MALARIA

infectious parasitic disease that can be either acute or chronic and is frequently recurrent. Malaria is common in Central and South America, the Mediterranean countries, Asia, and many of the Pacific islands. In the United States it was found in the South and less frequently in the northern and western parts of the country.

The primary causative organism, Plasmodium falciparum, requires both the Anopheles mosquito and humans to complete its life cycle: sexual reproduction of the protozoan occurs in the mosquito; an immature form is then transmitted to the human via the bite of the mosquito. In a person the parasite goes to the liver, replicates, and moves into the bloodstream, where it attacks red blood cells for their hemoglobin. Some of the plasmodia become sexually mature and are transmitted back to another biting mosquito. Three other Plasmodium species also infect humans.

Symptoms

At the onset of malaria, bouts of chills (ague) and fever lasting several hours and occurring every three or four days are the usual symptoms. If the disease is not treated, the spleen and the liver become enlarged, anemia develops, and jaundice appears. Death may occur from general debility, anemia, or clogging of the vessels of cerebral tissues by affected red blood cells. Cerebral malaria is most commonly seen in infants, pregnant women, and nonimmune travelers to endemic areas.

Immune Response

P. falciparum creates protein knobs on the surfaces of the red blood cells it attacks. These knobs attach the cell to the lining of the blood vessel, preventing its removal to the spleen for destruction. The parasite slows detection by the immune system by changing the makeup of the knobs periodically, substituting or rearranging its 150 "var" (variability) genes, a strategy unique to malaria. A pattern of remission and relapse results as the immune system learns each new "code" only to have it again changed. Patients with malaria gradually do develop immunity that modifies the course of the disease, but this immunity has a degree of strain specificity.

Treatment and Control

The bark of the cinchona and its product, quinine, have been used in the treatment of malaria for centuries. After World War II, they were largely replaced by the synthetic analog chloroquine. The use of chloroquine, in addition to the use of DDT for mosquito control, was expected to eradicate the disease, but a World Health Organization campaign (1955–69) to eradicate the disease globally (by controlling mosquitoes long enough to allow the human population to become disease free) proved unsuccessful. Despite that, spraying successfully eradicated the disease in some areas (Sardinia, Japan, and Taiwan).

In the 1960s several strains of the malarial parasite developed resistance to chloroquine. This, plus the growing immunity of mosquitoes to insecticides, has caused malaria to become one the of world's leading re-emerging infectious diseases, infecting an estimated half billion people a year and killing up to 2 million. Atovaquone and proguanil (Malarone) are used in areas where the disease has become highly resistant to the chloroquine and other alternative drug treatments. Artemisinin in combination with other drugs is an alternative treatment for resistant strains. Vaccines against malaria are still experimental. Spraying is still used to control malaria-transmitting mosquitoes, but fish that feed on mosquito larva also have been employed.

The Columbia Encyclopedia, Sixth Edition Copyright© 2004, Columbia University Press. Licensed from Lernout & Hauspie Speech Products N.V. All rights reserved.

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