JLE

Cahiers d'études et de recherches francophones / Santé

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Typology of malaria in Africa Volume 3, issue 4, Juillet-Août 1993

Authors
ORS-TOM, 213, rue La Fayette, 75010 Paris, France, DLMT/OMS, 1211 Genève 27, Suisse, Institut de médecine tropicale, Anvers, Belgique, OMS-Bureau régional pour l’Afrique, Brazzaville, Congo, Les Ulis, Yvelines, France.
  • Page(s) : 220-38
  • Published in: 1993

The highly variable outcomes of malaria campaigns have highlighted the disease’s epidemiological diversity and the need to tailor intervention strategies to various situations. This variability of outcome is due in part to differences in the species of parasite and responses of populations to infection. Its major cause, however, is the different forms of transmission, which provoke morbidity and defence reactions, depending on the circumstances. Transmission depends on the presence of more-or-less competent vectors, and their abundance and seasonal distribution; these in turn depend to varying degrees on the environment. At intercontinental level, biogeography governs the distribution of Anopheles mosquitoes. An. gambiae s.s., An. arabiensis and An. funestus are ubiquitous in intertropical Africa, and often occur sympatrically. This explains why Africa is a continuous focus of P. falciparum; Africa accounts for just 8% of the world’s population, but 85% of all cases of malaria. At the regional level, malaria in intertropical Africa has several epidemiological patterns: - in equatorial and tropical areas, the entire population is affected, but malaria is stable and immunity develops in the early years of life. However, high infant-adolescent mortality is the price paid for the relative immunity of surviving adults; - in sahalian areas, malaria shows intermediate stability; - in sahalo-saharan, austral and mountain areas, malaria is unstable: its irregular transmission precludes the development of immunity, consequently, epidemics affecting people of all ages can spread in years when environmental conditions (wet and/or hot) favour mosquito proliferation. Four of these forms of transmission occur in Madagascar, while the other periafrican islands fall into one of these categories. At the local level, the above patterns can be modified by local conditions such as water courses, relief and soil type. Concerning man and the environment, human activity modifies vegetation cover (especially deforestation) and water systems (drilling, dams, irrigation), and is accompanied by urbanization. Because malaria transmission is lower in urban than in rural areas, town-dwellers develop little immunity; if infected they often develop serious malaria. Malaria can be affected by events such as natural disasters, climate changes, migration of workers and political refugees, and insecticide campaigns. The increase in rapid transport risks dispersing parasites and vectors throughout the world. The impact of events is dependent on the areas where they occur. The introduction of irrigated rice fields triggered a malaria epidemic in Burundi (which lies in a mountain area where the disease is unstable), whereas it had little effect in Burkina Faso (a tropical area, where malaria is stable). The strata of diversification observed in Africa also occur on other continents.