Rats, fleas, and the bubonic plague firmly established pests in the annals of urban environmental health. Even today, a minor outbreak of plague can quickly make international headlines. (See Box 2.3.)However, for most wealthy urbanites of the developed world, diseases transmitted by pests are no longer a major concern. And in urban areas in the developing world, mosquitos and flies are far more important health threats than rats or fleas.

Neither mosquitos nor flies are particularly urban. Indeed, there is a far greater variety of habitats and species of these insects in rural areas. In addition, the opportunities for controlling such disease vectors are generally greater in urban areas. However, some types of disease-bearing mosquitos and flies have adapted well to particular urban habitats and find themselves relatively free of competition[1].

The Aedes aegypti mosquito breeds in small containers, such as flower vases and water drums, and has been carried from its East African home to every tropical continent, often following the used tire trade. Its Asian relative, Aedes albopictus, is now using the same trick and has recently invaded North and South America, the Mediterranean, and West Africa. Both of these species transmit dengue, a disease of increasing importance in Latin America and Southeast Asia.

In African and Indian cities, malaria is the most prevalent mosquito-borne disease and is often a prime cause of hospital admissions and deaths, particularly among children. In Accra, for instance, malaria accounted for more than 40 percent of reported illnesses at outpatient facilities from 1987 to 1990[2]. In both regions, urbanization has created important new breeding sites. In India, for instance, the Anopheles stephensi mosquito breeds in overhead water storage tanks.

Just as urban mosquito problems are intimately linked to water, urban fly problems are linked to waste. Various families of flies have adapted to the opportunities of urban ecology. The most obvious health risk is that they provide a shortcut on the fecal- oral route, although the extent of their contribution is still not clear. The housefly, along with several other species, is a filth feeder and breeder. Given poor sanitation, some flies are likely to be in contact with human feces and later land on human food, drink, or skin. Piles of garbage increase fly populations. Open food preparation and food storage areas provide opportunities for flies to land on food.

Some diseases are transmitted by mites, face flies, or other pests that thrive in dwellings or neighborhoods[3]. Chagas' disease is carried by triatomine bugs, leishmaniasis by sand flies, schistosomiasis by snails, scabies by mites, and yaws by face flies. Globally, these diseases are less serious urban health problems than those spread by mosquitos or the fecal-oral route diseases transmitted by flies. In many locations, however, they can be critical problems.

Some of the measures used to control insects and other pests indoors create their own environmental health threats. Mosquito coils and other substances burned to repel insects cause air pollution. Indoor spraying with aerosols and pump sprays exposes residents to potentially damaging pesticides. There may well be cases in which the health damage caused by using such measures outweighs their sometimes minimal effect on the spread of pest- borne disease.

References and notes

1. Jo Lines et al., "Trends, Priorities and Policy Directions in the Control of Vector-Borne Diseases in Urban Environments," Health Policy and Planning, Vol. 9, No. 2 (1994), p. 113.

2. George Benneh et al., Environmental Problems and the Urban Household in the Greater Accra Metropolitan Area (GAMA)--Ghana (Stockholm Environment Institute, Stockholm, Sweden, 1993), p. 75.

3. C.J. Schofield et al., "The Role of House Design in Limiting Vector-Borne Disease," in The Poor Die Young: Housing and Health in Third World Cities, Jorge E. Hardoy, Sandy Cairncross, and David Satterthwaite, eds. (Earthscan, London, 1990), p. 190.