Controlling disease vectors in the local environment

As Linking Environment and Health illustrated, vector-borne diseases such as malaria, dengue fever, yellow fever, and schistosomiasis cause enormous suffering and death throughout the developing world. Vaccines to prevent these diseases, or drugs to treat those diseases that cannot be prevented, hold out promise for future disease control. At this stage, however, vaccines and drugs are lacking for many of the major vector-borne diseases, in part because the disease organisms are formidable adversaries. Perhaps a greater reason is that these diseases are generally not a priority to large pharmaceutical companies, although there are important exceptions (53).

Even when drugs to combat diseases like dengue fever and malaria become available, vector control should remain a priority. Environmental interventions to contain the insects that transmit disease can occur at a range of levels, such as household, community, or region. Improvements in and around the home, for instance, can often make a tremendous difference in controlling vector populations and reducing the incidence of infectious diseases. In areas where Chagas disease remains high, for example, helping low-income families finance home improvements that remove the thatched roofs where the reduviid bugs hide can be a more effective use of public funds than investing in costly pesticide applications. In Brazil and Venezuela, replacing palm-thatched roofs with tiles or corrugated tin and repairing walls and floors were instrumental in reducing human contact with the disease vector and ultimately in reducing Chagas disease (54). These housing improvements also provided the added benefits of better ventilation, which improved indoor air quality at the same time (55).

Household water storage, used to make up for intermittent or inadequate piped water supplies, can also provide reservoirs for disease-bearing mosquitoes and other insects. Giving families fitted lids or screens to place over water containers can be a cheap, interim way to lower the risk of disease by reducing mosquito-breeding sites (56). Septic tanks and latrine pits can also be made mosquito-proof at low cost with polystyrene beads. These beads form a floating layer through which female mosquitoes cannot lay their eggs and mosquito larvae cannot breathe. Experience in Brazil, India, and Tanzania has shown that the layer will remain in place for more than 4 years (57). Similarly, providing nylon gauze with which to filter water drawn from shallow wells or surface water has proven successful in reducing the incidence of guinea worm (58). Making these types of interventions more widely available through primary health-care clinics or local health-care workers can help reduce disease incidence at low cost.

Community programs are an effective, often inexpensive, means of vector control. In such programs, community members take measures to improve the environmental conditions in and around their homes, thereby reducing reservoirs for mosquitoes, flies, and rats. Mass media campaigns can educate resident about the importance of removing garbage and other mosquito habitats from the community – creating a community “police” force against the disease (59). Other approaches tie vector control to income generation. In India, for example, several communities have controlled Anopheles mosquito breeding in shallow coastal lagoons by removing algae from larval breeding places and using it as manure or in papermaking. Other strategies involve introducing fish and shrimp that eat mosquito larvae into pools where mosquitoes might breed; these fish can later be sold (60).

The success of household or community interventions does not imply that governments can abdicate their responsibility in vector control. Clearly, many of these simple interventions would not be necessary if longer-term strategies were implemented, such as providing water and sanitation and improving garbage collection and disposal. Reliable piped water would reduce the need to store water in the first place, with the added benefit of reducing the burden of diarrheal diseases.

Furthermore, coordinated efforts among multinational agencies, national government, and private companies can reduce disease incidence much more effectively, and on a larger scale, than communities can on their own. The Onchocerciasis Control Programme in West Africa, for instance, has dramatically reduced the threat of river blindness. A multinational effort by the Food and Agriculture Organization of the United Nations (FAO), the United Nations Development Programme (UNDP), the World Bank, and WHO has controlled the blackfly by destroying its larvae with insecticides sprayed from the air. The environmental impact of the insecticides is continuously monitored. In addition, the U.S. pharmaceutical manufacturer Merck & Co. has committed to providing ivermectin – a drug that safely and effectively kills the larvae in the body – free of charge as long as river blindness exists. This program protects about 30 million people from river blindness, at an annual cost of less than US$1 per person (61). As part of a WHO-coordinated program to eliminate another tropical disease, lymphatic filariasis or elephantiasis, SmithKline Beecham announced in early 1998 that it would supply free of charge an antiparasitic drug to roughly one fifth of the world’s population (62).

For many other vectors–most notably the mosquito – both community involvement and government investment are needed to successfully reduce the burden of disease. A tremendous and often overlooked opportunity also exists to control disease vectors on a larger scale through careful planning of development projects and land use changes, as will be described later.