A Framework for Assessing the Risks of Waterborne Cryptosporidium: Application to Endemic RatesEPA Grant Number: U914961
Title: A Framework for Assessing the Risks of Waterborne Cryptosporidium: Application to Endemic Rates
Investigators: Perez, Joseph F.
Institution: Columbia University in the City of New York
EPA Project Officer: Edwards, Jason
Project Period: January 1, 1996 through February 1, 2000
Project Amount: $102,000
RFA: STAR Graduate Fellowships (1996) RFA Text | Recipients Lists
Research Category: Fellowship - Microbiology , Academic Fellowships , Biology/Life Sciences
The objective of this research project is to develop and apply, with available occurrence data, a framework for the comparison of risk assessment-derived estimates of infection and illness rates due to exposure to low levels of infective Cryptosporidium in tap water.
The application of risk assessment models utilizing available dose-response data for Cryptosporidium infection can provide estimates of population-based risks from tap water consumption. Comparisons of such risk assessment results with available occurrence data are complicated by the spectrum of clinical responses that is associated with infection with these organisms. It is important to relate infection rates to illness outcomes. Infections occurring in immunologically healthy persons may frequently be asymptomatic or mild, and they are self-limiting in their course. On the other hand, in immunocompromised hosts (persons with AIDS for example), infection with Cryptosporidium tends to be persistent and profoundly debilitating. Therefore, this risk assessment considered persons with AIDS separately. Infection risks were generated using two assumed levels of Cryptosporidium occurrence.
A source of data on infection rates is provided by epidemiologic surveillance programs, which generally consist of the facilitated reporting of infections from laboratories that perform stool examinations. Typically, testing for Cryptosporidium is conducted only at the request of physicians, with the result that most requests come from clinicians who treat AIDS patients and are aware of the disease. In contrast, cases occurring in the general population, even when associated with medical attention, are unlikely to receive a confirmed diagnosis via stool examination for Cryptosporidium. A sequence of events was derived to relate infection to illness severity, medical care utilization, diagnostic testing, and case reporting. Quantitative estimates of each of the component events for the AIDS and non-AIDS subgroups were derived, and were applied to the infection risk estimates. Results are presented in the form of projected case rates for populations with varying prevalence of AIDS.
Uncertainties regarding the actual occurrence of infective Cryptosporidium in tap water are substantial. However, the analysis was consistent with the premises that tap water may represent an exposure route for endemic Cryptosporidium infection, and case counts and rates derived from surveillance may substantially underestimate overall rates of infection and illness. The analysis also indicated that reported levels of cryptosporidiosis—and the preponderance of cases among persons with AIDS—could result from an exposure common to the entire population, irrespective of whether tap water is a source.