American Journal of Epidemiology Vol. 147, No. 3: 289-301
Copyright © 1998 by The Johns Hopkins University School of Hygiene and Public Health
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Cryptosporidium in Tap Water
Comparison of Predicted Risks with Observed Levels of Disease
1Division of Environmental Health Sciences, School of Public Health, Columbia University New York, NY
2Center for Environmental Research and Conservation, Columbia University New York, NY
Waterborne transmission of Cryptosporidium parvum is well-established as a source in outbreaks of cryptosporidiosis; however, the role of tap water in endemic disease is unclear. The authors applied a risk assessment approach incorporating uncertainty analysis to examine the potential role of tap water in the transmission of endemic C. parvum infection. The model had two components: exposure-infection, to relate low-dose exposure to infection; and infection-outcome, to include the probabilities of clinical outcomes leading to case detection and reporting. The population was divided into four subgroups: adults and children with and without acquired immunodeficiency syndrome (AIDS). Because of the high degree of uncertainty associated with available measures, a plausible baseline concentration of oocysts, 1 per 1, 000 liters, was assumed for input to the model. In the non-AIDS subgroups, the predicted median annual risk of infection was approximately 1 in 1, 000 (non-AIDS adults: 0.0009 infection/person/year, 95% confidence interval (Cl) 0.0003-0.0028), while in the AIDS subgroups the predicted risk was 2 in 1, 000 (AIDS adults: 0.0019 infection/person/ year, 95% Cl 0.0003-0.0130). When the risks were applied to the 1995 New York City population, more than 6, 000 infections were estimated, with 99% occurring in the non-AIDS categories. Estimates of the overall probabilities that an infection would result in a reported case predicted that three reported illnesses would occur out of every 10, 000 infections in non-AIDS adults (95% Cl 5 × 10–5 to 2 × 10–3), with a 10-fold higher probability in the non-AIDS pediatric subgroup. In contrast, the majority of infections occurring in the AIDS subgroup were predicted to result in reported cases (AIDS adults: probability = 0.61, 95% Cl 0.39-0.80). When the model was applied to the New York City population, the calculated number of tap-water-related cases per year in the non-AIDS subgroups was six (95% Cl 1-29), and in the AIDS subgroups it was 34 (95% Cl 6-240). Am J Epidemiol 1998; 147: 289-301.
acquired immunodeficiency syndrome; cryptosporidiosis; Cryptosporidium parvum; diarrhea; risk assessment; water; water microbiology
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