THE ENVIRONMENT

Environmental conditions help determine the intensity of malaria transmission. The optimal climate for sporogony, or parasite development in the mosquito, is a temperature of between 20°C and 30°C with humidity in excess of 60 percent. Sporogony ceases at temperatures below 16° C. Temperature also has a significant effect on mosquito development, and consequently affects vector density. Indeed, development from egg to adult may occur in 7 days at 31°C (88°F) but takes about 20 days at 20°C (68°F).

Mosquitoes benefit from rainfall. Even large amounts that flush away mosquito larvae can produce pools of water that serve as future larval development sites. In areas of unstable malaria transmission, such conditions often bring about an explosion of anopheline mosquitoes and frequently contribute to epidemics.

Human modification to the environment also can create larval development sites and “man-made” malaria. For example, massive logging in West Africa has resulted in a proliferation in certain areas of sunlit pools of water, an ideal habitat for An. gambiae. Road building and other types of infrastructure projects, as well as agriculture and irrigation, are among a number of human activities that can spread malaria and other vector-borne diseases. In some regions, human activities can have the opposite effect. For example, deforestation in Thailand has led to the disappearance of malaria in some areas.

EPIDEMIOLOGY

Despite being spread over large regions of the world, malaria is a “focal” disease. That is, because of the complex interactions among the human host, mosquito vector, malaria parasite, and the local environment, malaria affects discrete population groups in different ways. This lack of uniformity is a major reason why it is so difficult to design effective and all-encompassing control strategies.

Geographic Distribution

Malaria is primarily a disease of the tropics, but it is also found in many temperate regions of the world, including parts of the Middle East and Asia. Outbreaks are rare in temperate climates where eradication programs have been successful and in regions where environmental conditions are unfavorable for the anopheline mosquitoes, the malaria parasite, or both (Figure 2-6).

Of the four types of human malaria, that caused by P. vivax is the most widely distributed and the most common variety observed in temperate



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