Kissel, J., Samadpour, M., and Ongerth, J. (1996). "Comparison
of Estimated Risks of Giardia and Cryptosporidium in Drinking
Water with Observations in the Pacific Northwest," Proceedings of the
1996 Annual Conference of the American Water Works Association, Vol. C,
pp. 751-757.
Summary
Exponential models have been developed to estimate the risk of giardiasis and cryptosporidiosis posed by public water supplies. These models incorporate dose-response information from experimental studies and are in the following form:
where Pinf,t is the fractional probability of infection at time t, r is the fraction of ingested microorganisms, u is the Giardia cyst or Cryptosporidium oocyst concentration, and Q is the tap water ingestion rate. To illustrate the predictive capability of these models, Monte Carlo simulations of infection rates were generated using data from King County, Washington. Distributions for Giardia cyst and Cryptosporidium concentrations were assumed to be lognormal and were based on samples collected from the Cedar River, from which the city of Seattle receives roughly 70% of its water supply. Lognormal and beta distributions were used for Q and r, respectively, and were based on reported findings from previous studies. The model run for giardiasis cases yielded a distribution of outcomes that included values consistent with observed morbidity. However the range of simulated values spanned five orders of magnitude, reflecting the uncertainty in the efficacy of chlorination techniques. In the case of cryptosporidiosis, simulated results were not consistent with likely incidence rates. Some possible reasons for the discrepancy between elevated simulated rates and assumed "actual" rates (actual rates in Seattle are not known and assumptions were made based on rates seen in nearby Canadian locales) include: (1) large variability in the virulence of Cryptosporidium strains; (2) lack of dose-response data at low (actual) doses; (3) differences between reported infection and symptomatic (sub-clinical or clinical) rates; (4) overestimation of the assumed viable oocyst levels in delivered water; and (5) all recent cryptosporidiosis outbreaks in public systems in the US that derive their raw water from surface supplies have occurred in systems that utilize filtration. These findings suggest that a periodically malfunctioning filtration system may present greater cumulative risk than the absence of filtration in areas where raw water quality is high.
