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 3-5°C 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 30ºC (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 8°C, and larvae will not emerge from
eggs at temperatures below 12°C; the nymphal molt requires approximately 35
days at 25°C, and the adult molt requires 45 days at 25°C. Temperature also
affects the activity of ticks; a minimum threshold for activity is 4°C. 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.