Animal Waste, Water Quality and Human Health

GIARDIASISRose et al. (1991) fitted the “simple exponential model” to infections exhibited byvolunteers in studies reported by Rendtorff (1954) and Rendtorff & Holt (1954),using the exponential dose response model in which Pr infection =1– e −rd where d isagain the average dose given to each group of volunteers and r is the probability thata single Giardia cyst could cause infection. They obtained r = 0.01982. Thereforethe ID 50 = –ln(½)/r = 0.693/r ≈ 35.CRYPTOSPORIDIOSISComparative risk analysis 393Clinical trials for infectivity of oocysts of Cryptosporidium parvum were done as partof a set of three studies in the Medical School of the University of Texas. 9 Individualanalyses for each set have generally indicated that the appropriate dose-response curveis the single-parameter “simple exponential model”. But a meta-analysis has identifieddifferent infectivity levels when fitting a number of candidate curves to each trial’sdataset, such that the differences depend on the particular isolate used and on themethod of “passaging” the Cryptosporidium in the laboratory (Teunis et al. 2002a,2002b). Having regard to all these studies USEPA (2003), in developing its “LongTerm 2 Enhanced Surface Water Treatment Rule” for drinking water, concluded thatthe dose-response function (for infection, cf. illness) should indeed be of “simpleexponential” form, with a particular value of its single parameter (r = 0.09). Thisgives rise to ID 50 ≈ 8. However, two further studies have since been reported. 10Teunis (2009) has interpreted all five studies together, together with a sixth, 11 andthis (omitting the infectious TU502 Crypto. hominis data, because it has a humansource) leads to a conclusion that on average the ID 50 for Cryptosporidium can betaken as approximately the same as is inferred for Giardia (i.e., about 35).PATHOGENS IN ANIMAL EXCRETAThe following material has been particularly guided by information presented by Solleret al. (2010) and USEPA (2010), along with some other published literature. Chapter 3of this text gives further detailed information.Tables A10.1–A10.5 present summaries of studies of prevalence and concentrationof the five pathogens considered in this chapter, each including the four animal groups91011These studies were conducted for the TAMU, Iowa and ICP isolates (Okhuysen et al. 1999).The Moredun Crypto. parvum isolate (Okhuysen et al. 2002) and the TU502 Crypto. hominis isolate(Chappell et al. 2006).The 16W (Crypto. parvum) isolate.