Schistosomiasis, which is caused by five species of the trematode (flat worm) Schistosoma, requires water snails as an intermediate host. Worldwide prevalence has risen since the 1950s largely as a result of expansion of irrigation systems in hot climates where viable snail populations can survive and the parasite can find human parasite carriers (Hunter et al., 1993). All three genera of snail hosts (Bulinus, Biomphalaria, and Oncomelania) can tolerate a wide temperature range. At low temperatures, snails are effectively dormant and fecundity is virtually zero, but survival is good. At high temperatures, births (egg production) increase, but so does mortality (Table 9-3). However, snails are mobile and can move to avoid extreme temperatures within their habitats; water can act as an efficient insulator (Hairston, 1973; Gillett, 1974; Schiff et al., 1979). The precise conditions within water bodies that determine transmission depend on a host of environmental factors, including local geology and topography, the general hydrology of the region, the presence or absence of aquatic vegetation, and local agricultural usage (Appleton and Stiles, 1976; Appleton, 1977). In east Africa, colonies of Biomphalaria and Bulinus spp. persist at altitudes of 2,000 m or more, but transmission—if it occurs at all—is restricted to brief warm seasons. Climate change might allow schistosomiasis transmission to extend its range to higher altitudes. Conversely, increasing temperatures at sea level could decrease transmission unless the snails move to cooler refuges.

Water shortages resulting from climate change could create greater need for irrigation, particularly in arid regions. If irrigation systems expand to meet this need, host snail populations may increase (Schorr et al., 1984), leading to greater risk of human infection with the parasite. However, this impact could be reduced by constructing irrigation systems that are not conducive to snail breeding.

The geographical distribution of American trypanosomiasis (Chagas' disease) is limited to the Americas, ranging from the southern United States to southern Argentina and Chile (Carcavallo et al., 1998, 1999). Chagas' disease is transmitted by triatomine bugs (see Table 9-3). Temperature affects the major components of VC (reviewed by Zeledón and Rabinovich, 1981; Carcavallo, 1999). If temperatures exceed 30°C and humidity does not increase sufficiently, the bugs increase their feeding rate to avoid dehydration. If indoor temperatures rise, vector species in the domestic environment may develop shorter life cycles and higher population densities (Carcavallo and Curto de Casas, 1996). High temperatures also accelerate development of the pathogen, Trypanosoma cruzi, in the vector (Asin and Catalá, 1995). Many vector species are domesticated. Lazzari et al. (1998) found that in the majority of structures, differences between inside and outside temperature were small, although differences in humidity were significant. Triatomine dispersal also is sensitive to temperature (Schofield et al., 1992). Population density of domestic vectors also is significantly affected by human activities to control or eradicate the disease (e.g., replastering of walls, insecticide spraying). The southern limits of Triatoma infestans and Chagas' disease distributions recently have been moved significantly inside their climatically suitable limits by large-scale control campaigns (Schofield and Dias, 1999).

Plague is a bacterial disease that is transmitted by the bite of infected fleas (Xenopsylla cheopis), by inhaling infective bacteria, and, less often, by direct contact with infected animals (Gage, 1998). Plague exists focally in all regions except Europe. Notable plague outbreaks have occurred in several Asian, African, and South American countries in the past 10 years (John, 1996; WHO, 1997; Gage, 1998; PAHO, 1998). It is unclear whether climate change may affect the distribution and incidence of plague. There does appear to be a correlation between rainfall patterns and rodent populations (Parmenter et al., 1999; see also Section 9.7.9). Prospective field research studies must be conducted to confirm this.

Tick-borne diseases—in particular, Lyme disease, Rocky Mountain spotted fever, ehrlichiosis, and tick-borne encephalitis (TBE)—are the most common vector-borne diseases in temperate zones in the northern hemisphere. Ticks are ectoparasites; their geographical distribution depends on the distribution of suitable host species—usually mammals or birds (Glass et al., 1994; Wilson, 1998). Species that transmit these diseases have complex life cycles that require 3 years and three different hosts species—one for each stage of the cycle (larvae, nymph, and adult). Climate directly and indirectly influences the tick vector, its habitat, host and reservoir animals, time between blood meals, and pathogen transmission. Bioclimatic threshold temperatures set limits for tick distribution and are of importance for the magnitude of disease occurrence (Table 9-3). Temperatures must be sufficiently high for completion of the tick's life cycle. Humidity must be sufficient to prevent tick eggs from drying out. Temperatures above the optimum range reduce the survival rate of ticks. In temperate countries, tick vectors are active in the spring, summer, and early autumn months.

Over the past 2 decades, marked increases have been reported in the abundance of ticks and the incidence of tick-borne disease in North America and Europe. In North America, these changes have been attributed to an increase in awareness of tick-borne diseases and increased abundance of wild tick hosts (principally deer), as reforestation has expanded areas of suitable habitat (Dennis, 1998). There is some evidence that the northern limit of distribution of the tick vector (Ixodes ricinus) and tick density increased in Sweden between the early 1980s and 1994, concurrent with an increased frequency of milder winters (Talleklint and Jaenson, 1998; Lindgren et al., 2000). In New York state, Ixodes scapularis has expanded its geographic distribution northward and westward in the past 10 years. The reasons for this expansion are unknown.

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