Animal Waste, Water Quality and Human Health

80Animal Waste, Water Quality and Human Health(cattle, sheep, goats, and horses) and various other domestic and wild animals(Hunter & Thompson 2005, Xiao & Fayer 2008). As explained in Chapter 2,what was historically classified as Crypto. parvum in a variety of differentlivestock hosts has been shown more recently to be a group of different speciesor genotypes of Cryptosporidium, some of which appear to be relativelyhost-adapted and possibly of low infectious potential for humans. This has madeaccurate quantification of the environmental loading rate of DNA-confirmedCryptosporidium parvum from the many livestock species found throughout theworld problematic and subject to periodic revision. Prevalence and intensity datafor presumptive Cryptosporidium parvum from the 1970s through the early2000s probably overestimates the amount of oocyst shedding in domesticanimals. Recent reviews or catchment-scale modeling of cryptosporidial loadingby livestock struggle with this problem by tabulating oocyst loading data listedas Cryptosporidium (e.g., Dorner et al. 2004, Ferguson et al. 2009, Robertson2009) or stratified by Cryptosporidium species (Starkey et al. 2007).With these caveats in mind, high prevalence rates and high intensities offaecal shedding of presumptive Cryptosporidium parvum oocysts have beendocumented in young dairy calves from various regions of the world for severaldecades, with point prevalences of faecal shedding ranging from ∼10 to ∼80%in groups of one- to four-week old dairy calves and mean intensities rangingfrom 1 × 10 5 to 6 × 10 7 oocysts per gram among groups of infected animals(Goodgame et al. 1993, Xiao & Herd 1994, Atwill et al. 1998, Kuchzynska &Shelton 1999, Uga et al. 2000, Nydam et al. 2001, Moore et al. 2003; Applebeeet al. 2005, Starkey et al. 2005, Santin et al. 2008). Figure 3.1 is an example ofa group of young dairy calves shedding oocysts after oral inoculation ofwild-type dairy calf Cryptosporidium parvum. Assuming that the body massduring the first 30 days of life ranges from 40 to 80 kg and mean daily faecalexcretion is 3.3% of body mass (Table 3.2), the environmental loading rate forCryptosporidium parvum oocysts for a group of young dairy calves couldfeasibly range from a few billion to hundreds of billions of oocysts per daydepending on infection levels in the calves. Attempting to develop a singlevalue for the environmental loading rate of Cryptosporidium parvum in this highrisk population might convey a false sense of accuracy given the potentialinfluence of animal husbandry practices, biosecurity of feed and water supplies,calving pen hygiene, and manure management practices on the prevalence offaecal shedding of oocysts in dairy calves (Maldonado et al. 1998, Sischo et al.2000, Starkey et al. 2006, Silverlås et al. 2009). The duration of oocystshedding has been associated with the number of ingested oocysts, suggestingthat interventions that reduce the environmental load of Cryptosporidiumparvum oocysts to which young dairy calves are exposed may lead to reduced