Changes in Human Ecology and Behavior in Relation to the Emergence of Diarrheal Diseases, Including Cholera
MYRON M. LEVINE AND ORIN S. LEVINE
In a broad sense, one can divide the world into two distinct ecologies with respect to the occurrence of enteric diseases, with a developing country ecology at one extreme and an industrialized world ecology at the other. Between these extremes, one encounters gradations and exceptions. In each of these settings, changes in human ecology and behavior are having an impact on enteric infections. In developing areas, some of the sweeping changes in demographics and population distribution that are underway are creating environments of amplified transmission of enteric pathogens or are attenuating the protective features of some traditional practices. In industrialized areas, the changes underway to meet the ever-increasing demands of consumption-based economies are creating opportunities for the importation of pathogens that improvements in infrastructure were believed to have eliminated. Finally, even the gains in infrastructure and commercialization of food production, food distribution, and food retailing, with all of their positive impact on health, have by their very nature created environments in which we have seen the emergence of enteric pathogens.
Myron M. Levine is professor and director of the Center for Vaccine Development at the University of Maryland School of Medicine, Baltimore, Maryland.
Orin S. Levine is a researcher in the National Center for Infectious Diseases at the Centers for Disease Control and Prevention, Atlanta, Georgia.
DEVELOPING WORLD ECOLOGY
Much of the developing world's population lives in substandard housing, under crowded conditions, without piped water or sanitation. Under these conditions of pervasive fecal contamination, the various bacterial, protozoal, and viral agents that cause diarrheal illness are readily transmitted (1–6). In low-socioeconomic-level populations in the developing world, infants and toddlers experience from 4–10 episodes per child per yr during the first 2 yr of life (1–3). Up to 20% of life experience of infants may be spent suffering from diarrheal illness (1–3) and up to one-third of deaths among children <2 yr of age are due to diarrheal dehydration, persistent diarrhea, and other complications of diarrheal illness (7).
A striking feature of pediatric diarrheal illness in developing countries is the large proportion associated with bacterial enteropathogens. Among the most important agents are enterotoxigenic Escherichia coli , enteropathogenic E. coli, and Shigella (1–6, 8–11). Enteroaggregative E. coli, a recently described category of diarrheagenic E. coli has been incriminated as an important cause of persistent diarrhea in developing countries (12–15). Consequent to clinical infections caused by the various agents and their antigenic varieties, infection-derived immunity is acquired (4, 16–18); this is reflected in markedly lower incidence rates in older children and adults and in increased prevalence of antibody with age (18).
INDUSTRIALIZED WORLD ECOLOGY
Quite distinct patterns of diarrheal disease are encountered in industrialized regions of the world, where there is access to microbiologically monitored drinking water, flush toilets, wastewater treatment, and adequate housing with little or no crowding. In these relatively affluent settings, viral agents of diarrhea predominate in pediatric populations (19). Nevertheless, some notable exceptions occur in specific settings in which hygienic conditions are less adequate (e.g., pediatric day care, custodial institutions for the mentally retarded), with the net effect of creating environments similar to those observed in less developed countries (20, 21).
CHANGES IN HUMAN BEHAVIOR AND ECOLOGY THAT PROMOTE THE EMERGENCE AND/OR AMPLIFIED TRANSMISSION OF ENTERIC INFECTIONS
Urbanization. At one extreme, we may consider the diarrheal disease problem among the most isolated and primitive populations
alive today (such as the Yanomami Indians in the Orinoco River basin) as an example of the burden of diarrheal disease in the absence of any urbanization. A lack of crowding, relatively small population, and prolonged breast-feeding of infants and young children for several years are believed to account for this. Even among the Mayan Highlanders in Guatemala, a much less isolated traditional society, the incidence of diarrheal disease does not peak until the second year of life (1), likely due to breast-feeding patterns. This timing is in contrast to the pattern observed in periurban slums in many developing areas where diarrheal disease incidence peaks in the second semester of the first year of life.
As another example, we can consider the concurrent emergence of typhoid fever as a public health problem and the rise of cities during the Industrial Revolution of Europe in the 1800s (22). Not to despair, typhoid fever also serves as an example of how a single change in infrastructure can rapidly and markedly diminish the incidence of enteric infections (22–24).
In most large cities of Western Europe and North America in the last quarter of the 19th century typhoid fever was highly endemic with incidence rates of 200–500 cases per 100,000 population being common (23, 24). In the late 19th and early 20th centuries, the introduction of a single intervention, treatment of municipal water supplies (with chlorine, sand filtration, or both) drastically and precipitously reduced the incidence of typhoid fever wherever these interventions were applied (22–24).
Decreased breast-feeding. Breast-feeding constitutes one of the most effective interventions to diminish the occurrence of diarrheal illness in young infants, particularly with respect to bacterial diarrheas (25–28). Sociologic changes that are increasingly encountered in urban areas of developing countries lead women to abandon breast-feeding early, as they must return to work long hours in situations where they cannot nurse their infants. In the 1960s and 1970s, aggressive advertising by some manufacturers of infant formulas, inadvertently or otherwise, led many mothers of low socioeconomic level to abandon breast-feeding and to administer diluted formula to their infants. Often these formulas were diluted with contaminated water, and a lack of refrigeration fostered heavy bacterial contamination. Ultimately, the World Health Assembly attempted to deal with this problem by passing a resolution that provides a restrictive code and guidelines on advertising to be followed by artificial formula manufacturers (29).
Shifting agricultural patterns toward cash crop production. As developing countries attempt to industrialize and become a part of the global economy, they are forced to generate large sums of foreign exchange to finance their development. As a result, many farmers have switched
from growing an array of crops for their own needs to cultivation of a single cash crop (e.g., sugar, rice, coffee), which may be of little or no nutritional value. In many cases, children have suffered nutritionally from this switch (30). The adverse consequences and the interaction of malnutrition and diarrheal disease are well described (1).
Wars and political upheavals. No human behavior so impacts the public's health as war. The impact of war on diarrheal disease is no exception. Former U.S. President Jimmy Carter estimates that there are no less than 30 wars currently under way worldwide (31). With the proliferation of advanced weapons during recent decades and their distribution among much of the developing world, the potential for devastation of infrastructure and health resources is at an unprecedented level.
The impact of war and political unrest on diarrheal disease is illustrated all too clearly by the current situations in Somalia and Bosnia–Herzegovina. In Somalia, where large segments of the population have been displaced and forced to live in refugee camp settings, mortality rates among children <5 yr as high as 69.4 per 105 per day have been reported (32); dysentery caused by multiresistant Shigella dysenteriae was one of the two most important causes of mortality during this period (32).
Civil turmoil can revert an industrialized country to developing country status with respect to inadequate provision of piped water supplies and sanitation. This abrupt change in ecology has occurred in Bosnia–Herzegovina, for example, where epidemics of diarrheal illness and typhoid fever have resulted.
Commercialization of food production and food service. In industrialized countries, agriculture is characterized by massive farms where vegetables are cultivated, extensive ranches where herd animals are raised, and enormous animal husbandry operations where poultry are raised. Additionally, a uniquely Western phenomenon has arisen: enormous fast-food chains have come to exist that serve millions of meals daily in the United States and Europe with all outlets in each chain adhering to uniform food-preparation techniques. While such commercialization of food production and food service have achieved impressive economies of scale, it has also been observed how such large-scale standardization can unwittingly foster and facilitate the emergence of certain diarrheal pathogens such as Salmonella enteritidis and enterohemorrhagic Escherichia coli.
Recent epidemics of infection with Sa. enteritidis through raw or
partially cooked eggs in restaurants provide one example to illustrate this point (33). Through molecular epidemiologic techniques, a recent outbreak of Sa. enteritidis was traced to a single egg-producing farm that housed >400,000 egg-laying hens in five henhouses (a veritable poultry city!) (34, 35).
Infections due to Sa. enteritidis have been increasing throughout Europe and the United States so rapidly that by 1990, Sa. enteritidis surpassed the venerable Salmonella typhimurium as the most commonly isolated serotype from humans in the United States (36, 37). With 80,000 hens per henhouse, often with cages stacked one upon another, commercial egg-producing farms create a permissive environment for the rapid transmission of enteric pathogens such as Salmonella and Campylobacter jejuni among animals, thereby creating a reservoir of infection for humans.
Part of the appeal of fast-food chains is the consumer's expectation that the food prepared in all outlets will conform to uniform procedures, using comparable ingredients (often from the same source). Thus, a high level of consistency is demanded and expected by the consumers from each fast-food chain, irrespective of the state or country in which a particular outlet is found. This integral part of the appeal of fast food also makes it capable of disseminating infection widely through a common vehicle. Recent outbreaks of hemorrhagic colitis and hemolytic uremic syndrome due to enterohemorrhagic E. coli of serotype O157:H7 provide an excellent example (38–42).
Enterohemorrhagic E. coli first came to be recognized in the U.S. as an enteric pathogen in 1982 when a multistate outbreak of an unusual clinical syndrome was seen in several midwestern and western states associated with the consumption of undercooked hamburgers (38). The clinical syndrome observed, hemorrhagic colitis, consisted of watery bloody diarrhea, without fever or fecal leukocytes (38). An unusual serotype of E. coli, O157:H7, was isolated from cases that had not previously been incriminated as a cause of diarrheal illness. Examination of the O157:H7 organism by various investigators revealed that this pathogen had amassed a fascinating array of virulence properties, including elaboration of phage-encoded potent Shiga-like toxins (43), a 60-MDa virulence plasmid associated with expression of novel fimbriae (44), and a chromosomal gene (45) that induces attaching and effacing lesions of intestinal mucosa (46). It was subsequently shown that serotype O157:H7 is a prototype within a new category of diarrheagenic E. coli, referred to as enterohemorrhagic E. coli; multiple other O:H serotypes also fall within this category, most notably O26:H11 and O111:NM (42), among others.
It rapidly came to be recognized that ≈1–2% of individuals with
bloody diarrhea caused by enterohemorrhagic E. coli go on to develop the hemolytic uremic syndrome characterized by the triad of hemolytic anemia, acute renal failure, and thrombocytopenia (42). Finally, it is now appreciated that cattle serve as the main reservoir of enterohemorrhagic E. coli infection that can be passed to humans by consumption of improperly cooked beef products or by close contract with infected cattle, particularly calves (as in a petting zoo for children) (42, 47, 48).
Two large multistate outbreaks of enterohemorrhagic E. coli disease have resulted from the consumption of undercooked beef hamburgers served by large fast-food chains. In the original outbreak of 1982, as well as in some recent outbreaks, epidemic investigations have shown that the restaurants' cooking practices were insufficient to kill the bacteria (38, 42, 49, 50). Enterohemorrhagic E. coli is becoming increasingly important as an enteric pathogen in North America and Europe, while it remains distinctly uncommon in developing countries.
Importation of food from developing countries. Occasionally, imported foods may lead to outbreaks of unexpected bacterial diarrheal illness. For example, an outbreak of cholera in Maryland followed the ingestion of frozen coconut milk imported from Thailand (51). Investigation of the packaging process in Thailand indicated a high degree of fecal contamination of the environment with several possible opportunities for contamination of the food. Taylor et al. (51) point out that ''In a global economy, the control of cholera and other food-borne diseases will require globally recognized regulatory standards for manufacturers of imported products."
Increase in day-care center attendance. In the United States, there are more single-parent families than ever. In addition, more women than ever are pursuing careers that require them to seek day care for their children. As the number of young children and infants in day care has grown, we have seen the re-emergence of diarrheal diseases caused by certain bacterial and protozoal pathogens among these children. The reason for the emergence of these enteric pathogens that are so common to developing areas is the compromised hygienic conditions found in these day-care settings. Infections due to Shigella and the protozoa Giardia lamblia, for instance, are common in day-care center attenders (21, 52, 53), where children may be crowded together sharing toys and sanitary facilities.
Shigella sonnei is by far the predominant serotype of Shigella isolated in day-care settings and is the main serotype found in industrialized countries, in general. A relationship has been noted between level of development and the serogroup of Shigella that is prevalent (54). Sh. dysenteriae is found in the least developed ecologies, and Shigella flexneri is found in somewhat further developed environments; Sh. sonnei
predominates in those niches where shigellosis persists in industrialized settings (54). A possible explanation has been proposed to explain these observations (4). O antibodies are believed to mediate protection against sonnei shigellosis (55). Some strains of Plesiomonas shigelloides, an autochthonous bacterial species of surface waters (56), express a polysaccharide O antigen identical to that of Sh. sonnei (57). Among populations living in less-developed ecologic conditions, the repeated antigenic stimulation by Plesiomonas bacteria consequent to ingestion of untreated surface waters may stimulate cross-protection against Sh. sonnei.
Institutions that care for the aged. In industrialized countries the proportion of the population that is elderly is steadily increasing, as is life expectancy. Improved geriatric health care has resulted in the survival of many elderly patients with chronic illnesses, many of whom live in institutions for the aged. The result is an unusual ecologic niche where there exist many elderly chronically infirm individuals with diminished host defenses (e.g., hypochlorhydria) who are fed institutional food from large-scale kitchens. Should there occur a breakdown of food hygiene in such a setting, large outbreaks of gastrointestinal disease may ensue with high attack rates and fatalities (58, 59).
International travel. Approximately 30–50% of individuals from industrialized areas who travel to developing areas experience at least one episode of diarrhea due to enterotoxigenic E. coli during a 2-week stay (60). Potentially, travelers may transport highly virulent enteric pathogens back with them, either knowingly—e.g., the outbreak of cholera due to crabs smuggled in luggage from South America (61)—or unwittingly—e.g., the outbreak of cholera among passengers on a transcontinental airline flight from South America (62).
CHANGES IN THE EPIDEMIOLOGIC BEHAVIOR OF CHOLERA,IN PART DUE TO CHANGES IN HUMAN BEHAVIOR
One of the characteristics of cholera is its propensity for pandemic spread, wherein many countries come to be affected over many years. The seventh pandemic of cholera, due to the El Tor biotype of Vibrio cholerae O1 began in Sulawesi in 1961 and progressively spread to other continents over many years. The last large developing world populations that had so far escaped the ravages of cholera were finally touched when the enormous and explosive epidemic began in South America in 1991 (63).
In recent years it has become increasingly recognized that an important reservoir for V. cholerae O1 exists in the environment in brackish waters where the vibrios adsorb to zooplankton and other chitinous fauna (64). After the onset of the South American outbreak of cholera, it
came to be recognized that cargo ships can inadvertently disseminate V. cholerae O1 to new receptive brackish water environments through their practice of using ballast water to balance the vessel. Thus, ballast water of ships from South America approaching U.S. ports was found to contain the outbreak strain of V. cholerae O1 (65).
In late 1992 epidemic cholera due to a newly recognized serogroup of V. cholerae (O139) broke out in India and swiftly spread to Bangladesh (66, 67). The high rates of illness in adults in these cholera-endemic areas suggest that prior immunity to V. cholerae O1 does not afford protection against the new serogroup. Preliminary analysis of virulence properties of the O139 serogroup shows virtual identity to El Tor strains (68). What is remarkable about this new strain is its rapidity of spread. O139 serogroup has already caused cholera in Thailand, China, Pakistan, Kazakhstan, and Malaysia, and cases have been imported into the United States (69) and the United Kingdom (70). Moreover, one indigenous case has been observed in the United Kingdom in an individual who did not travel. This extraordinarily rapid spread is being attributed to high-volume air travel between the subcontinent and developing as well as industrialized areas of the world.
Changes in human behavior and ecology clearly impact on the emergence or disappearance of enteric infections. We have limited our attention to certain changes in ecology and human behavior that we believe to be of primal importance but have failed to mention others. Nevertheless, the picture from those presented is clear. With each advance in human development there arise consequences. The commercialization of food production and service, the shift in the role of women from home to work force, the increase in pediatric populations attending day-care centers and of elderly populations housed in institutions for the aged, the linking of distant places by rapid intercontinental travel, and the interdependence of economies have all impacted to modify the epidemiology of diarrheal disease. In some instances the net result is a diminution in the transmission of diarrheal diseases; in other cases the effect is a fostering of the emergence of certain enteric infections.
Human populations throughout the world can be found in diverse conditions. A proportion of the population of developing countries lives in deprived conditions characterized by ramshackle housing, lack of piped water and sanitation, and widespread fecal contamination of the
environment. Enteric infections, particularly due to bacterial pathogens, are readily transmitted under these circumstances. In contrast, the majority of inhabitants of industrialized countries live in a sanitary environment that generally discourages the transmission of enteric pathogens, particularly bacteria. In both these ecologic niches, changes in human ecology and behavior are leading to the emergence of certain enteric infections. Relevant factors in developing areas include urbanization (leading to periurban slums), diminished breast-feeding, and political upheaval that results in population migrations. In industrialized areas, large-scale food production (e.g., enormous poultry farms), distribution, and retailing (e.g., fast-food chains) create opportunities where widespread and extensive outbreaks of food-borne enteric infection can ensue if a breakdown in food hygiene occurs.
1. Mata, L. J. (1978) The Children of Santa Maria Cauque: A Prospective Field Study of Health and Growth (MIT Press, Cambridge, MA).
2. Black, R. E., Brown, K. H., Becker, S., Abdul Alim, A. R. M. & Huq, I (1982) Am. J. Epidemiol. 115, 315–324.
3. Guerrant, R. L., Kirchhoff, L. V., Shields, D. S., Nations, M. K., Leslie, J., de Sousa, M. A., Araujo, J. G., Correia, L. L., Sauer, K. T., McClelland, K. E., Trowbridge, F. L. & Hughes, J. M. (1983) J. Infect. Dis. 148, 986–997.
4. Ferreccio, C., Prado, V., Ojeda, A., Cayazzo, M., Abrego, P., Guers, L. & Levine, M. M. (1991) Am. J. Epidemiol. 134, 614–627.
5. Levine, M. M., Ferreccio, C., Prado, V., Cayazzo, M., Abrego, P., Martinez, J., Maggi, L., Baldini, M. M., Martin, W., Maneval, D., Kay, B., Guers, L., Lior, H., Wasserman, S. S. & Nataro, J. P. (1993) Am. J. Epidemiol. 138, 849–869.
6. Cravioto, A., Reyes, R. E., Ortega, R., Fernandez, G., Hernandez, R. & Lopez, D. (1988) Epidemiol. Infect. 101, 123–134.
7. Nichols, B. & Soriano, H. (1977) Am. J. Clin. Nutr. 30, 1457–1472.
8. Black, R. E., Lopez de Romana, Brown, K. H., Bravo, N., Bazalar, O.-G. & Kanashiro, H. C. (1989) Am. J. Epidemiol. 129, 785–799.
9. DeMol, P., Brasseur, D., Hemelhof, W., Kalala, T., Butzler, J. P. & Vis, H. L. (1983) Lancet i, 516–518.
10. Bhan, M. K., Bhandari, N., Sazawal, S., Clemens, J., Raj, P., Levine, M. M. & Kaper, J. B. (1989) Bull. W.H.O. 67, 281–288.
11. Levine, M. M., Prado, V., Robins-Browne, R. M., Lior, H., Kaper, J. B., Moseley, S., Gicquelais, K., Nataro, J. P., Vial, P. & Tall, B. (1988) J. Infect. Dis. 158, 224–228.
12. Bhan, M. K., Raj, P., Levine, M. M., Kaper, J. B., Bhandari, N., Srivastava, R., Kumar, R. & Sazawal, S. (1989) J. Infect. Dis. 159, 1061–1064.
13. Bhan, M. F., Khoshoo, V., Sommerfelt, H., Raj, P., Sazawal, S. & Srivastava, R. (1989) Pediatr. Infect. Dis. J. 8, 499–502.
14. Cravioto, A., Tello, A., Navarro, A., Ruiz, J., Villafan, H., Uribe, F. & Eslava, C. (1991) Lancet 337, 262–264.
15. Wanke, C., Schorling, J. B., Barrett, L. J., DeSouza, M. A. & Guerrant, R. L. (1991) Pediatr. Infect. Dis. J. 10, 746–751.
16. Glass, R. I., Becker, S., Huq, I., Stoll, B. J., Khan, M. U., Merson, M. H., Lee, J. V. & Black, R. E. (1982) Am. J. Epidemiol. 116, 959–970.
17. Clemens, J. D., Van Loon, F., Sack, D. A., Rao, M. R., Ahmed, F., Chakraborty, J., Kay, B. A., Khan, M. R., Yunus, M., Harris, J. R., Svennerholm, A.-M. & Holmgren, J. (1991) Lancet 337, 883–884.
18. Mosley, W. H., Beneson, A. S. & Barui, R. (1968) Bull W.H.O. 38, 327–334.
19. Kotloff, K. L., Wasserman, S. S., Steciak, J. Y., Tall, B. D., Losonsky, G. A., Nair, P., Morris, J. G. & Levine, M. M. (1989) Pediatr. Infect. Dis. J. 7, 753–759.
20. DuPont, H. L., Gangarosa, E. J., Reller, L. B., Woodward, W. E., Armstrong, R. W., Hammond, J., Glaser, K. & Morris, G. K. (1970) Am. J. Epidemiol. 92, 172–179.
21. Pickering, L., Evans, D. G., DuPont, H. L., Vollet, J. J., III & Evans, D. J., Jr. (1981) J. Pediatr. 8, 539–547.
22. Levine, M. M. (1993) in Proceedings of Eastern Pennsylvania Branch–American Society for Microbiology Symposium on the Migration of Infectious Diseases: Five Hundred Years After Columbus, ed. Evangelista, A. T. (Plenum, New York), in press.
23. Wolman, A. & Gorman, A. (1931) The Significance of Water-Borne Typhoid Fever Outbreaks (Williams & Wilkins, Baltimore).
24. Anonymous (1919) J. Am. Med. Assoc. 72, 997–999.
25. Glass, R. I., Svennerholm, A.-M., Stoll, B. J., Khan, M. R., Hossain, K. M. B., Huq, M. I. & Holmgren, J. (1983) N. Engl. J. Med. 13, 89–92.
26. Levine, M. M. & Edelman, R. (1984) Epidemiol. Rev. 6, 31–51.
27. Clemens, J. D., Stanton, B., Stoll, B., Shahid, N. S., Banu, H. & Chowdhury, A. K. M. A. (1986) Am. J. Epidemiol. 123, 710–720.
28. Gunn, R. A., Kimball, A. M., Dutta, S. P., Matthew, P. P., Mahmood, R. A., Pollard, R. A., Feeley, J. C., Levine, M. M. & Feldman, R. A. (1979) Lancet ii, 730–732.
29. World Health Assembly (1981) International Code of Marketing of Breast Milk Substitutes (W.H.O., Geneva), WHA Resolution 34.22.
30. Kennedy, E., Bouis, H. & von Braun, J. (1992) Soc. Sci. Med. 35, 689–697.
31. Carter, Jimmy (1993) Talking Peace: A Vision for the Next Generation (Dutton Children's Books, New York).
32. Centers for Disease Control (1992) Morbid. Mortal. Wkly. Rep. 41, 913–917.
33. Mishu, B., Griffin, P. M., Tauxe, R. V., Cameron, D. N., Hutcheson, R. H. & Schaffner, W. (1991) Ann. Intern. Med. 115, 190–194.
34. Vugia, D. J., Mishu, B., Smith, M., Tavris, D. R., Hickman-Brenner, F. W. & Tauxe, R. V. (1993) Epidemiol. Infect. 110, 49–61.
35. Alterkruse, S., Koehler, J., Hickman-Brenner, F., Tauxe, R. V. & Ferris, K. (1993) Epidemiol. Infect. 110, 17–22.
36. Rodrigue, D. C., Tauxe, R. V. & Rowe, B. (1990) Epidemiol. Infect. 105, 21–27.
37. Centers for Disease Control (1991) Salmonella Surveillance Report 1990 (U.S. Public Health Service, Atlanta).
38. Riley, L. W., Remis, R. S., Helgerson, S. D., McGhee, H. B., Wells, J. G., Davis, B. R., Herbert, R. J., Olcott, E. S., Johnson, L. M., Hargrett, N. T., Blake, P. A. & Cohen, M. L. (1983) N. Engl. J. Med. 308, 681–685.
39. Spika, J., Parsons, J., Nordenberg, D., Wells, J. G., Gunn, R. A. & Blake, P. A. (1986) J. Pediatr. 109, 287–291.
40. Neill, M., Tarr, P., Clausen, C., Christie, D. L. & Hickman, R. O. (1987) Pediatrics 80, 37–40.
41. Centers for Disease Control (1993) Morbid. Mortal. Wkly. Rep. 42, 85–86.
42. Griffin, P. M., Ostroff, S. M., Tauxe, R. V., Greene, K. D., Wells, J. G., Lewis, J. H. & Blake, P. A. (1988) Ann. Intern. Med. 109, 705–712.
43. Strockbine, N., Marques, L., Newland, J., Smith, H. W., Holmes, R. K. & O'Brien, A. D. (1986) Infect. Immun. 53, 135–140.
44. Karch, H., Heeseman, J., Laufa, R., O'Brien, A. D., Tacket, C. O. & Levine, M. M. (1987) Infect. Immun. 55, 455–461.
45. Yu, J. & Kaper, J. B. (1992) Mol. Microbiol. 6, 411–417.
46. Tzipori, S., Karch, H., Wachsmuth, K. I., Robins-Browne, R. M., O'Brien, A. D., Lior, H., Cohen, M. L., Smithers, J. & Levine, M. M. (1987) Infect. Immun. 55, 3117–3125.
47. Wells, J. G., Shipman, L. D., Greene, K. D., Sowers, E. G., Green, J. H., Cameron, D. N., Downes, F. P., Martin, M. L., Griffin, P. M. & Ostroff, S. M. (1990) J. Clin. Microbiol. 29, 985–989.
48. Montenegro, M. A., Bulte, M., Trumpf, T., Aleksic, S., Reuter, G., Bulling, E. & Helmuth, R. (1990) J. Clin. Microbiol. 28, 1417–1421.
49. Kotula, A. W., Chesnut, C. M., Emswiler, B. S. & Young, E. P. (1977) J. Anim. Sci. 45, 54–58.
50. Belongia, E. A., MacDonald, K. L., Parham, G. L., White, K. E., Korlath, J. A., Lobato, M. N., Strand, S. M., Casale, K. A. & Osterholm, M. T. (1991) J. Infect. Dis. 164, 338–343.
51. Taylor, J. L., Tuttle, J., Pramukul, T., O'Brien, K., Barrett, T. J., Jolbitado, B., Lim, Y. L., Vugia, D., Morris, J. G., Jr., & Tauxe, R. V. (1993) J. Infect. Dis. 167, 1330–1335.
52. Weissman, J. B., Gangarosa, E. J., Schmerler, A., Marier, R. L. & Lewis, J. N. (1975) Lancet i, 88–90.
53. Black, R. E., Dykes, A. C., Sinclair, S. O. & Wells, J. G. (1977) Pediatrics 60, 486–491.
54. Kostrzweski, J. & Stypulkowska-Misiurewicz, H. (1968) Arch. Immunol. Ther. Exp. 16, 429–451.
55. Cohen, D., Green, M. S., Block, C., Rouach, T. & Ofek, I. (1988) J. Infect. Dis. 157, 1068–1071.
56. Arai, T., Ikejima, N., Itoh, T., Sakai, S., Shimada, T. & Sakazaki, R. (1980) J. Hyg. 84, 203–211.
57. Basu, S., Tharanathan, R. N. & Kontrohr, T. (1985) FEMS Microbiol. Lett. 28, 7–10.
58. Levine, W. C., Smart, J. F., Bean, N. H. & Tauxe, R. V. (1991) J. Am. Med. Assoc. 266, 2105–2109.
59. Carter, A. O., Borczyk, A. A., Carlson, J. A. K., Harvey, B., Hockin, J. C., Karmali, M. A., Krishnan, C., Korn, D. A. & Lior, H. (1987) N. Engl. J. Med. 317, 1496–1500.
60. Black, R. E. (1986) Rev. Infect. Dis. 8, Suppl., S131–S111.
61. Centers for Disease Control (1991) Morbid. Mortal. Wkly. Rep. 40, 287–289.
62. Centers for Disease Control (1992) Morbid. Mortal. Wkly. Rep. 41, 134.
63. Swerdlow, D. I., Mintz, E. D., Rodriguez, M., Tejada, E., Ocampo, C., Espejo, L., Greene, K. D., Saldana, W., Seminario, L. & Tauxe, R. V. (1992) Lancet 340, 28–33.
64. Colwell, R. R. & Spira, W. M. (1992) in Cholera, eds. Barua, D. & Greenough, W. B., III (Plenum, New York), pp. 107–127.
65. McCarthy, S. A., McPhearson, R. M., Guarino, A. M.& Gaines, J. L. (1992) Lancet 339, 624–625.
66. Ramamurthy, T., Garg, S., Sharma, R., Bhattacharya, S. K., Nair, G. B., Shimada, T., Takeda, T., Karasawa, T., Kurazano, H. & Pal, A. (1993) Lancet 341, 703–704.
67. Cholera Working Group (1993) Lancet 342, 387–390.
68. Hall, R. H., Khambaty, F. M., Kothary, M. & Keasler, S. P. (1993) Lancet 342, 430.
69. Centers for Disease Control (1993) Morbid. Mortal. Wkly. Rep. 42, 501–503.
70. Public Health Laboratory Service (1993) Comm. Dis. Rep. Wkly. 3, 173.