Malaria Parasite, Life Cycle

by National Institute of Health

(Page 2 of 5)

Malaria is caused by a one-celled parasite from the genus Plasmodium. than 100 different species of Plasmodium exist, and they produce malaria in many types of animals and birds, as well as in people.

Four species of Plasmodium infect humans. ve appearance under the microscope, and each one produces a somewhat different pattern of symptoms. Two or more species can live in the same area and can infect a single individual at the same time.

Plasmodium falciparum is responsible for most malaria deaths, especially in Africa. The infection can develop suddenly and produce several life-threatening complications. With prompt treatment, however, it is almost always curable.

Plasmodium vivax, the most geographically widespread of the species and the cause of most malaria cases diagnosed in the United States, produces less severe symptoms. Relapses, however, can occur for up to 3 years, and chronic disease is debilitating. P. vivax is now found mostly in the tropics, especially throughout Asia.

Plasmodium malariae infections not only produce typical malaria symptoms but they also can persist in the blood for very long periods, possibly decades, without ever producing symptoms. A person with asymptomatic (no symptoms) P. malariae, however, can infect others, either through blood donation or mosquito bites. P. malariae has been wiped out from temperate climates, but it persists in Africa.

Plasmodium ovale is rare, can cause relapses, and generally occurs in West Africa.

Life Cycle

The human malaria parasite has a complex life cycle that requires both a human host and an insect host. In Anopheles mosquitoes, Plasmodium reproduces sexually (by merging the parasite's sex cells). In people, the parasite reproduces asexually (by cell division), first in liver cells and then, repeatedly, in red blood cells.

When an infected female Anopheles mosquito bites a human, she takes in blood. At the same time, she injects saliva that contains the infectious form of the parasite, the sporozoite, into a person's bloodstream.

The thread-like sporozoite promptly invades a liver cell. There, during the next week or two (depending on the Plasmodium species), each sporozoite develops into a schizont, a structure that contains thousands of tiny rounded merozoites (another stage of the parasite). When the schizont matures, it ruptures and releases the merozoites into the bloodstream.

Alternatively, some P. vivax and P. ovale sporozoites turn into hypnozoites, a form that can remain dormant in the liver for months or years. If they become active again, the hypnozoites cause relapses in infected individuals.

Merozoites released from the liver rapidly invade red blood cells where they fuel their activities by consuming hemoglobin, the oxygen-carrying part of the blood. Within the red blood cell, most merozoites go through another round of asexual reproduction, again forming schizonts filled with yet more merozoites. When the schizont matures, the cell ruptures and merozoites burst out.

The newly released merozoites invade other red blood cells, and the infection continues its cycle until it is brought under control, either by medicine or the body's immune defenses.

The Plasmodium parasite can complete its life cycle through the mosquito because some of the merozoites that penetrate red blood cells do not develop asexually into schizonts. Rather, they change into male and female sexual forms known as gametocytes. These circulate in the person's bloodstream, awaiting the arrival of a blood-seeking female Anopheles.

When she bites an infected person, the female mosquito sucks up gametocytes along with blood. Once in the mosquito's stomach, the gametocytes develop into sperm-like male gametes or large, egg-like female gametes. Fertilization produces an oocyst filled with infectious sporozoites. When the oocyst matures, it ruptures and the thread-like sporozoites migrate, by the thousands, to the mosquito's salivary (saliva-producing) glands. And the cycle starts over again when she bites her next victim.

Spread of Malaria

Many biological and environmental factors shape the character of malaria in a given location. Nearly all the people who live in endemic areas are exposed to infection repeatedly. Those who survive malaria in childhood gradually build up some immunity. They may carry the infection, serving as reservoirs for transmission by mosquitoes without developing severe disease. In other areas, where the infection rate is low, people do not develop immunity because they rarely are exposed to the disease. This makes them more susceptible to the ravages of an epidemic. An epidemic can occur when conditions, such as those discussed below, allow the mosquito population to suddenly increase.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.