Animal Waste, Water Quality and Human Health

380Animal Waste, Water Quality and Human Healthfor sheep except when the “supershedder” option is selected. In that case,enteropathogenic E. coli is evaluated to be the greatest source of risk overall.Modification of the weightings (see: Table 10.2) could change the relative riskestimates and predicted relative importance of each pathogen in each animalspecies. Epidemiologically, Cryptosporidium scores the highest as cause ofdetected waterborne infections (Coffey et al. 2007). The relative severity ofillness might also be made less subjective by deriving estimates of DALYs lostfor each pathogen but was not undertaken at this time. The qualitativedescriptions of severity for the five pathogens applied in this example are inaccordance with those presented by Goss and Richards (2008).Table 10.2Weighting factors applied when assessing hazard relative importance.Hazard CharacteristicQualitativeDescriptionNumerical weightassignedSeverity of infection Severe 1Moderate 0.1Mild 0.01Survival (in faeces, water) Long (L) 1Medium (M) 0.1Short (S) 0.01Prevalence of pathogen in faeces High (H) 1Medium (M) 0.1Low (L) 0.01Concentration of pathogen in faeces High (H) 1Medium (M) 0.1Low (L) 0.01The estimation of risk also relies on consideration of the dose ingested and thelikelihood that the dose will lead to a symptomatic infection. The relative riskcalculations in the model assume that there is a direct proportionality betweenthe dose ingested and the probability of illness. This is generally in accord withthe predictions of the dose response models considered herein (single-parametersimple exponential or two-parameter beta-Poisson, as used in the discussion ofID 50 values in the appendix to this chapter). If the dose is rather lower than theID 50 , the risk can be considered to be directly proportional to the dose (Haas1996, Gale 2001a&b, 2003), because the dose-response relationship is linear atlow doses. The assumption of proportionality is incorrect, however, if the dosein significantly greater than the ID 50 for the pathogen of interest. This is becausethe dose-response curve for probability of infection is asymptotic and non-linear