Animal Waste, Water Quality and Human Health

390Animal Waste, Water Quality and Human Healthand 41 added to avoid generating negative values under some other scenarios.Similarly, the calculated value is rounded to the nearest integer. This results in ascale of relative risk from 1 to 38. (Note that the upper value can be increased ifthe effect of other risk-increasing factors is included using the additionalquestion.) Because the scale is logarithmic, every unit increase in the relativerisk score corresponds to a ten-fold increase in risk, that is, due to the combinedeffect of probability of infection and the expected severity of infection.To calculate the relative population health risk, the individual risk is multipliedby the population size (Question 12). To set the risk calculation on a similar scale,the logarithm of the population size is added to the individual risk index.The model is available for download from: http://www.foodsafetycentre.com.au/risk-assessment.php10.5 CONCLUSIONSComparative risk assessment is an approach for evaluating and quantifying riskswithout resorting to the complex, time-intensive quantitative microbial riskassessment process. It also provides a relative quantitative aspect not availablein the qualitative risk assessment process, relying instead on some narrative todescribe risk when dealing with different types or sources of exposure (e.g. low,medium, high for animal excretion rates, as in Table 10.1). The comparative riskmodel in this chapter makes use of an interactive spreadsheet programme thatcan be applied in a form that is readily understood and easy to use.While the model presented here has been developed for very specific zoonoticpathogens, it might have other applications which may be very attractive forevaluating risk under various situations. For instance, risk differences betweenlocal, regional or larger areas can be evaluated using the comparative riskmodel, thereby providing water resource managers with a means to prioritizewhere they should apply the greatest risk reduction efforts and in what order.The model could also provide risk managers with a means to determine the mosteffective treatment or management options regarding public health risksassociated with recreational activities. Furthermore, it may provide a tool forrisk managers, wherein various scenarios might be developed and evaluated todetermine which approach provides the greatest public health protection. Lastly,the spreadsheet approach for applying the model may be very useful as atraining tool for those not entirely familiar with the risk assessment process.Although the comparative risk model sacrifices some of the detailed aspects of thequantitative microbial risk assessment paradigm, the relative nature of this approachis valuable for examining many of the issues associated with risk assessment. Themodel presented here should be considered a prototype for determining risk posed