previously endemic regions, thereby causing significant morbidity and mortality.

Arthropod-borne parasitic diseases, such as malaria, filariasis, onchocerciasis, trypanosomiasis, and leishmaniasis, remain major human threats. An estimated 120 million people suffer from lymphatic filariasis, and approximately 18 million people are afflicted with onchocerciasis, of whom about 340,000 are blind and an equal number visually impaired. More than 10 million people are afflicted with leishmaniasis, and 50,000 to 100,000 individuals die of visceral leishmaniasis each year in India alone. In Latin America, an estimated 20 million people have Chagas disease. In Africa, approximately 45 million people are at risk for African trypanosomiasis, which has virtually precluded domestic livestock production in a geographic region of Africa larger than the United States and now is tragically resurgent in humans in areas in East Africa.

Ecological and environmental conditions are key determinants of the transmission and persistence of vector-borne pathogens. Ecological conditions can increase the risk of infection by altering human exposure to vectors or changing their distribution, abundance, longevity, activity, and habitat associations, and thereby increasing or decreasing the overall potential for the vector population to transmit the pathogen to humans. Mosquito abundance and transmission of pathogens are typically associated with rainy seasons, since juvenile mosquitoes develop in aquatic habitats. Dengue transmission, for example, typically occurs during the rainy season, although the prior month’s temperature has been shown to affect transmission during the rainy season (Focks et al., 1995). Populations of floodwater- and container-breeding mosquitoes (e.g., Ae. aegypti) are dramatically affected by environmental conditions; their abundance is directly linked to rainfall (or snowmelt for temperate-zone mosquitoes), which induces the eggs to hatch. In contrast, transmission of Saint Louis encephalitis virus may be greatest in relatively dry periods after the rainy season (Shaman et al., 2002). The preferred breeding site of the principal vector, Culex quinquefasciatus, is stagnating pools of water with concentrated nutrient materials, and thus mosquito abundance increases during drier conditions, which favor the formation of such breeding sites. The emergence and reemergence of vector-borne pathogens are linked to changes in temperature (which determines how long it takes the parasite to develop), wind speed, and relative humidity (all of which affect vector feeding frequency); the amount and diversity of vegetation; and the presence of alternative hosts (which can alter the rate of blood feeding on humans). In particular, as previously discussed, global warming could theoretically result in dramatic alterations in the incidence and distribution of vector-borne diseases.

The movement of goods and people can also support the movement of vectors, allowing them to become established in new areas (see the later



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