Simulating cryptosporidium exposures in drinking water during an outbreak Available to Purchase

This paper predicts exposures to Cryptosporidium parvum oocysts through drinking water under conditions which are consistent with a waterborne outbreak. Sources of variation which contribute to the variation in oocyst exposures include the oocyst densities in the raw waters, the efficiency of oocyst removal by treatment and the daily consumption of unboiled tap water. Even under outbreak conditions the majority of consumers may not ingest any oocysts each day. Of those who are exposed, some ingest just one oocyst/d while others ingest higher doses, which in a small proportion approach the ID₅₀ for C parvum. Ignoring this variation and using a single point average exposure predicts that a much larger proportion of the population is exposed each day but only ever to very low doses of oocysts. The impact of ignoring this variation on the predicted risks depends on the nature of the dose-response curve and, in particular, the assumptions made about the low dose extrapolation. The heterogeneity of oocyst densities in drinking water during an outbreak could contribute to the failure to detect oocysts in some waterborne outbreaks.