Cryptosporidium parvum (C. parvum) is a protozoal parasite that can cause gastrointestinal illness in a wide variety of mammals, including humans, livestock, companion animals, and wildlife. New species of Cryptosporidium are constantly being discovered, such as C. canis and C. felis, but their significance relative to the large role that C. parvum plays in livestock and human cryptosporidiosis is still unclear. In the majority of livestock species, clinical disease and shedding of C. parvum typically occurs in youngstock under a few months of age, but fecal shedding of oocysts can also occur in healthy older animals which can then serve as a source of infection for these younger animals. In humans, clinical disease and shedding can appear at all ages, but is typically more common among children. The predominant clinical sign is profuse, watery diarrhea lasting from a few days to several weeks in normal (immunocompetent) individuals, but can be prolonged and life threatening among immunocompromised hosts such as AIDS patients. Modes of transmission range from direct fecal-oral transmission, as might occur between infected and susceptible calves during lay behavior, or ingestion of food or water inadvertently contaminated with oocysts from the feces of an infected host.
Waterborne transmission of the pathogenic protozoa, Cryptosporidium parvum, has emerged as an important public health concern. Because the infectious stage of C. parvum (oocysts) is resistant to conventional water treatment processes, public health agencies and water districts are actively seeking methods of reducing surface water contamination with this parasite. Protection of source water such as rivers and lakes has the potential to reduce the risk of transmission to humans and animals through drinking water, as well as through human recreational contact with untreated water. Given that the parasite readily infects a large number of mammalian hosts (Fayer et al. 1997), there are a number of possible contributing sources of oocysts present for any given watershed. Unfortunately, the primary quantitative sources of waterborne C. parvum oocysts are not well defined, and our methods of prioritizing point and non-point vertebrate sources of this zoonotic parasite are lacking.
Our objective is to develop a standardized methodology for comparing environmental loading rates for different populations of vertebrate hosts for C. parvum. Such a comparison would help form the basis of a rational decision making process for evaluating land use practices and vertebrate populations with respect to their relative environmental loading rates for important waterborne microbial pathogens. Both domestic and wild animal populations are infected by and can shed in their feces the infectious stage of this parasite. Attempting to characterize or assess the risk of point and non-point source protozoal contamination requires numerous parameters to be estimated, the most important being a valid and precise estimate of the oocyst loading rate per animal unit (Atwill et al. 2001; Hoar et al. 2000). The oocyst loading rate, which can be defined as the total number of oocysts excreted by a defined cohort of animals for a specific period of time, can be calculated directly by measuring the kinetics of total oocyst shedding, that is, duration and intensity per Kg feces, multiplied by fecal production. This direct measurement method is very difficult for free-ranging wildlife and some species of livestock. An alternative approximation for determining the oocyst loading rate for cohorts of mammals is to measure the prevalence of infection and the intensity of shedding using cross-sectional surveys of the mammalian population, and then relying on experimental or laboratory estimates of fecal production (Hoar et al. 2000). We applied these concepts to a variety of domestic and wild animal species to generate a set of comparative loading rates for the waterborne pathogen, C. parvum.