BOX 3-4 An Outbreak of Cryptosporidiosis

Cryptosporidiosis, a waterborne intestinal infection caused by Cryptosporidium spp., produces potentially life-threatening disease in those who are immunocompromised and mild to chronic diarrhea in others (Fayer and Ungar, 1986). In 1993, an estimated 403,000 crytposporidiosis infections occurred among residents of and visitors to Milwaukee, Wisconsin (MacKenzie et al., 1994). Cryptospordium oocysts in untreated water from Lake Michigan had apparently been inadequately removed by the coagulation and filtration process in a portion of the Milwaukee water treatment plant. The source of the oocysts leading to the outbreak remains speculative. Possible sources include cattle along two rivers that flow into the Milwaukee harbor, slaughterhouses, and human sewage. Various vertebrates (e.g., cows and wild deer) are naturally infected by Cryptosporidium spp. (Navin and Juranek, 1984; Simpson, 1992; Tzipori et al., 1981). Perhaps considerable rainfall, combined with a high concentration of animal runoff near the municipal water supply, triggered this transmission event. Genotypic and experimental infection data may suggest a human rather than bovine source (Peng et al., 1997). In the 2 years following this contamination of the water supply, it was estimated that 54 deaths (85 percent among people with AIDS) may have resulted from the 1993 outbreak (Hoxie et al., 1997). In addition to contaminated drinking water, outbreaks of cryptosporidiosis in the United States and abroad have been linked to chlorinated and unchlorinated recreational water facilites, such as public swimming pools, water parks, lakes, and rivers (Carpenter et al., 1999).

the emergence and spread of infectious disease would likely be substantially impacted. A recent National Research Council (NRC) report, Under The Weather: Climate, Ecosystems, and Infectious Disease, addresses the impact of climate and weather change in further detail (NRC, 2001).

CHANGING ECOSYSTEMS

The abundance and distribution of plants and animals can, conversely, impact on components of the physical environment. Forest growth, for example, usually reduces evapotranspiration; cropping often increases local relative humidity; and the development of large urban areas generally leads to an accumulation of atmospheric particulates and warmer air temperatures. Even very minor ecological changes, such as implementing a new farming technique, can confront pathogens with new environments and significantly alter the transmission patterns of infectious diseases. Of course, the pathogens must have sufficient genetic variation to adapt to such ecological changes and new environments. But most pathogenic evolutionary changes that result in a potentially new disease still require an ecological



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