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Malaria, a protozoan disease transmitted by the bite of the Anopheles mosquito, is one of mankind's most feared and serious afflictions. It is a leading cause of morbidity and mortality in many tropical areas of the world, especially in Africa. Approximately 55% of the world's population is exposed to the infection, which exerts its toll mainly on the young and the pregnant. Malaria is endemic or sporadic throughout most of the tropics and subtropics below an altitude of 1500 m, excluding the Mediterranean littoral, the United States, and Australia. Malaria is perhaps the most significant disease acquired through international travel to the tropics.
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Five species of the genus Plasmodium infect humans: Plasmodium vivax, Plasmodium ovale, Plasmodium malariae, Plasmodium falciparum, and Plasmodium knowlesi. In 2012, there were an estimated 207 million cases of symptomatic malaria worldwide (80% of which occurred in Africa), with about 627,000 deaths, 90% of which occurred in Africa, and with nearly 77% of those occurring in young children.1 The great majority of malaria deaths are due to P. falciparum infections, although both P. vivax and P. knowlesi can also cause fatal disease.
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The incidence of malaria has decreased dramatically in some countries in recent years as a result of intensive control efforts (e.g., South Africa), but elsewhere there has been little change, and some areas have experienced increases or epidemics (e.g., Algeria, Venezuela). In the United States, more cases of imported malaria were identified in 2011 than in any year since 1971.2 Obstacles to successful reduction of malaria worldwide include human factors (poverty, war, inadequate international cooperation), capacities of the mosquito vector (changing temperatures, insecticide resistance) and parasite characteristics (antimalarial drug resistance).
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In an area of intense transmission, most malaria infections occur in children, who gradually acquire a considerable degree of immunity. In such areas, a high percentage of children may have asymptomatic parasitemia (malaria infection), and illness due to malaria (malaria disease) is commonly mild and only occasionally severe or life threatening. Nevertheless, the absolute burden of mortality is large because nearly all children are infected. Adults in such areas rarely develop severe or fatal disease.
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By contrast, malaria due to P. falciparum is a medical emergency in a nonimmune host of any age, because the infection, if untreated, is likely to progress and to become life threatening. Once a P. falciparum infection has reached the stage of severe disease, there is a 5% to 30% risk of a fatal outcome, even if optimal treatment is then begun. The early clinical features of malaria are nonspecific, and malaria can mimic many other infections. A diagnosis of malaria must be considered in any person returning from the tropics with an unexplained febrile illness and must be considered in any resident in the tropics who develops a fever.
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Malaria transmission occurs in large ...