REPORTS - SPECIAL REPORTS

The Regional Impacts of Climate Change


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Encephalitides: Of reported encephalitis cases in North America, many are mosquito related, including Saint Louis encephalitis, which has occurred as far north as Windsor, Ontario (1975); LaCrosse encephalitis; and western, eastern, and Venezuelan equine encephalomyelitis (Shope, 1980). The elderly are at highest risk for Saint Louis encephalitis, and children under 16 years are at greatest risk of LaCrosse encephalitis.

Although mosquito longevity diminishes as temperatures rise, viral transmission rates (similar to dengue) rise sharply at higher temperatures (see Figure 8-11) (Hardy, 1988; Reisen et al., 1993). From field studies in California (Reeves et al., 1994), researchers have suggested that a 3-5C temperature increase could cause a northern shift in western equine and Saint Louis encephalitis outbreaks, with the disappearance of western equine encephalitis in southern endemic regions. Also to be considered in these types of impact assessments is the impact of projected climate change on mosquito habitat (e.g., freshwater hardwood swamps for the eastern equine encephalomyelitis vector Culiseta melanura-which may well be eliminated from the southeast United States).

Outbreaks of Saint Louis encephalitis are correlated with periods of several consecutive days in which temperature exceeds 30C (Monath and Tsai, 1987). For example, the 1984 California epidemic followed a period of extremely high temperatures. In addition, eastern equine encephalitis has been associated with warm, wet summers along the east coast of the United States (Freier, 1993). Computer analysis of monthly climate data has demonstrated that excessive rainfall in January and February, combined with drought in July, most often precedes outbreaks of eastern equine encephalitis (Bowen and Francy, 1980). Such a pattern of warm, wet winters followed by hot, dry summers resembles many of the GCM projections for climate change over much of the United States.

Tickborne diseases: Ticks transmit Lyme disease-the most common vector-borne disease in the United States, with more than 10,000 cases reported in 1994-along with Rocky Mountain spotted fever (RMSF), and Ehrlichiosis. Involved tick and mammal host populations are influenced by land use and land cover, soil type, and elevation, as well as the timing, duration, and rate of change of temperature and moisture regimes (Mount et al., 1993; Glass et al., 1994). The relationships between vector life-stage parameters and climatic conditions have been verified experimentally in field and laboratory studies (Goddard, 1992; Mount et al., 1993). Ixodes scapularis-an important hard-backed tick vector in North America-will not deposit eggs at temperatures below 8C, and larvae will not emerge from eggs at temperatures below 12C; the nymphal molt requires approximately 35 days at 25C, and the adult molt requires 45 days at 25C. Temperature also affects the activity of ticks; a minimum threshold for activity is 4C. Ticks also are highly dependent on a humid environment. Climate change, therefore, could be expected to alter the distribution of these diseases in both the United States and Canada (Grant, 1991; Canadian Global Change Program, 1995; Environment Canada et al., 1995; Hancock, 1997). For example, any tendency toward drying would suggest a reduction in the incidence of these diseases.

 


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