Animal Waste, Water Quality and Human Health

370Animal Waste, Water Quality and Human Health10.2.1 The diseasesThe severity of consequences of exposure depend on a number of factors related tothe pathogen and the host including the infectiousness of the pathogen, the doseingested, and the severity of disease that is the usual outcome. In turn, the severityof the disease depends on the susceptibility of the host to infection by thepathogen. As discussed above, there is variability in each of these risk-affectingfactors but in the risk assessment tool developed here we adopt average values tocharacterise the relative risk. As noted above 4 , if conservative or worst-casevalues are taken for each variable, the resulting risk estimate is characterized byan extremely improbable event. It should also be noted that point estimates basedon measures of central tendencies, for example, average, or mode, will notnecessarily lead to an answer that represents the most likely outcome and can leadto large errors (Cassin et al. 1996). Nonetheless, we have included differentcategories in the risk ranking tool where appropriate to be able to distinguishsituations when risks are systematically higher or systematically lower. Forexample, children are often more susceptible to infection than adults and for thisreason we have included options in the tool that can differentiate this risk, or ifthere is some correlation between sporadic contamination and the likelihood thatpeople will be exposed to the recreational water. (Differential susceptibility isdiscussed in greater detail below.) In the case of differential exposure due to age,the population exposed can be selected from “general”, “children” only or “adults”only. More sub-categories could easily be included in the tool if there were datathat showed that specific populations were physiologically more likely to becomeinfected. Note, also, that some populations are more likely to be exposed due tocustom, location, and so on. but this aspect is addressed in a separate questionconcerning frequency of exposure to the recreational water resource being assessed.10.2.2 Assessing infectivityThe infectiousness of a pathogen is sometimes characterised as “the infectiousdose”. This is inappropriate because there is variability in the number ofpathogens required to cause infection or illness (depending on the pathogenitself and the susceptibility of the consumer). Recognising this, infectiousness isoften characterised by the ID 50 : the number of cells of the pathogen that resultsin 50% of the exposed population becoming infected. The relationship betweenprobability of infection and dose ingested is described as the dose-responsecurve and can be described by a variety of mathematical equations. A detailedreview of dose-response relationships for infection processes, both from abiological and mathematical perspective, is presented in FAO/WHO (2003).