Risk assessment model for a waterborne outbreak of cryptosporidiosis

Counts of Cryptosporidium oocysts in 100L volumes of treated water are simu lated for conditions representative of a waterborne outbreak in a surface w ater-derived supply. Assuming oocysts act independently during infection, t he risk of infection is directly related to the arithmetic mean oocyst dens ity in the water supply, which is in turn related to the total number of oo cysts which break through treatment. Spatial/temporal heterogeneity of oocy st concentrations in the treated water contributes to monitoring programmes based on "spot-sampling" underestimating the arithmetic mean oocyst densit y and hence the risk of infection. This could contribute to the reported la ck of a clear association between oocyst concentrations measured in drinkin g water supplies and the risk of waterborne outbreak of cryptosporidiosis i n the population. An increase in spatial heterogeneity of oocysts during tr eatment could also contribute to an overestimation of the net oocyst remova l by treatment.