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Medical Author:

Mary Nettleman, MD, MS, MACP

Mary Nettleman, MD, MS, MACP

Mary D. Nettleman, MD, MS, MACP is the Chair of the Department of Medicine at Michigan State University. She is a graduate of Vanderbilt Medical School, and completed her residency in Internal Medicine and a fellowship in Infectious Diseases at Indiana University.

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Medical Editor:

Charles Patrick Davis, MD, PhD

Charles Patrick Davis, MD, PhD

Dr. Charles "Pat" Davis, MD, PhD, is a board certified Emergency Medicine doctor who currently practices as a consultant and staff member for hospitals. He has a PhD in Microbiology (UT at Austin), and the MD (Univ. Texas Medical Branch, Galveston). He is a Clinical Professor (retired) in the Division of Emergency Medicine, UT Health Science Center at San Antonio, and has been the Chief of Emergency Medicine at UT Medical Branch and at UTHSCSA with over 250 publications.

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Malaria is caused by protozoan of the genus Plasmodium. There are several stages in the life cycle of Plasmodium, including sporozoites, merozoites, and gametocytes. Sporozoites are the form that is injected by the mosquito into humans. Infection begins with a bite from an infected mosquito. After being injected into the human host by the mosquito, the parasite travels into the bloodstream and eventually makes its way to the liver, where the parasite begins to reproduce and develop into merozoites. The merozoites leave the liver and enter red blood cells to reproduce. Soon, new parasites burst out in search of new red blood cells to infect.

Sometimes, the reproducing Plasmodia will create a form known as a gametocyte in the human bloodstream, which is infectious to mosquitoes. If a mosquito takes a blood meal when gametocytes are present, the parasite begins to reproduce in the insect and create sporozite forms that are infectious to people, completing the life cycle.

There are five species of Plasmodium that infect humans:

• P. vivax: Though it's most common in India and Central and South America, it's found worldwide. Infections can sometimes lead to life-threatening rupture of the spleen. This type of malaria can hide in the liver and return later to cause a relapse years after the first infection. Special medications are used to eradicate P. vivax from the liver.
• P. ovale: It's rarely found outside Africa. Symptoms are similar to those of P. vivax. Like P. vivax, P. ovale can hide in the liver for years before bursting out again to cause symptoms.
• P. malariae: It's found worldwide but is less common than the other forms. This form of malaria is hard to diagnose because there are usually very few parasites in the blood. If untreated, the infection can last many years.
• P. falciparum: This is the most life-threatening species of malaria. Although present throughout much of the tropical and subtropical world, it is particularly common in sub-Saharan Africa. P. falciparum is resistant to many of the older drugs used to treat or prevent malaria.
• P. knowlesi: Found in Malaysia, this species can cause high levels of parasites in the blood, leading to organ failure or death.

Clinicians who treat malaria in the United States are sometimes asked, "Is it contagious?" The answer is that malaria is not spread directly from person to person. A few cases have occurred in other countries through blood transfusion, intravenous drug abuse with shared needles, or organ transplant. An infected mother can spread malaria through the placenta to her unborn child.

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See the Entire Malaria Topic Guide »