Freshwater: Diarrheal diseases in North America can be caused by a large variety of bacteria (e.g., salmonella, shigella, and campylobacter), viruses (e.g., rotavirus), and protozoa (e.g., giardia lamblia, toxoplasma, and cryptosporidium). Climatic effects on the distribution and quality of surface water, including increases in flooding or water shortages, can impede personal hygiene and impair local sewage systems. For example, extreme precipitation contributed to an outbreak of toxoplasmosis in British Columbia in 1995 when excessive runoff contaminated a reservoir with oocysts from domestic and wild cats (British Columbia CDC, writ. comm. 1995).
Cryptosporidiosis, which causes severe diarrhea in children and can be fatal to immunocompromised individuals, is the most prevalent waterborne disease in the United States (Moore et al., 1995). Natural events (e.g., floods, storms, heavy rainfall, and snowmelt) often can wash material of fecal origin, primarily from agricultural nonpoint sources, into potable water. The Milwaukee cryptosporidiosis outbreak in 1993 resulted in 403,000 reported cases; it coincided with unusually heavy spring rains and runoff from melting snow (MacKenzie et al., 1994).
Factors enhancing waterborne cryptosporidiosis will depend on hydrological responses to climate change and the degree of flooding in water catchment areas. Flushing from heavy rains may be more important than actual flooding, especially for private wells influenced by surface water. Land-use patterns also determine contamination sources (e.g., agricultural activities) and therefore must be considered.
In addition, intensification of heavy rainfall events (as suggested by some scenarios) could lead to more rapid leaching from hazardous-waste landfills, as well as contamination from agricultural activities and septic tanks. This leaching or contamination represents a potential health hazard-particularly at times of extensive flooding, which can lead to toxic contamination of groundwater or surface drinking water. Improvements in water treatment facilities and technologies could help ameliorate this situation.
Marine: Warm water favors the growth of toxic organisms such as red tides, which cause paralytic shellfish poisoning, diarrheic shellfish poisoning, and amnesic shellfish poisoning. For example, one species of toxic algae previously confined to the Gulf of Mexico (Gymnodinium breve) extended northward in 1987 after a "parcel of warm Gulf Stream water" reached far up the east coast, resulting in human neurologic shellfish poisonings and substantial fish kills (Tester, 1991). Domoic acid, a toxin produced by Nitzchia pungens diatom that causes amnesic shellfish poisioning, appeared on Prince Edward Island for the first time in 1987. The outbreak coincided with an El Niņo year, when warm eddies of the Gulf Stream neared the shore and heavy rains increased nutrient-rich runoff (Hallegraeff, 1993).
Zooplankton, which feed on algae, can serve as reservoirs for Vibrio cholerae and other enteric pathogens, particularly gram-negative rods. Quiescent forms of V. cholerae have been found to persist within algae; these quiescent forms can revert to a culturable (and likely infectious) state when nutrients, pH, and temperature permit (Huq et al., 1990). V. cholerae occur in the Gulf of Mexico and along the east coast of North America. With warmer sea surface temperatures, coastal algal blooms therefore could facilitate cholera proliferation and transmission.
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