Animal Waste, Water Quality and Human Health

84Animal Waste, Water Quality and Human HealthBeef and dairy cattle have been shown to be commonly infected with Giardiaduodenalis (Buret et al. 1990; Deshpande & Shastri 1981, O’Handley et al. 1999,Olson et al. 1997a, Olson et al.1997b, Paul et al. 2009, Quilez et al. 1996,Taminelli & Eckert 1989, Xiao et al. 1993, Wade et al. 2000). It would appear,however, that the majority of infections in cattle are of Assemblage E(non-zoonotic) and a smaller percentage of Assemblage A (human infective).Recent studies by Trout et al. (2004 through 2007) have shown that 13% to15%, 3%, and 2% of calves, yearlings, and adult dairy cows, respectively, wereshedding cysts of Assemblage A. The prevalence rate of faecal shedding ofAssemblage E is much higher than that of Assemblage A, which may have ledearly investigators and public health officials to over-estimate the importance oflivestock as a source of zoonotic Giardia duodenalis. Young cattle exhibithigher prevalence rates of faecal shedding compared to adult cattle. Forexample, previous studies have found that the prevalence rate of Giardiaduodenalis shedding for beef and dairy calves ranges from 17 to 55% and foradult beef and dairy cattle from 0 to 17% (Fayer et al. 2000, Wade et al. 2000,Huetink et al. 2001, Appelbee et al. 2003, McAllister et al. 2005, Gow &Waldner 2006). A survey of 22 feedlots in seven states in the USA (California,Washington, Colorado, Oklahoma, Texas, Nebraska, and South Dakota) found19% (1006/5260) of fresh faecal pats with detectable Giardia duodenalis cysts,with higher prevalence rates among recent arrivals and during winter timeconditions (Hoar et al. 2009). The arithmetic mean intensity for the positivefaecal pats was ∼10,700 cysts/g faeces (Figure 3.2), or ∼2030 cysts/g faecesfor the entire sample of faecals (positives and negatives) from across middle andwestern US feedlots, with a small subset of samples exhibiting >500,000cysts/g faeces (super-shedders). Assuming feedlot steers defecate between 10and 40 kg, this would generate an environmental loading rate between 2 × 10 7and 8 × 10 7 cysts per animal per day. About 34% (168/495) of Canadian beefcalves had detectable Giardia duodenalis cysts, but almost all isolates were nothuman infective (Applebee et al. 2003). In an earlier study, faecal samples fromcow-calf operations had a mean intensity of 5801 cysts/g faeces (Heitman et al.2002). Approximately 25% of sheep and 36% of goats in Belgium were foundpositive for Giardia duodenalis, with a mean intensity of ∼4600 and ∼18,000cysts/g faeces, respectively (not adjusted for % recovery) (Geurden et al. 2008).Assuming adult sheep produce 0.7 kg faeces per day, this would generate amean daily environmental loading rate of 3.2 × 10 6 cysts per sheep per day.Interested readers are encouraged to read recent reviews to better understandthe ongoing challenge of establishing a definitive role for livestock as a causeof human giardiasis (Thompson & Monis 2004, Hunter & Thompson 2005,Slifko et al. 2000).