Malaria, Geography of

Malaria is an infectious, vector-borne disease caused by the plasmodium parasite, which is spread from person to person by the anopheles mosquito. Acute symptoms of the disease include high fever, chills, headache, and fatigue, with common complications such as coma, anemia, and liver damage. Historically, malaria was widespread globally, but today it is confined mainly to the tropics, where it continues to exact a high toll on human health and development, with approximately 1 million deaths annually, countless infections and complications, and associated social and economic burdens. The vast majority of malarial infections and deaths occur in sub-Saharan Africa, as well as in tropical South and Central America, and South and Southeast Asia. The geography of malaria can be understood as the interaction between three interrelated geographies: (1) that of the anopheles mosquito, the vector for the disease; (2) that of the plasmodium parasite, the direct cause of malaria; (3) and that of human resistance to malaria, whether innate, acquired, or developed through culture and technology.

Since anopheles mosquitoes are the only means of transmitting the parasite from one person to another, their distribution has been the key factor shaping the geography of malaria. While there are several hundred species of anopheles mosquitoes, only about 40 are considered efficient vectors for malaria due to their ability to withstand the parasite and their anthropophilic feeding characteristics (i.e., a tendency to favor human over animal blood). Thus, in some locales where climatic conditions would seem to favor the presence of malaria, the absence of an efficient vector has left them malaria free. Nevertheless, malaria vectors are found in a wide array of landscapes, from tropical forests and savannas, to coastal marshes, to agricultural areas and cities. The most dangerous malaria vector, Anopheles gambiae, endemic to central Africa, sustains the world's highest rates of malarial infection and mortality.

Malaria parasites spend the sexual phase of their life cycle inside the anopheles mosquito. Insofar as the mosquito is cold-blooded, [Page 1817]the survival and reproduction of the parasite are sensitive to ambient temperature. Different species of the parasite are better equipped to handle low temperatures. Plasmodium falciparum, the deadliest form of the parasite, is less resistant to cold and as a result has always been most prevalent in the tropics. Another species, Plasmodium vivax, is capable of withstanding cooler and highly seasonal climates, which historically has allowed for a broader geographical reach into temperate zones such as the Southern United States, Southern England, Italy, and northwestern Argentina.

Finally, differences in human resistance to malarial infection help explain the heterogeneous geography of the disease. Genetic factors offer complete or partial resistance to infection for some ethnic groups. Most famously, the sickle-cell trait, found in about 20% of people in sub-Saharan Africa, reduces the severity of malarial symptoms. Many ethnic groups, including most Africans, have genetic immunity to P. vivax infection. In addition, partial yet impermanent resistance to malaria can also be acquired through exposure to infection. In areas where the prevalence of malaria is epidemiologically “stable,” such as sub-Saharan Africa, environmental conditions are optimal for year-round transmission of the parasite, but rates of immunity after childhood are very high. As a result, in such “endemic” regions, children suffer the vast majority of deaths from endemic malaria.

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