Simulating Cryptosporidium exposures in drinking water during an outbreak

This paper predicts exposures to Cryptosporidium parvum oocysts through dri nking water under conditions which are consistent with a waterborne outbrea k. Sources of variation which contribute to the variation in oocyst exposur es 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 ooc ysts each day. Of those who are exposed, some ingest just one oocyst/d whil e others ingest higher doses, which in a small proportion approach the ID50 for C parvum. Ignoring this variation and using a single point average exp osure predicts that a much larger proportion of the population is exposed e ach 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-res ponse curve and, in particular, the assumptions made about the low dose ext rapolation. The heterogeneity of oocyst densities in drinking water during an outbreak could contribute to the failure to detect oocysts in some water borne outbreaks. (C) 1998 IAWQ Published by Elsevier Science Ltd. All right s reserved.