Animal Waste, Water Quality and Human Health

78Animal Waste, Water Quality and Human Healthoocysts per squirrel per day, a rate that is 6 to 15 times greater in oocyst productionthan adult beef cattle despite being less than one thousandth the size of a beef cow(Atwill et al. 2004). Probabilistic modeling and Monte Carlo estimation proceduresfor this parameter can generate uncertainty intervals for the oocyst loading rate(Dorner et al. 2004, Starkey et al. 2007). This rate can be further adjusted foranimal numbers per acre or hectare (A), resulting in the calculated pathogenproduction rate for a specific animal stocking rate per unit area, fPI p A. Pathogenproduction rates per acre or hectare can allow a more accurate match between theexpected pathogen load at a site and the effectiveness or log 10 reduction capacityof a load or transport intervention strategy (Tate et al. 2004). Lastly, the spatialpattern of faecal deposition by livestock predicts where the pathogen loads on alandscape will be deposited for extensive livestock production systems anddraught animals. In general, this spatial pattern is not randomly distributed acrossthe landscape, but often highly clustered or positively correlated with preferredforaging sites, loafing areas, staging areas for draught animals, drinking water,shade in warmer climates, and feed supplements (e.g., salt block, concentrates)(Tate et al. 2003, Lewis et al. 2005). It is likely that only a portion of the totalspatial pathogen load is hydrologically connected, suggesting that the portionthat has little risk of reaching water can be discounted or under-weighted inlandowner- and catchment-scale modeling of waterborne pathogen transmission(Tate et al. 2003, Ferguson et al. 2005).Accurate estimates of the environmental loading rate can also be generatedby daily faecal sampling of representative groups of animals and calculatingpathogen loading across the entire infectious cycle (Nydam et al. 2001), but theeffort needed to generate this type of data precludes its widespread use, especiallyfor extensive animal populations at low stocking densities. Alternatively, it maybe cheaper to estimate the environmental loading rate as the mean intensity of apathogen for pooled faecal matrices from all animals in a group (positive andnegative animals) matched by age or weight class (I p,n ) × daily faecal productionper animal for that age or weight class ( f ), fI p,n . One concern about this approachis that pooling of positive and negative faecal samples will lower pathogenconcentrations in the faecal matrix, and depending on assay sensitivity, generateexcessive false negatives and under-estimates of pathogen loading.Lastly, the advent of molecular fingerprinting tools has sharpened ourunderstanding of host-adapted strains and virulence factors that are responsiblefor the majority of human illness, which has the effect of reducing boththe prevalence of faecal shedding (↓P) and the mean intensity of human-infectivepathogens excreted among positive animals (↓I p ), thereby reducing the calculatedenvironmental loading rate for many of the priority pathogens. Examples where asubset of strains is responsible for much of the burden of human illness from a