Weather factors such as temperature, rainfall, and humidity are capable of assisting or interrupting the biology and population dynamics of vector mosquitoes (Reeves et al., 1994), thereby influencing their abundance and distribution. Rainfall (or lack of it) plays a crucial role in the epidemiology of arboviral diseases, as it provides the medium for the aquatic stages of the mosquito life-cycle. Temperature impacts on mosquito productivity and on viral replication. Humidity affects mosquito survival, and hence the probability of transmission (Sellers, 1980; Reiter, 1988; Leake 1998).

Temperature directly affects the distribution and nutritional requirements of mosquitoes. Extreme temperatures will kill mosquito populations - for example, Culex annulirostris larvae die at temperatures below 10°C and above 40°C (Lee et al., 1989). Consequently, mosquitoes are limited both in latitudinal and altitu-dinal range. High temperatures (up to a limit) reduce the period needed for larval development, meaning that more generations can fit into a given time period. C. annulirostris has an egg-to-adult time of 12-13 days at 25°C, and of only 9 days at 30°C (Kay and Aaskov 1989). A 10°C increase in temperature can reduce the development time of Schistosoma mansoni, a human pathogen in an intermediate host, by more than half - from 35 to 12 days (Harvell et al., 2002).

Mosquitoes have a high surface area to mass ratio, and are susceptible to loss of body water if a rise in ambient temperature is not accompanied by a rise in humidity. This has particular implications for arid regions, and to a lesser extent for temperate ones. High humidity influences the survival of mosquitoes (Reeves et al., 1994). As the proportion of old mosquitoes in the population increases, so does the risk of pathogen transmission (older female mosquitoes are more likely than younger ones to have had two or more blood meals). For example, Hales and others (2002) have shown that the single climatic variable, vapor pressure, could predict the current global distribution of dengue fever epidemics with 89 percent accuracy.

Water is essential for the breeding cycle of mosquitoes, given both the larval and pupal stages are aquatic. The effect of rainfall on mosquito breeding, however, is not always direct and positive. The pattern of precipitation is critical for mosquito survival. A moderate increase in rainfall can be beneficial (Lindsay and Mackenzie, 1997), while excessive increases can wash away the mosquito larvae or dormant eggs and interrupt the transmission cycle - a particular problem for species that prefer to breed in still water. The length of a rainfall event (i.e. the number of contiguous days with recorded rainfall), the number and duration of events, and the total amount of rain that falls are all factors that differently affect the breeding of mosquito vectors of Rift Valley fever in epizootic regions (Davies et al., 1985). The timing of rainfall across the year (seasonality) is also decisive in determining whether disease outbreaks occur, and whether they are epidemic in proportion (Woodruff et al., 2002).

Box 14.2 provides a summary of the sensitivity of infectious disease pathogens, vectors, and reservoir host species to changes in climatic conditions.

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