Cryptosporidiosis Susceptibility and Risk: Assessment Results and Validation. A. Makri, R. Modarres, and R. T. Parkin, The George Washington University
The assessment of human health risks from drinking water exposure to microbial pathogens has been a regulatory priority, triggered by outbreaks and regulatory provisions in the 1996 Safe Drinking Water Act Amendments. Developing effective risk models is limited by the availability and reliability of data, and by ongoing debate about defining and incorporating susceptibility. We developed and validated risk estimates of cryptosporidiosis at a regional scale, based on the microbial risk framework published by ILSI. We used water quality and endemic illness occurrence data from a metropolitan area with high AIDS prevalence. A multiplicative model was constructed with distribution functions representing Cryptosporidium in tap water, amounts of tap water consumed per day, dose-response, and probability of illness conditional on infection. Point estimates were used for the conditional probability of prolonged illness, and three conditional probabilities describing the likelihood that a case will be detected by active surveillance: physician visit, ova and parasite examina tion, and Cryptosporidium testing. We accounted for susceptibility for AIDS patients by applying AIDS status specific parameters for dose-response, both illness end-points, and the probabilities related to case detection. We accounted for susceptibility due to age by applying age specific estimates of water consumption. The resulting daily and annual risk estimates were expressed by age group and AIDS case status. They were combined with demographic data to derive expected cryptosporidiosis incidence (number of cases and incidence rate per 100,000 persons) at two geographical scales. To evaluate predictive ability the estimates were compared with observed data from active cryptosporidiosis surveillance. This builds on prior work by Teunis and Havelaar and Perz et al., by performing region-specific validation of estimates developed with data on the water concentration of Cryptosporidium.
Supported by Cooperative Agreement CX 826396-01-0 with the EPA Office of Water, Health, and Ecological Criteria Division.
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