Anopheles stephensi is the most important vector of urban malaria on the Indian subcontinent. Nowhere else in the world is there a malaria vector so well adapted to urban life. It breeds in wells, cisterns, fountains, ornamental tanks, barrels, and buckets (Bang and Shah, 1988).
During the initial planning of the Indian National Malaria Eradication Program (NMEP), cities and towns with a population of more than 40,000 were considered to have little malaria transmission or to be malaria free (Sharma and Mehrotra, 1986), and malaria control in these communities was turned over to the local government. The result was a precipitous drop in malaria transmission in rural areas with no corresponding drop in urban areas. The proportion of malaria cases occurring in urban areas increased in many parts of India. In the state of Tamil Nadu, for example, 50 percent of malaria cases reported in 1961 were from urban areas; by 1963, this proportion had increased to 95 percent (Sharma and Mehrotra, 1986). The presence of pockets of urban malaria in India was an important contributor to the dramatic resurgence of the disease in the late 1960s. An in-depth evaluation of the NMEP concluded that the absence of control strategies for urban malaria was the program's most important failure (Sharma and Mehrotra, 1986).
At least one other Anopheles species, An. arabiensis, found in Nigeria, seems to be adapting to living in town or city centers (World Health Organization, 1988). In most of Africa, Latin America, and Asia, however, malaria vectors are primarily confined to rural areas. In these regions, malaria is an important urban disease only in peripherally located communities that are similar in some respects to rural sites. In a study of malaria transmission and urbanization in Brazzaville, Congo, human settlement was initially found to favor the spread of An. gambiae, but subsequent increases in population density tended to eliminate larval development sites as open spaces were gradually built up and standing water became too contaminated for Anopheles larvae to survive in (Trape and Zoulani, 1987). Since Anopheles species breed in fairly clean collections of standing water, such as rice paddies, swamps, and lakes, an increase in fecal or other contamination ruins their suitability for larval development. Malaria, therefore, is more of a problem in semiurban than in truly urban areas.
The proportion of people living in urban and semiurban areas is expanding rapidly. In 1950, almost 30 percent of the world's 2.5 billion inhabitants lived in urban areas. By the year 2000, half of the world's projected 6.2 billion people will live in urban settings, many of them in slums and squatter settlements (World Health Organization, 1988). Although most anopheline mosquitoes do not thrive in urban areas, the fact than an increasing share of the world's population will reside in urban or semiurban locales provides impetus for increased malaria control in those areas.
Finally, it should be pointed out that the social and economic factors