Author of Response: Dan Shelton, Research Leader, Environmental Microbial Safety Lab, Agricultural Research Service, U.S. Department of Agriculture, sheltond@ba.ars.usda.gov The livestock pathogens most likely to be found in surface water and to pose a health risk to humans are various pathogenic bacteria and protozoan parasites. Among the bacterial pathogens, probably the most common waterborne pathogens coming from livestock are the various toxigenic Escherichia coli. Although all toxigenic strains can cause illness in humans, of greatest concern are the enterohemorrhagic E. coli (EHEC), of which E. coli O157:H7 is the most prevalent. EHEC infections typically result in bloody diarrhea and, in a minority of cases, can cause kidney failure, which may be fatal. EHEC are routinely found in the digestive tract of dairy and beef cattle. Since EHEC do not cause disease in cattle, it is impossible to ascertain when, or in what amounts, they are being excreted. Limited research indicates that watersheds containing high densities of grazing cattle can be contaminated with EHEC. Epidemiological data suggest that fewer than 100 EHEC cells are required for infection. Note that recreational water quality standards typically allow for approximately 100 E. coli per 3 ounces of water. Consequently, surface waters considered safe for recreational use could still pose a serious threat to human health. Other bacterial pathogens of concern include Salmonella and Campylobacter. Various Salmonella strains are excreted by cattle, swine, and poultry, while Campylobacter is most commonly associated with poultry. Neither pathogen is typically considered to pose a serious waterborne threat to human health. Although there is significant potential for the contamination of surface waters from manure-borne Salmonella, the relatively high number of Salmonella cells required for infection (approximately 100,000) limit its impact as a waterborne threat to human health. In the case of Campylobacter, although fewer cells are required for infection (approximately 500), poultry manure management practices decrease the likelihood of Campylobacter contaminating surface waters. Among the protozoan parasites, the most common parasites coming from livestock are various Cryptosporidium species and Giardia lamblia. Recent research indicates that there are several species of Cryptosporidium and strains of G. lamblia with differing host ranges and that only a few of these are infectious for humans. For example, pre-weaned calves typically shed C. parvum that is highly infectious for humans, frequently at high levels, while older calves are more likely to shed species of this parasite that are not infectious for humans. Among species and strains infectious for humans, relatively few organisms are required for infection (approximately 50 to 100), indicating that these organisms pose a significant waterborne threat. Additional Reading: Waterborne Pathogens in Agricultural Watersheds, USDA Watershed Science Institute Technical Note 2 Waterborne Pathogens in Agricultural Watersheds

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