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Over the past 3 years, reports in the news media have focused on a multistate episode of bloody diarrhea and hemolytic uremic syndrome in people who had ingested hamburgers contaminated with a particular serotype of Escherichia coli, O157:H7.^1-3 Other cases of this disease have been epidemiologically linked to unpasteurized milk.^4,5 Because cattle were identified as the source of the contaminated milk and ground beef, practices used in the production of milk and beef are being examined. The following is a hypothetical situation in which a veterinarian is asked by a dairy farm client relevant questions about this E coli serotype. Rather than present here the most typical dairy farm, this situation was created to provide as many questions as possible about how this serotype may impact cattle production and public health.

The dairy farm is owned and operated by a family of 4, and they hire an additional worker. They maintain a 60-cow herd and replace half their cull cows with their own best heifers and the other half with heifers purchased from other farms, dealers, and auctions. They market all their bull calves and the heifer calves not selected for replacements. They sell milk to a grade A dairy plant. A small volume of state-certified raw milk is sold at the farm to people living in the community. The family also gives some of the raw milk to their employee, and they drink their own raw milk in the home. A herd that had been providing some of their replacement heifers for the past 5 years was found to be positive for E coli O157:H7 when fecal samples are tested, because that farm was the source of beef consumed by people affected in an episode of O157:H7-associated illnesses. The operators of the 60-cow herd ask their veterinarian the following questions:

Q: What is E coli O157:H7?

A: The O157:H7 designates a serotype of the E coli bacteria that was first identified as a cause of human illness in 1982.⁶ This serotype had seldom been isolated before that time. Serotype O157:H7 and several others are classified as enterohemorrhagic E coli (EHEC) because they cause bloody diarrhea and produce potent toxins, but are not invasive.⁷ Escherichia coli O157:H7 is the ehec serotype most often associated with human disease episodes and the ehec most studied in food producing animals.

Q: What illnesses are caused in human beings and animals by E coli O157:H7?

A: In human beings, the main symptom associated with this serotype is bloody diarrhea attributable to hemorrhagic colitis.⁸ Some people infected with this organism, however, may appear normal or have abdominal cramps and watery diarrhea without any evidence of blood in the stool. A serious consequence of infection may be hemolytic uremic syndrome with possible renal failure.⁸

Escherichia coli O157:H7 has been isolated only rarely from animals with diarrhea,^9,10 and it was not known whether the E coli was the cause of the diarrhea. Therefore, no animal illnesses have been conclusively attributed to E coli O157:H7. Other ehec serotypes, however, have been isolated from calves with bloody diarrhea.¹¹ The lack of observable illness in food-producing animals that are shedding E coli O157:H7 in their feces, makes it more difficult to identify carrier animals so that they can be removed from the human food chain.

Q: What types of animals have been found to be test-positive for E coli O157:H7?

A: Most attempts to isolate this organism have been done in dairy and beef cattle, because they are most often associated with human disease episodes. In a national study by Hancock et al(a) of over 1,000 dairy herds, the prevalence of fecal-positive calves was low: 0.36% of calves and 1.8% of herds.(a) A review of all the cattle studies indicated that dairy heifers and calves had higher prevalence of O157:H7 shedding than did adult dairy cattle.¹² In a study in beef cattle,¹³ the prevalence of shedding among adult cattle was slightly higher than that found in studies of dairy cattle. Disease episodes in human beings have not been traced to animals other than cattle, with the possible exception of turkeys.⁵ However, E coli O157:H7 has been documented to colonize the gastrointestinal tract of pigs^14,15 and chickens under experimental conditions.^16,17 Preliminary evidence suggests that E coli-positive cattle periodically shed O157:H7 in their feces and that the excretion period ranges from hours to weeks.¹⁸

Results of surveys of retail meats and poultry provide indirect evidence that species other than cattle may have E coli O157:H7 in their gastrointestinal tract. In one study of samples from Wisconsin and Alberta, Canada,¹⁹ pork, poultry, lamb, and beef were test-positive.

Q: How do people and animals become infected?

A: Most multiple case episodes and sporadic cases in human beings have been linked to ingestion of undercooked beef and unpasteurized milk.^1-5 Serotype O157:H7 has been isolated on numerous occasions from samples of beef and milk incriminated in human multiple-case episode. Multiple cases have also been traced to supposedly pasteurized milk,²⁰ potatoes packed in peat that could have been contaminated with manure,²¹ unpasteurized apple cider,²² and water.⁵ People have also become infected after direct or indirect contact with calves shedding O157:H7²³ and with infected persons.⁵

To the author's knowledge, definitive evidence of E coli O157:H7 colonization and attachment in the bovine intestinal tract and infection under natural conditions have not yet been reported. The observation that this organism can be shed by cattle over a period of several weeks is indirect evidence of bacterial colonization. It is probable that animals acquire O157:H7 in their gastrointestinal tract from environmental sources such as manure. Various management and feeding practices are being examined for possible links to the presence of E coli O157:H7.

Q: How do milk and food products become contaminated?

A: Escherichia coli normally reside and multiply in the intestines of animals and people; thus most contamination can be traced directly or indirectly to this source. Owing to current food industry technology, products can be contaminated at many points, from the live animal through slaughter, processing, distribution, storage, kitchen preparation, and serving. Contamination can originate on the production farm or from other human beings, such as family members, associates, and food handlers. A food product can become cross-contamined during food processing and preparation by contact with a table surface or equipment that has been previously contaminated.

Q: How do we know whether our herd is positive for O157:H7?

A: It is possible to examine fecal samples by use of bacteriologic methods to identify O157:H7. Slow fermentation of sorbitol²⁴ and latex agglutination using serogroup O157-specific serum²⁵ are the bases of most laboratory testing. Serologic tests in cattle and rapid cow-side tests for this organism are not available. It is important to recognize that individual animals in a test-positive herd may not be affected and others may only intermittently shed the bacteria. Therefore, adequate numbers of animals should be tested to identify a test-positive herd. For an individual animal test, it would be desirable to examine at least 3 samples, collected several days apart, before the animal can be considered to be test-negative.

Q: How can we prevent our herd from becoming O157:H7 positive?

A: You should discontinue buying replacement heifers from the dairy herd identified as the source of the beef incriminated in the episode of O157:H7-associated illnesses. If that herd becomes test-negative, you could resume purchasing heifers. Although the exact source responsible for O157:H7-positive herds is usually not known, you can use measures that are effective in preventing infection by other pathogenic bacteria such as raising all your replacements, testing of replacement animals prior to purchase, control of vermin and insects, cleaning and sanitation, and not permitting any contact with untreated human fecal waste.

Q: If our herd is identified as test-positive for O157:H7, what are the consequences?

A: At this time, there is no requirement to report herds that test positive for O157:H7. If your herd is associated with human cases of disease, regulatory agencies will be involved and they may request your cooperation in further testing. Little is presently known about the natural history of O157:H7 in the bovine gastrointestinal tract, and proven control methods cannot be specified. Therefore, the procedures used to control enteric infections, in general, can be tried: isolate test-positive animals until retesting indicates that they are test-negative, feed colostrum to the calves, use probiotic feed supplements,²⁶ control known infective agents that might heighten susceptibility to colonization by O157:H7, and optimize nutrition. It is known that feed deprivation, as occurs during weaning and shipment, can increase fecal excretion of E coli,²⁷ thus permitting exposure of others in the herd as well as possible contamination of milk and meat products during slaughter and processing. Maintaining your herd in a milk- and dairy beef quality-assurance program will help to optimize management of the herd and avoid of further enteric problems.

Q: Suppose we do not test our herd, but in reality, it is O157:H7-positive, how can we protect against public health consequences?

A: Because of the purchase of heifers from a herd known to be test-positive for O157:H7, you should stop selling, distributing, or consuming unpasteurized milk. You should also discontinue selling breeding heifers to other farms unless they are tested and found to be E coli O157:H7-negative. Cattle manure should not be used on gardens because of the possibility that some of your cattle are shedding O157:H7, which could result in contaminated fresh vegetables. The meat from these animals should be handled in a manner to prevent contamination and it should be properly cooked prior to consumption.^28,29

References

1. CDC. Multistate outbreak of Escherichia coli O157:H7 infections from hamburgers--western United States, 1992-1993. MMWR Morb Mortal Wkly Rep 1993; 42:258-263.

2. Dorn CR. Review of foodborne outbreak of Escherichia coli O157:H7 infection in the western United States. J Am Ved Med Assoc 1993;203:1583-1587.

3. Bell BP, Goldoft M, Griffin PM, et al. A multistate outbreak of Escherichia coli O157:H7-associated bloody diarrhea and hemolytic uremic syndrome from hamburgers. JAMA 994;272:1349-1353.

4. Dorn CR. Hemorrhagic colitis and hemolytic uremic syndrome caused by Escherichia coli in people consuming undercooked beef and unpasteurized milk (letter). J Am Ved Med Assoc 1988;193:1978.

5. Griffin PM, Tauxe RV. The epidemiology of infections caused by Escherichia coli O157:H7, other enterohemorrhagic E coli and the associated hemolytic uremic syndrome. Epidemiol Rev 1991;13:60-98.

6. Riley LW, Remis RS, Helgerson SD, et al. Hemorrhagic colitis associated with a rare Escherichia coli serotype. N Engl J Med 1983;308:681-685.

7. Levine MM. Escherichia coli that cause diarrhea: enterotoxigenic, enteropathogenic, enteroinvasive, enterohemorrhagic, and enteroadherent. J Infect Dis 1987;155:377-389.

8. Karmali MA. Verocytotoxin-producing Escherichia coli. Clin Microbiol Rev 1989;2:15-38.

9. Orskov F, Orskov I, Villar JA. Cattle as a reservoir of verotoxin-producing Escherichia coli. Lancet 1987;2:276.

10. Blanco J, Gonzalez EA, Garcia S, et al. Production of toxins by Escherichia coli strains isolated from calves with diarrhea in Galicia (Northwestern Spain). Vet Microbiol 1988;18:297-311.

11. Dorn CR, Francis DH, Angrick EJ, et al. Characteristics of Vero cytotoxin producing Escherichia coli associated with intestinal colonization and diarrhea in calves. Vet Microbiol 1993;36:149-159.

12. Centers for Epidemiology and Animal Health. Escherichia coli O157:H7 issues and ramifications. Fort Collins, Colo:USDA:APHIS:VS, 1994.

13. Hancock DD, Besser TE, Kinsel ML, et al. The prevalence of Escherichia coli O157:H7 in dairy and beef cattle in Washington State. Epidemiol Infect 1994;113:199-207.

14. Francis DH, Collins JE, Duimstra JR. Infection of gnotobiotic pigs with an Escherichia coli O157:H7 strain associated with an outbreak of hemorrhagic colitis. Infect Immun 1986;51:953-956.

15. Tzipori S, Karch H, Wachsmuth KI, et al. Role of a 60-megadalton plasmid and Shiga-like toxins in the pathogenesis of infection caused by enterohemorrhagic Escherichia coli O157:H7 in gnotobiotic piglets. Infect Immun 1987;55:3117-3125.

16. Berry JT, Doyle MP, Schoeni JL, et al. Colonization of chicken caeca by Escherichia coli associated with hemorrhagic colitis. Appl Environ Microbiol 1985;49:310-315.

17. Starvic S, Buchanan B, Gleeson TM. Intestinal colonization of young chicks with Escherichia coli O157:H7 and other verotoxin-producing serotypes. J Appl Bact 1993;74:557-563.

18. Centers for Epidemiology and Animal Health. Details missing in developing E coli management plans. Feedstuffs 1994;66:1,18-

19. Doyle MP, Schoeni JL. Isolation of Escherichia coli O157:H7 from retail fresh meats and poultry. Appl Environ Microbiol 1987;53:2394-2396.

20. Upton P, Cola JA. Outbreak of Escherichia coli O157:H7 infection associated with pasteurized milk supply. Lancet 1994;344:1015.

21. Morgan GM, Newman C, Palmer SR, et al. First recognized community outbreak of haemorrhagic colitis due to verotoxin-producing Escherichia coli O157:H7 in the U.K. Epidemiol Infect 1988; 101:83-91.

22. Besser RE, Lett SM, Weber JT, et al. An outbreak of diarrhea and hemolytic uremic syndrome from Escherichia coli O157:H7 in fresh-pressed apple cider. JAMA 1993;269:2217-2220.

23. Renwick SA, Wilson JB, Clarke RC, et al. Evidence of direct transmission of Escherichia coli O157:H7 infection between calves and a human. J Infect Dis 1993;169:792-793.

24. Farmer JJ, Davis BR. H7 antiserum-sorbitol fermentation medium: a single tube screening medium for detecting Escherichia coli O157:H7 associated with hemorrhagic colitis. J Clin Microbiol 1985;22:620-625.

25. March SB, Ratnam S. Latex agglutination test for detection of Escherichia coli serotype O157. J Clin Microbiol 1989;27:1675-1677.

26. Starvic S, Buchanan B, Gleeson TM. Competitive exclusion of Escherichia coli O157:H7 from chicks with anaerobic cultures of faecal microflora. Letters Appl Microbiol 1992;14:191-193.

27. Whipp SC, Rasmussen MA, Cray WC. Animals as a source of Escherichia coli pathogenic for human beings. J Am Ved Med Assoc1994;204:1168-1175.

28. Kotula AW, Chesnut CM, Emswiler BS, et al. Destruction of bacteria in beef patties by cooking. J Animal Sci 1977;45:54-58.

29. Food and Drug Administration. Escherichia coli O157:H7 foodborne outbreak--FDA recommended cooking temperatures for ground beef. Federal State Relations, FDA, Rockville, Maryland, Jan 28, 1993.