult to differentiate because the symptoms are often similar (15). The acute clinical characteristics are diarrhea, abdominal pain, fever, and weight loss; and the acute pathologic features include a constant flux of neutrophils into inflamed mucosa, eventually penetrating the epithelium into the intestinal lumen. The chronic spontaneously relapsing disorder exhibits many of the symptoms of the acute state; however, this phase has an average symptom duration of 3.2 years before correct diagnosis. Abdominal abscesses are a common and dangerous complication of Crohn disease, while in ulcerative colitis, abdominal perforations may lead to peritonitis. Crohn disease involves the ileum or colon (anaerobes are important), while ulcerative colitis appears restricted to the colon (aerobes are important). Nationality and familial associations suggest a genetic predisposition for the disease (4,15). Although the cause of inflammatory bowel disease and the mechanism(s) for spontaneous exacerbations and remissions are unknown, much research has focused on transmissible agents, including foodborne pathogens. An association between bacterial L-forms and inflammatory bowel disease has been sporadically reported, with isolation of Pseudomonas, Mycobacterium, Enterococcus fecalis, and E. coli from affected tissue but not from appropriate controls. There is considerable debate as to whether L-forms are pathogenic in humans or persist in affected tissue. Mycobacterium paratuberculosis, the causative agent of Johne disease in ruminants, may be associated with Crohn disease through the production of L-forms of the bacterium. Subclinically infected cows shed M. paratuberculosis, and the organism has been identified in pasteurized milk by polymerase chain reaction specific for the M. paratuberculosis insertion sequence IS900. The pathogen model suggests that a susceptible human neonate first contracts the organism after ingesting commercial dairy products. This invokes an antigen-poor (lacking a cell wall) L-form that grows slowly and persists in the lamina propria, stimulating a chronic low-grade inflammation. The immune response increases in severity over years without bacterial replication, ultimately producing the pathologic features of Crohn disease (15,16). Another model proposes an autoimmune phenomenon mediated by alterations in inflammatory cytokine profiles, possibly as a result of infection (4). Recent immunocytochemical techniques demonstrated antigens to Listeria monocytogenes, E. coli, and Streptococcus spp. in Crohn disease tissues. Macrophages and giant cells immunolabelled for antigen specific to these organisms were found beneath ulcers, around abscesses, along fissures, within the lamina propria, in granulomas, and in germinal centers of mesenteric lymph nodes (17). Superantigens and Autoimmunity In contrast to conventional antigens, superantigens interact with the variable side of the Vß chain of the T-cell receptor by recognizing elements shared by a subset of T cells. Depending on the type of interaction, recognition can have different consequences, including proliferation and expansion, suppression (clonal deletion), or, alternatively, induction of prolonged unresponsiveness (anergy) or cell death (apoptosis) (18-21). Superantigens from several foodborne bacteria (e.g., Staphylococcus, Streptococcus, Yersinia, and Clostridium) have been isolated and characterized. Many are thought to be associated with several autoimmune disorders, for example, rheumatic heart disease, rheumatoid arthritis, multiple sclerosis, Graves disease, Sjogren syndrome, autoimmune thyroiditis, psoriasis, Kawasaki disease, Crohn disease, and insulin-dependent diabetes mellitus (6,18-24). Although it is accepted that superantigens have a role in autoimmune disorders, the acceptance is based on extensive animal model studies (6,18-24) but limited human clinical studies. In human diseases where superantigens have been clearly demonstrated as the cause, for example, toxic shock syndrome, initial T-cell proliferation and T-cell receptor-mRNA up-regulation have been observed, but the long-term sequelae in terms of T-cell function are unknown (22). Recent studies suggest that superantigens may also cause an acute flare of a disease within patients in remission from a preexisting autoimmune disorder. Renal Disease After colitis caused by E. coli O157:H7 and other enterohemorrhagic strains of E. coli, hemolytic uremic syndrome develops in some patients (1,2,25). The syndrome is characterized by a triad of symptoms: acute renal failure, thrombocytopenia, and microangiopathic hemolytic anemia. Acute renal failure is the leading cause of death in children, and thrombocytopenia is the leading cause of death in adults. Hemolytic uremic syndrome is a worldwide problem that mirrors the distribution of E. coli O157:H7 and other Shiga and Shiga-like toxin-producing microorganisms. Outbreaks of hemorrhagic colitis and subsequent cases of hemolytic uremic syndrome have developed as a result of various food vehicles. Besides O157:H7, other Shiga-like toxin-producing E. coli, Citrobacter, Campylobacter, Shigella, Salmonella, and Yersinia have been linked to the disorder (1,2,25,26). The toxin-mediated damage to the kidneys may not be limited to the glomerular endothelial cells as once thought but may include the tubular epithelial cells (26-28). Binding studies showed the toxins to be specific for the glycosphingolipid globotriaosylceramide (Gb3), which is present on renal but not umbilical endothelial cells. This may account for the differential sensitivity of renal cells to toxin-induced damage, since Gb3 was present in the glomeruli of infants under 2 years of age but not in the glomeruli of adults. Thus, the presence of Gb3 in the pediatric renal glomerulus may be a risk factor for development of hemolytic uremic syndrome (28). Characterization of the Shiga toxin receptor has led to a potential preventive treatment (4). Neural and Neuromuscular Disorders Guillain-Barré syndrome is a subacute, acquired, inflammatory demyelinating polyradiculoneuropathy that frequently occurs after acute gastrointestinal infection. The disease is characterized by alexia, motor paralysis with mild sensory disturbances, and an acellular increase in the total protein content in the cerebrospinal fluid. The disease occurs worldwide and is the most common cause of neuromuscular paralysis. Cases have three dominant characteristics: the predilection to nerve roots, mononuclear infiltration of peripheral nerves, and eventual demyelination (primary a xonal degeneration) (29). Severe cases tend to occur in the summer and have been linked to previous infection with C. jejuni, although other enteric pathogens may trigger the syndrome. Some controversy exists regarding whether Guillain-Barré syndrome is an autoimmune disease. Although adequate data exist to classify the syndrome as an autoimmune disease (four major Rose-Witebsky criteria are almost completely met), the immunologic mechanisms at work in Guillain-Barré syndrome triggered by C. jejuni are likely to be complex (29-31). Studies of the relationship between Guillain-Barré syndrome and C. jejuni support the hypothesis of molecular mimicry, since peripheral nerves may share epitopes with surface antigens of C. jejuni (32). This has been supported by studies in which anti-GM1 IgG antibodies recognized surface epitopes on intact C. jejuni, and the reaction was strain-specific for certain Penner serotypes. There are inconclusive data with regard to Guillain-Barré syndrome and HLA, although some studies have shown a predilection for the HLA-B35 haplotype (29-31). Cytokines may be responsible for inducing the inflammatory process and probably play a role in the response leading to nerve demyelination. Complement has a role in the process leading to nerve damage, possibly through the production of activation products, which lead to an increase in the permeability of the blood nerve barrier, which perpetuates the inflammation. Although Guillain-Barré syndrome might be considered an autoimmune response, it also serves as an example of a disease with an infectious origin, a disease that entails the integrated actions of both humoral and cellular immunity. Ciguatera poisoning is the most common foodborne disease related to the consumption of fin fish; this distinctive clinical syndrome is characterized by a plethora of gastrointestinal, neurologic, and sometimes cardiovascular features (33,34). Two toxins are involved in toxicosis. Ciguatoxin-1 (cig-1), the principal toxin, is a heat stable, lipid-soluble polyether that is not inactivated by heat, cold, or gastric juices, nor eliminated by drying, salting, smoking, or marinating. Cig-1 induces membrane depolarization in nerve and muscle tissue by opening voltage-dependent sodium channels. A second toxin, maitotoxin, is water-soluble and opens calcium channels. The role of this second toxin in the pathophysiology of the disease is less well understood. The acute symptoms of the toxicosis are varied and include paresthesia of the extremities, circumoral paresthesia, reversal of hot and cold sensations, dental pain, myalgias, arthralgias, generalized pruritus, cranial nerve dysfunction, and dysuria. Severe acute symptoms require urgent care with parenteral atropine for bradyarrhythmias. Mannitol is often administered to counter the effect of the toxin on the sodium channels; however, the mechanism of action is unknown, and the therapy is useless after 24 hours. Many of these symptoms may remain chronic and are often misdiagnosed as chronic fatigue syndrome, brain tumors, or multiple sclerosis. The management of the chronic symptoms is frustrating for the patient and clinician. Interventions include amitriptyline, tocaidine, or mexilitine to modulate sodium channels in conjunction with calcium channel blockers such as nifedipine. Antidepressants such as Prozac also appear to be useful. Patients with chronic symptoms frequently report waxing and waning of symptoms. Activities such as sexual intercourse and drinking alcohol significantly exacerbate expression. Some women with chronic symptoms report worsening during menses. Mood levels, weather conditions, and dietary constituents often exacerbate symptoms. Some clinicians advocate a strict diet that avoids all seafood, fish byproducts, nuts, and alcohol, and in some cases, patients are asked to abstain from sex. One distinctive feature in this toxicosis is that one episode of ciguatera poisoning does not confer immunity. In fact, it is likely to sensitize the patient to otherwise subthreshold doses of toxin (33,34). Amnesic shellfish poisoning is caused by domic acid, a conjugate of kainic and glutamic acid (35). In small quantities domic acid has an excitatory effect, but in large amounts it is neurotoxic. The toxicosis is first characterized by gastrointestinal symptoms followed by neurologic dysfunction. Severe cases may be prolonged and chronic; sequelae include confusion with disorientation, paucity of speech, lack of response to deep pain due to blocking of receptors in the spinal cord, autonomic nervous system dysfunction, seizures, abnormal ocular movements, grimacing posture, myoclonus, loss of reflexes, and coma. Other prominent chronic sequelae include loss of visual-spacial recall and mononeuropathies without dementia, mimicking Alzheimer's disease. The toxicosis is particularly serious in the elderly, and any deaths usually occur within this population. Valium, calcium channel blockers, phenobarbital, diazapam, thiobarbiturates, and hypothermia are treatments for patients with severe and chronic cases. General Immunity, Organ Impairment, and Neurologic Disorders Toxoplasmosis due to Toxoplasma gondii is a chronic latent parasitic infection (36,37). In humans the parasite exists in two forms: the tachyzoite, the rapidly multiplying stage that actively invades host cells and represents the principal form of the acute phase of the disease; and the bradyzoite, the form that multiplies very slowly in host cells, resulting in the formation of cysts that persist in tissues. Toxoplasma infection in humans is usually asymptomatic because of effective immunity involving antibodies, T cells, and cytokines. Activated macrophages, CD4 and CD8 lymphocytes, and the cytokines IFN-g, TNFa, and IL-6 play a major role in control of both the acute infection and maintenance or prevention of the chronic stage (37,38). Indeed, treatment with IFN-g is used to control passage into the chronic stage, and treatment with anti-IFN-g reactivates chronic infection (39). The production of nitric oxide may have opposing effects. Nitric oxide production protects against T. gondii and at the same time limits the immune response, probably contributing to the establishment of the chronic state (40). The incidence of congenital toxoplasmosis is uncertain but may be as high as 9,500 cases a year (1). The percentage attributable to food is uncertain; however, consumption or contact with contaminated meat is more important as a cause than is contact with cats (1). Congenital impairments associated with maternal toxoplasmosis infection passed to the fetus include hearing loss, visual impairment (retinal lesions, strabismus), and slight to severe mental retardation. These impairments are still present in 80% of persons who reach the age of 20 years (1). Chronic toxoplasmic encephalitis may occur when a person's immune system is impaired. Indeed, toxoplasmic encephalitis marked by dementia and seizures has become the most commonly recognized cause of central nervous system opportunistic infections in AIDS patients. Additionally, it appears that certain cancer treatments weaken the immune system, and old infections in the muscles can become reactivated, causing severe complications or death (1,41,42). Helminth parasites can cause serious disease in infected persons (42). The impact of helminth infections is due less to the severity of the diseases they cause, than to the vast number of persons infected. For example, more than one billion people are infected with the largest intestinal nematode, Ascaris lumbricoides. Although there is usually no overt clinical sign of infection, disease can arise from an overwhelming infection or an inappropriate immune response. Additionally, infected persons frequently harbor more than one parasite for years. Most intestinal helminth parasites have direct life cycles, with no intermediate host or vector, and are transferred by contaminated food. Some species, such as Trichuris (whipworm) and Enterobius (pinworm), are restricted to the gut, but others, such as Ascaris, have tissue-migrating phases. All, however, induce a dramatic expansion of the Th2 lymphocyte subset. It remains unclear whether these Th2-derived responses (induction of interleukin-4 [IL-4] and down-regulation of IFNg), resulting in stimulation of IgG1 and IgE isotypes, eosinophilia, and mastocytosis are responsible for the immune-mediated pathologic response. Immunologic lesions may occur where early infection is associated with a strong T-cell proliferative response that becomes down-regulated in established chronic disease (evidence of a Th1 defect in the chronic disease). In ascariasis, an allergic response generated by the lung migratory phase (chronic immune sensitization) can cause pneumonia and, in animal models, spontaneous development of idiopathic bronchial asthma. A formative influence on the response of the immune system is the antigenic environment during pregnancy. Children born to infected mothers may have significantly higher susceptibility to the same infection later in life (42). Viral agents induce autoimmune disorders, and one potential mechanism of induction is molecular mimicry (43,44)