Animal Waste, Water Quality and Human Health

Comparative risk analysis 37910.4.4.1 Identifying the sourceQuestion 1 asks the user to select the type of farmed animal population thatcontributes most to the source of contamination of the recreational/workingwater body. Doing so determines both the pathogen considered to present thegreatest risk from that animal species and also the relative risk, based oninformation presented in Table 10.1, as described below. The overall hazardpresented by the pathogen is considered to arise from:• its relative prevalence among herds/flocks of the animal selected;• the concentration of the pathogen in faeces of the species selected;• the relative survival of the pathogen in the environment (expressed as T 90 );• the relative severity of the disease caused by the pathogen;• the infectiousness of the pathogen as expressed by the ID 50 .For each animal group a simplified index of “hazardousness” for each of the fivepathogens considered is calculated by the following formula:Relative pathogen risk = (relative prevalence × relative concentration× relative survival (or T 90 )× relative disease severity)/ID 50The maximum of the values generated for each pathogen for the animal speciesselected is the relative risk value assigned as the answer to Question 1 and isused in further calculation of relative risk. It should be noted that this approachis based on several subjective decisions and assumptions. The most apparentis the translation of qualitative assessment of pathogen prevalence andconcentration (see Table 10.1) into relative quantities. Table 10.2 indicates thevalues, or relative weights, that were applied. The weightings are based onfactors of ten for simplicity but could be altered if reliable, representative,quantitative data were available.To allow the estimation of the consequences of the effects of more extremehazards, for example, a “supershedder”, or an epidemic level of pathogenexcretion within a herd/flock, an additional choice reflecting a higher level ofpathogen excretion is added to the range of responses to Question 1. Currently,selection of this option only has the effect of increasing the modelledconcentration of E. coli in the faeces of cattle by 1000-fold, but other optionscould be included.In practice, the combination of the above weights and ID 50 values results inCryptosporidium representing the greatest level of hazard when cattle or poultryare selected, Giardia and Cryptosporidium when pigs are selected, and EHEC