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Geographic Distribution of Malaria

Up with your antennae nurses! When parasites like Plasmodium vivax pop up in the suburbs, you need to read the signals. This organism, the culprit in mosquito-transmitted malaria, showed up in patients in Florida last summer and in Virginia in 2002. Such locally acquired cases serve as a reminder that insectborne infections can appear in unexpected places. What’s more, locally acquired cases of malaria have appeared virtually every summer in the U.S. for the past decade, and almost half the people in the world are at risk for the disease.^1,2

Although a few cases of malaria in the United States don’t merit panic, they do remind us that malaria is mobile. With 300 million to 500 million cases diagnosed worldwide every year, the 1,200 that occur on average in the United States every year are mostly in immigrants and travelers from malaria-risk areas.³ However, since 1997, mosquito-transmitted outbreaks of malaria not attributed to travel or other risk factors, such as blood transfusion, organ transplantation, or needle sharing, have been reported in Florida, Virginia, Georgia, and New York.³ There also have been cases of airport malaria in unexpected areas around the world in which mosquitoes arrive via airplane and infect inhabitants on arrival.³

Virginia cases

According to Holly Williams, RN, MN, PhD, an anthropologist / epidemiologist for the Malaria Epidemiology Branch of the Centers for Disease Control and Prevention, the cases in Virginia were probably not caused by an immigrating insect. They occurred too far from the airport. “The flying range [for mosquitoes] is about two to three kilometers from their breeding sites,” she explains. “They found habitats with larva in the area,” Williams notes of the investigation that ensued after the Virginia outbreak was discovered.

On a hot August night in the summer of 2002, three teens happened to be within a block of one another when they were infected. Although it is not known if one mosquito bit all three people, it is possible, according to Williams. “After a mosquito bites, her eggs develop over the course of several days. She’ll lay her eggs, then go off in search of blood once more. However, feeding can be interrupted, causing her to bite a second or third person in a given night,” she explains.

As for how the local mosquito picked up the Plasmodium in the first place, it is highly plausible that it took blood from someone in the vicinity who had acquired malaria overseas. “PCR [polymerase chain reaction] of the three parasites indicates that the parasite most likely came from the New World, the Americas, with a high probability that all three came from the same source,” Williams notes.

The fact that two cases were identified during the summer of 2002, and another much later in March 2003 in Virginia, adds to the intrigue but is not particularly mysterious.² Symptoms of malaria can begin 10 days to four weeks after infection, but can be delayed for a year. Plasmodium vivax, the parasite in this case, can relapse or rest in the liver for four years after a person is bitten by an infectious mosquito.⁴ Although rare, a relapse, or recrudescent malaria, can occur up to 30 years after the initial infection.⁴

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Reproduced with permission from JAMA (2004; 291:2664).
Copyright 2004, American Medical Association.

Health threat

Malaria occurs mostly in poor tropical and subtropical parts of the world, mainly affecting areas in Asia and Africa south of the Sahara Desert, where an estimated 90% of deaths due to malaria occur.5 Financial constraints in these impoverished areas impede controlling this preventable infection, and the incidence of disease and death appears to be getting worse.

In countries where malaria is not endemic, such as the U.S., malaria also poses a health threat because most people living in nonendemic areas have no protective immunity. When they get malaria they can develop a rapidly severe, even fatal, disease.

To add to the problem, health care providers are unfamiliar with malaria, and this can cause delayed or incorrect diagnosis and treatment. Under certain conditions, people infected with malaria can transmit parasites to local mosquitoes. They can, in turn, infect local residents. Left unchecked, this course of events can reintroduce malaria in a previously malaria-free area such as the United States.