Tropical Diseases - Malaria

Malaria, one of the most common diseases in the world, gets its name from an Italian word for “bad air” because of an ancient belief that a mysterious substance in the air was the cause of the ailment. It is now known that the disease is caused by any of at least four parasites carried by Anopheles mosquitoes. According to the World Health Organization (WHO), some 200 million persons are affected by the disease, including one-fourth of the adult population on the continent of Africa. WHO estimates that at least one million children die each year of malaria. The disease was relatively rare in the United States until the 1960s, when hundreds of cases began to appear among military personnel who apparently contracted the disease in southeast Asia but did not develop symptoms until they returned to the United States; the disease later occurred in soldiers who had never left the United States, apparently transmitted by domestic Anopheles mosquitoes that had become infested with the malaria parasites.


The symptoms of malaria differ somewhat among various patients because the four known kinds of plasmodia, or protozoa, that cause the infection do not produce the same specific effects. However, the general symptoms common to all forms of malaria are fever, chills, headache, muscle pains, and, in some cases, skin disorders such as cold sores, hives, or a rash. A malaria attack may begin with a severe chill that lasts from twenty minutes to an hour, followed by a fever lasting from three to eight hours with temperature rising to more than 104° F. The fever usually is accompanied by profuse sweating, and the afflicted person is left exhausted by the cycle of chills and fever. The attacks become more or less successively milder, less frequent, and more irregular, and finally cease, although there may be relapses.

One kind of malarial organism seems to cause attacks that occur every other day, while another type produces attacks that appear quite regularly on every third day; still another type of malaria plasmodium seems to cause a fever that is continuous. While the liver seems to be a favored target organ, other body systems can be involved, with related complications. If the organism reaches the brain, the patient may suffer convulsions, delirium, partial paralysis, or coma. If the organism invades the lungs, there may be coughing symptoms and blood-stained sputum. In some cases, there may be gastrointestinal symptoms with abdominal pain, vomiting, or diarrhea.

Medical examination of malaria patients frequently reveals signs of anemia, an enlarged spleen, liver abnormalities, and edema, or swelling because of fluid accumulation. Blood studies may show the malaria parasites in the blood, damaged red blood cells, and an abnormal white blood cell count. The four species of malaria organism are distinctive enough to be identified in laboratory tests.


Treatment includes administration of antimalarial drugs such as quinine, chloroquine, or primaquine. Newer antimalarial drugs are sometimes used in combinations because of the development of drug-resistant strains of the organism in South America and Asia. There is no vaccine that protects against malaria.


The protozoa that cause malaria are carried by the Anopheles mosquito, but humans are the intermediate host. This means that both infected humans and infected mosquitoes are needed to continue the life cycle of the organism. The disease therefore can be controlled if Anopheles mosquito populations are eradicated and humans are not carrying the protozoa in their blood. When these organisms get into human blood, they invade the red blood cells and multiply until the blood cells rupture to release offspring called daughter cells . When the mosquito bites a human for a blood meal, the daughter cells enter the mosquito stomach, where they complete their life cycle and migrate to the mosquito's salivary gland to be injected into the next human, and so on. It takes from ten days to six weeks following a mosquito bite for the first malaria symptoms to develop, the time differences varying with the species of protozoa involved. The malaria mosquito in recent years has developed resistance to insecticides, and areas of infestation have spread in some countries where irrigation for farming has been expanded.

How Malaria Is Transmitted

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