A disease caused by members of the protozoan genus Plasmodium, a widespread group of sporozoans that parasitize the human liver and red blood cells. Four species can infect humans: P. vivax, causing vivax or benign tertian malaria; P. ovale, a very similar form found chiefly in central Africa that causes ovale malaria; P. malariae, which causes malariae or quartan malaria; and P. falciparum, the highly pathogenic causative organism of falciparum or malignant tertian malaria. Malaria is characterized by periodic chills, fever, and sweats, often leading to severe anemia, an enlarged spleen, and other complications that may result in loss of life, especially among infants whose deaths are almost always attributed to falciparum malaria. The infective agents are inoculated into the human bloodstream by the bite of an infected female Anopheles mosquito, more than 60 species of which can carry the infection to humans. The disease is found in all tropical and some temperate regions, but it has been eradicated in North America, Europe, and Russia. Despite control efforts, malaria has probably been the greatest single killer disease throughout human history and continues to be a major infectious disease.Epidemic The vast reproductive capacity of Plasmodium parasites is illustrated by their life cycle, which begins as a series of asexual divisions in human liver and then red blood cells. Transfer of the parasites to the mosquito host depends on the rate of sexual multiplication that begins in the infected human red blood cells and is completed in the mosquito stomach, followed by asexual multiple division of the product of sexual fusion. Clinical malaria usually begins 7–18 days after infection with sporozoites. Red cell infections tend to follow a remarkably synchronous division cycle. The parasite progresses from merozoite to a vegetative phase (trophozoite) to a division stage (schizont), ending with the new generation of merozoites ready to break out in a burst of parasite releases and initiate the chills-fever-sweat phase of the disease. The sequence of chills, fever, and sweats is the result of simultaneous red cell destruction at 48- or 72-h intervals.Sporozoa Chloroquine remains the drug of choice for prevention as well as treatment of vivax, ovale, and malariae malaria. However, most strains of falciparum malaria have become strongly chloroquine-resistant. For prevention of chloroquine-resistant falciparum malaria (and in some areas vivax malaria is now chloroquine-resistant as well) a weekly dose of mefloquine beginning a week before, then during, and for 4 weeks after leaving the endemic area is recommended. Chloroquine-resistant malaria is chiefly treated with the oldest known malaricide, quinine, in the form of quinine sulfate, plus pyrimethamine-sulfadoxine. Drug resistance Failure of earlier efforts to eradicate malaria and the rapid spread of resistant strains of both parasites and their mosquito vectors necessitated renewed interest in prevention of exposure by avoidance of mosquito bites using pyrethrin-treated bednets, coverage of exposed skin during active mosquito periods (usually dawn, dusk, and evening hours), and use of insect-repellent lotions. A balance between epidemiological and immunological approaches to prevention, and the continued development of new drugs for prophylaxis and treatment are recognized as the most effective means to combat one of the most dangerous and widespread threats to humankind from an infectious agent. Medical parasitology |
