One of the earliest recorded human diseases, malaria is a widespread and often fatal protozoal disease. It occurs mostly in tropical regions of the world, such as Africa, Central America, and Southeast Asia. Experts estimate there are more than 270 million new malaria infections worldwide each year and an estimated 2 million deaths.
Malaria means "bad air" in Italian, reflecting the pre-1880 view that it was caused by gases from the swampy regions where many cases occur. In 1880, Charles Laveran observed a protozoal parasite in the blood of an afflicted patient. In 1898, Ronald Ross found that the bite of the female Anopheles mosquito transmits the parasites into the bloodstream.
The parasites, once in the bloodstream, travel to the liver and reproduce asexually in cells in the liver. These cells burst and release the parasites back into the bloodstream. The parasites then enter red blood cells, where they reproduce asexually and feed on the hemoglobin in the cells. The red blood cells burst, releasing both asexual and potential sexual forms of the parasite. The sexual forms develop in the gut of Anopheles mosquitoes that have fed on infected blood, thus completing the parasite's life cycle. This cycle causes the symptoms of intermittent fever and chills in malaria victims. Four types of Plasmodium parasitesÑbelonging to the subphylum SporozoaÑcause malaria, and the cycle of each type is completed in a different time period. Symptoms occur about every 24 hours when a victim is infected with P. falciparum, at 48-hour intervals with P. vivax and P. ovale, and at 72-hour intervals with P. malariae. Other malarial symptoms include headache, weakness, and an enlarged spleen.
Various human races have developed genetic defenses against malaria. African and Mediterranean peoples, for instance, have genes for altered hemoglobin, on which the parasite cannot thrive as easily as it does in normal hemoglobin. People having one normal hemoglobin gene and one altered gene lessen their chances of acquiring malaria. Unfortunately, people who inherit two altered genes are subject to either sickle-cell disease or thalassemiaÑparticularly Cooley anemiaÑdepending on the type of gene inherited.
Cure and Prevention
No feasible methods of treatment for malaria were available before the 1630s, when Spanish missionaries discovered an extract from the bark of the South American cinchona tree. This extract, the alkaloid quinine, and a related drug, quinidine, were the only antimalarial drugs until the first half of the 20th century. Current treatment includes the drug chloroquine as the first choice, as well as pyrimethamine and chloroguanide. Parasites have developed resistance to chloroquine, however, although combined use of another drug, desipramine, is helping to overcome this resistance. Fears of similar development of resistance have been expressed concerning the worldwide distribution in the later 1980s of another drug, halofantrine, and may lead to limitations being placed on a further new drug, mefloquine. Researchers are exploring the usefulness of yet another drug, arteether, derived from a chemical called qinghaosu that was isolated by Chinese chemists from the herb Artemisia annua. This herb has been used for centuries in traditional Chinese medicine to treat malaria and fever.
The need for a malaria vaccine is urgent. In 1984, using genetic engineering techniques, researchers made large quantities of malaria antigen available for testing. A method was developed for diagnosing malaria on a large scale, using a genetic probe for P. falciparum. By the mid-1990s, a vaccine that offers partial protection was developed and tested by Colombian researchers. Testing continues in Africa and Asia, as well as research to make the vaccine more effective.
People who have had malaria often suffer relapses, which may occur years after the initial infection. One theory concerning the origin of relapses is that they result from ersistence of parasites in the liver; another theory is that relapses are caused by parasitized red blood cells that circulate in the blood for prolonged periods of time.