may be pathogens of concern in nonpoint sources. Giardia is found in 90 percent to 100 percent of the muskrat populations and is prevalent in beavers (Erlandsen et al. 1988). Cryptosporidium parvum is found widely distributed in mammals, and zoonotic (animal to human) transmission has been well documented (Current 1987). Infections in cattle along with rainfalls, which washed the oocyst (the environmentally resistant and infectious form of the organism) into the water supply, were hypothesized as contributing to a large outbreak in the United Kingdom, resulting in 55,000 illnesses (Smith and Rose 1990). Cattle and sheep may represent a large reservoir for human infections. Both Cryptosporidium and Giardia can be found at prevalences of 68 percent and 29 percent, respectively, in polluted waters (waters receiving sewage and agricultural discharges) and 39 percent and 7 percent, respectively, in pristine waters (Rose et al. 1991b). In one watershed, animals were the major source of the contamination rather than sewage discharges (Rose et al. 1988). These studies suggest that domestic sewage discharges are a larger source of Giardia, while animals may be the major source of Cryptosporidium (Rose et al. 1991b).

Among the bacteria, Salmonella, Yersinia, and Campylobacter are associated with animal reservoirs. Salmonellae are common in poultry (chickens, turkeys, ducks) and in gulls, pigeons, and doves but have been identified in other wild birds much less frequently (Feachem et al. 1983). Between 15 and 50 percent of domestic animals and 10 percent of mice and rats may be infected. Wild mammals do not appear to be a major source for human infections. Both wild and domestic animals may serve as reservoirs for Yersinia enterocolitica. The organism has been identified in foxes and beavers as well as cattle, sheep, and pigs. Campylobacter has been found in a wide variety of animals. Domestic animals (cattle, sheep, and pigs) and birds (poultry and caged birds) have been documented as sources of infections in humans.

Animals may also contribute significant numbers of indicator bacteria (total coliforms, fecal streptococci, and enterococci) to waters (Crane et al. 1983). Gannon and Busse (1989) suggested that animals were the source of the elevated indicator bacterial levels in storm water. An epidemiological study of recreational waters has suggested that the indicator bacteria arising from agricultural inputs are not associated with human bacterial and viral infections (Calderon et al. 1991).

Toxins in Shellfish and Fish

Several illnesses are associated with the consumption of shellfish and fish as a result of toxic algal blooms (NRC 1991), including neurotoxic shellfish poisoning, paralytic shellfish poisoning, and scromboid poisoning (Table B.4).

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