Assessing the Public Health Threat Associated with Waterborne Cryptosporidiosis: Report of a Workshop

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Reported by CDC and EPA.

On September 22-23, 1994, the Center for Disease Control and Prevention's National Center for Infectious Diseases (NCID) convened a workshop entitled, Prevention and Control of Waterborne Cryptosporidiosis: An Emerging Public Health Threat," for the purpose of assisting CDC and state public health departments in providing guidance on these issues. Representatives from 40 states and from regulator and public health agencies, water utility companies, and advocacy groups discussed approaches to avoiding unnecessary boil-water advisories (i.e., statements to the public advising persons to boil water before drinking it) and preventing and controlling waterborne cryptosporidiosis. Work groups at the meeting addressed four issues: 1) surveillance systems and epidemiologic study designs; 2) public health responses when oocysts are detected in drinking water; 3) cryptosporidiosis in immunocompromised persons; and 4) water sampling methods and interpretation of results. The work groups defined the problems associated with these issues and developed strategies that could be used initially to manage these problems. The work group conclusions are for considerations by persons and organizations who must assist with these issues and by those who seek to advance understanding of waterborne cryptosporidiosis.


Cryptosporidium parvum has been recognized as a human pathogen since 1976. During 1976-1982, the disease was resorted rarely and occurred predominantly in immunocompromised persons. In 1976-1982 the number of reported cases began to increase as a result of the acquired immunodeficiency syndrome (AIDS) epidemic. Initially, the increase in incidence was limited to immunocompromised persons; however, outbreaks and sporadic infections in immunocompetent persons were identified with the aid of newly developed laboratory diagnostic techniques.

Cryptosporidium is a protozoan parasite transmitted by ingestion of oocysts that have been excreted in the feces of infected humans and animals. The infection can be transmitted through person-to-person or animal-to-person contact, ingestion of fecally contaminated water or food, or contact with fecally contaminated environmental surfaces. Several municipal waterborne outbreaks of cryptosporidiosis (1-7), including the 1993 outbreak in Milwaukee, have focused attention and concern on the potential for waterborne transmissions.

Recent studies indicate that Cryptosporidium oocysts are present in 65%-97% of surface water (i.e., rivers, lakes, and streams) tested throughout the United States (8-10). Because Cryptosporidium is highly resistant to chemical disinfectants used to treat drinking water, physical removal of the parasite from water by filtration is an important component of the municipal water treatment process. However, many cities in the United States do not use filtration as part of their water treatment process, and no current method can guarantee complete removal `d oocysts. The risks for transmission can be reduced by water filtration if the filters are properly operated and maintained.

In the United States, all outbreaks of waterborne cryptosporidiosis detected from 1984 through 1993 occurred in communities where water utilities met state and federal standards for acceptable drinking water quality, and all surface water supplies implicated in those outbreaks had been filtered. These outbreaks indicate that utility compliance with Environmental Protection Agency (EPA) water treatment standards did not adequately protect against waterborne cryptosporidiosis. The EPA turbidity standards have been strengthened since the Milwaukee outbreak, and the finished (i e., tap) water in Milwaukee at the time of the outbreak would not have met the new standards. Nevertheless, recentreports of Cryptosporidium oocysts in fully treated (i.e., disinfected and filtered) municipal water that was meeting these new standards indicate small; numbers of oocysts breached water treatment filters in 27%-54% of the communities evaluated (11, 12). …