II.

The Parasites That Cause Malaria

Four species of Plasmodium parasites cause malaria in humans: Plasmodium falciparum, P. vivax, P. ovale, and P. malariae. Each causes a different form of the disease. P. vivax and P. ovale cause the mildest forms; P. falciparum, the severest and most deadly form. Other Plasmodium species infect primates, rodents, birds, and lizards. Several of these species, particularly those that infect rodents, have been used in experimental studies and for testing malaria drugs and vaccines.

To understand malaria, it is necessary to understand the life cycle of the parasite that causes it. Plasmodium parasites undergo many stages of development, reproducing inside mosquitoes and also in the liver and red blood cells of human beings. The multiplication of the parasites in human blood cells produces the recurrent attacks of fever and chills that are symptoms of the disease.

A.

Life Cycle in Humans

Malaria infection begins when a female mosquito of the Anopheles genus bites a human and injects infectious cells known as sporozoites into the person’s bloodstream. The sporozoites travel in the bloodstream to the liver, where they multiply for one to two weeks to form cells called merozoites. The merozoites leave the liver to invade red blood cells. Inside these blood cells, the merozoites multiply rapidly until the blood cells burst. As the cells burst, they release merozoites that go on to infect other red blood cells and begin the cycle again. Some merozoites divide to form gametocytes, immature male and female gametes (cells involved in sexual reproduction). The gametes are involved in transmission of the disease.

B.

Life Cycle in Mosquitoes

When a mosquito bites a human who has malaria, it takes in blood containing Plasmodium gametes. Inside the mosquito’s stomach, the gametes quickly mature. The male and female gametes unite to produce a zygote, which multiplies to create thousands of sporozoites. The sporozoites migrate to the mosquito’s salivary glands. When the mosquito bites another human, the life cycle of the malaria parasite begins again in the human body.