U.S. malaria cases increase with international travel

In 2011, 1,925 malaria cases were reported in the U.S., according to data published in the Morbidity and Mortality Weekly Report released Nov. 1 by the Centers for Disease Control and Prevention (CDC). This number is the highest seen since 1971 and represents a 48 percent increase from five years ago in 2008.

The majority of malaria infections in the U.S. occur among people who have traveled to regions with malaria endemic. Of the 1,925 malaria cases in 2011, only five of the cases were not classified as imported.

Malaria is a disease caused by a parasite transmitted through the saliva of infected female Anopheles mosquitoes. The initial symptoms of this disease are often compared with those of a severe case of the flu, so people will often experience fever, chills, headaches and nausea.

In 2011, five people died in the U.S. from malaria or associated complications, but malaria is a curable disease if diagnosed and treated promptly with the proper medications.

"The root of transmission is really from a human to a mosquito and from a mosquito to a human," said Kristin Michel, an associate professor of biology at Kansas State University who studies the Anopheles gambiae s.s. mosquitoes, the species most responsible for malaria transmission in Africa.

A mosquito can become infected when they take a blood meal on an infected human and transfer that disease to another human. The mosquito acts as a vector, but different from humans, the mosquito vector does not suffer from the presence of the parasite.

Globally the number of malaria cases in endemic regions is decreasing, but international travel appears to be growing steadily and the use of appropriate prevention measures by travelers is still inadequate.

Michel said that because virtually all of the malaria cases detected in the U.S. are acquired outside of the U.S., international travelers are increasing their risk of acquiring malaria when they visit malaria endemic areas. This elevated risk also includes U.S. military personnel stationed in malaria endemic areas. Anyone traveling internationally needs to be more cautious about where and when they travel.

More than two-thirds of the 2011 cases were imported from Africa and the cases show seasonal peaks in January and August, which are likely correlated with peak travel times to African destinations related to winter and summer holidays.

"The CDC provides actual maps and very good recommendations for travelers," said Michel. "Anybody who travels outside of the U.S. into potentially endemic area needs to consult the CDC website and/or their physician and ensure that they have the right prophylaxis."

Michel says that to prevent malaria transmission, travelers can use a bed net at night when they sleep, try to avoid going out in the evenings when mosquitoes are most likely to bite and use repellents or insecticide on their clothing, but chemical prophylaxis is still by far the most effective prevention tool.

Chemical prophylaxis involves taking an antimalarial medication before and during traveling to an endemic area to build up a significant level of drug in the human body the entire time of travel she said. If infected mosquitos bite travelers, the parasite is killed immediately before it is able to infect their system.

The number of people who take the necessary measures to protect themselves from malaria are increasing but still less than half. This is why researchers, like Michel are looking for new ways to control and eventually eradicate malaria. With funding from the National Institutes of Health and K-State Agricultural Experiment Station, her lab is currently working directly with the mosquitoes that transmit malaria.

"Malaria control is really 90 percent or more mosquito control," said Michel. "What we are trying to figure out is what factors are present in the mosquito that either enable or hinder parasite development. So, specifically we work on the immune system of the mosquitos, because like we do, the mosquito has an immune reaction against parasites when it is infected. And what happens is a mosquito kills a lot of the parasites but not all of them, so the ones that make it through can get transmitted."

Through research, her lab is trying to understand why some parasites can get through and if it is possible to prevent this by chemical means. If found, this information could help develop a new way to fight malaria.