Rangeland Cattle and the Risk of Waterborne Cryptosporidium Parvum Infection in Humans

BACKGROUND Cryptosporidium parvum (C. parvum) is a tiny protozoal parasite that can cause gastrointestinal illness in a wide variety of mammals, including humans, cattle, sheep, goats, pigs, and horses. It also occurs in various wildlife species such as deer, raccoons, opossums, and rabbits (Fayer et al. and Ungar 1990). In cattle, clinical disease and shedding of the parasite is usually limited to calves under a few months of age (National Animal Health Monitoring System 1994, Kirkpatrick 1985, Anderson and Hall 1982). Although not confirmed by studies done in the U.S., researchers in England and in Spain have reported the shedding of C. parvum in adult beef cattle (Scott et al. 1994, Lorenzo Lorenzo et al. 1993). In humans, clinical disease and shedding can appear at all ages, but is typically more common among children (Ungar 1990). The predominant clinical sign is profuse, watery diarrhea lasting from a few days to several weeks in normal (immunocompetent) calves (Kirkpatrick 1985) and humans (Jokipii and Jokipii 1986). While this disease is usually self-limiting in immunocompetent calves and humans, it can be prolonged and life-threatening among immunocompromised people such as AIDS patients. An effective treatment for eliminating this parasite from the gastrointestinal track still does not exist (White et al. 1994, Goodgame et al. 1993). A few antibiotics may show some promise in reducing the amount of oocyst shedding in AIDS patients, but further clinical trials are needed to fully evaluate their efficacy (White et al. 1994, Goodgame et al. 1993). The severity of this disease for the immunosuppressed and the fact that this parasite was implicated in recent large scale water-borne outbreaks of gastroenteritis in humans (MacKenzie et al. 1994, Hayes et al. 1989) has prompted the U.S. Environmental Protection Agency (U.S. EPA), Centers for Disease Control and Prevention, state and local public health agencies and regional water districts to seek ways to reduce surface water contamination of this parasite. Some of this attention has focused on identifying the primary sources of C. parvum in surface water. Cattle are often perceived to be a leading environmental source of water-borne C. parvum. For example, the U.S. EPA has explicitly warned that inclusion of C. parvum into the proposed Enhanced Surface Water Treatment Rule will likely result in new restrictions being placed on the location and management of livestock operations situated within watershed regions (U.S. EPA 1994). Presented below is a brief summary of the medical ecology of C. parvum in calves and in humans and the existing scientific evidence that addresses the claim that grazing of cattle on watershed regions puts humans at significant risk for water-borne infection of C. parvum.