Malaria

I

Introduction

Malaria, disease caused by infection with single-celled parasites of the genus Plasmodium. Anopheles mosquitoes transmit these parasites from one person to another in their bites. Malaria is characterized by periodic bouts of severe chills and high fever. Serious cases of malaria can result in death if left untreated. More than a million people die of the disease each year, most of them in Africa, according to the World Health Organization (WHO).

Malaria was once widespread in North America and other temperate regions. Today, the disease occurs mostly in tropical and subtropical regions, particularly in sub-Saharan Africa and Southeast Asia. The disease is also found in Central and South America, Oceania, and on some Caribbean islands. Public health officials had hoped to wipe out malaria during the 20th century. However, malaria parasites have developed defenses against many antimalarial drugs. This response, known as drug resistance, makes the drugs less effective. In addition, the Anopheles mosquitoes that transmit the disease have become resistant to many insecticides.

Malaria remains a global health problem, and public health efforts today focus on controlling it. In addition, a worldwide effort is under way to develop a vaccine that protects people against the disease. In the meantime, research by the WHO has found that sleeping under bed nets treated with insecticide can greatly reduce deaths from malaria, especially among children.

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.

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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.

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