Malaria Fact Sheet -- The Disease (WHO-DOCS 5)

Malaria is one of the most serious and complex health problems facing humanity in the 20th century. Approximately 300 million of the world's people are infected by the disease and between 1 and 1.5 million people die from it every year. Previously extremely widespread, the disease is now mainly confined to the poorer tropical areas of Africa, Asia and Latin America. The problems of controlling malaria in these countries are aggravated by inadequate health structures and poor socioeconomic conditions. The situation has become even more complex over the last few years with the increase in resistance to the drugs normally used to combat the parasite that causes the disease.

Malaria is caused by protozoan parasites of the genus Plasmodium. Four species of Plasmodium can produce the disease in its various forms - Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale and Plasmodium malaria. P. falciparum is the most widespread and dangerous of the four: untreated it can lead to fatal cerebral malaria.

Parasites are transmitted from one person to another by the female anopheline mosquito. The males do not transmit the disease as they feed only on plant juices. There are some 380 species of anopheline mosquito, but only 60 species are able to transmit the parasite. Like all other mosquitos, the anophelines breed in water, each species having its preferred breeding grounds, feeding patterns and resting place. Sensitivity to insecticides is also highly variable.

The parasites develop in the gut of the mosquito and are passed on in the saliva of an infective mosquito each time it takes a new blood meal. The parasites are then carried by the blood in the victim's liver where they invade the cells and multiply. After 9-16 days they return to the blood and penetrate the red cells, where they multiply again, progressively breaking down the red cells. This induces bouts of fever and anaemia in the infected individual. In cerebral malaria, the infected red cells obstruct the blood vessels in the brain. Other vital organs can also be damaged often leading to the death of the patient.

Malaria is diagnosed by the clinical symptoms and microscopic examination of the blood. It can normally be cured by antimalarial drugs. The symptoms, fever, shivering, pain in the joints and headache, quickly disappear once the parasite is killed. In certain regions, however, the parasites have developed resistance to certain antimalarial drugs, particularly chloroquine. Patients in these areas require treatment with other more expensive drugs. Cases of severe disease including cerebral malaria require hospital care.

In endemic regions, where transmission is high, people are continuously infected so that they gradually develop immunity to the disease. Until they have acquired such immunity, children remain highly vulnerable. Pregnant women are also highly susceptible since the natural defence mechanisms are reduced during pregnancy. A detailed knowledge of the ecology of the local insect vector and the behaviour of people in endemic areas is essential for determining the most effective methods to control the disease.

This page was created by Arnaud Bresson, bresson@hei.unige.ch, under the supervision of Dr. Isabelle Nuttall , MD, DPH, nuttalli@who.ch on August 8, 1995.

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