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Where are the mosquitoes that spread malaria in the U.S.? Officials aren't sure

The U.S. does not routinely track mosquitoes that spread malaria "because we haven't been worried about them," one expert said. Concerns over the insects, however, are growing.
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A ninth case of malaria diagnosed in a person who had not traveled out of the U.S. has experts on alert — and calling for more surveillance of the mosquitoes that spread the illness.

"The time to think about the next mosquito-borne disease is not when we find a sick person. It's now," said Dan Markowski, technical adviser to the American Mosquito Control Association, a nonprofit organization representing groups that monitor mosquito activity.

While public health officials maintain that Americans' risk of contracting malaria remains quite low, some experts say the country should increase its surveillance of the specific type of insects responsible for malaria spread: Anopheles mosquitoes.

"We have not been tracking them in the United States because we haven’t been worried about them," said Dr. Photini Sinnis, an expert at the Johns Hopkins Malaria Research Institute in Baltimore.

It's been decades since U.S. health authorities have had malaria on their radar. Malaria, a potentially deadly illness that causes flu-like symptoms such as fever, body aches and extreme chills, sickened thousands of Americans in the early part of the 20th century. The disease spreads via a parasite that gets transmitted to a person through a mosquito bite.

Insecticides and elimination of standing water where mosquitoes like to breed wiped out the mosquito-borne illness from the U.S. in the early 1950s.

Since then, the vast majority of cases in the country — more than 2,000 each year — have been among people returning from travel abroad. A case of locally acquired malaria hadn't been reported in two decades until now.

2023 has been different. This year, nine cases of malaria have been detected in people infected on U.S. soil: one in Texas and seven in Florida announced earlier this year, and one case detected in Maryland last Friday. The cases are not believed to be linked state-to-state, and no deaths have occurred.

"We've had these cases in the past because these mosquitoes are here. It's a low probability of events, but they can happen," said Sinnis, also a professor at the Johns Hopkins Bloomberg School of Public Health. "But when they start happening more frequently, you say, 'Maybe things are changing. Maybe this is here to stay.'"

Sinnis said the U.S. needs more data and more time to know for sure. "Next summer will be interesting. The next five summers will be interesting," she said.

Counties across the U.S. routinely trap and study mosquitoes for their ability to transmit disease. Experts usually zero in on Culex mosquitoes, which transmit West Nile virus. That mosquito-borne illness is most concerning to public health officials because it infects thousands of people each year. Other bugs found in traps — moths, flies and other types of mosquitoes — are usually tossed aside because they do not cause widespread illness. 

Anopheles mosquitoes that can carry the parasites responsible for malaria spread are often discarded, said Markowski, of the American Mosquito Control Association, because local health authorities don’t have the funding to study the ones that so rarely cause disease in the U.S.

Still, "we've always had these vectors here," said Dr. Jill Weatherhead, an assistant professor of tropical medicine and infectious diseases at Baylor College of Medicine in Houston. "We need to make sure that those mosquito control programs reporting surveillance are strong enough to prevent ongoing transmission."

Dr. Monica Parise, a medical epidemiologist with the Centers for Disease Control and Prevention, said the agency is exploring the idea of increasing surveillance of Anopheles mosquitoes.

"But if you really want to home in on the risk," Parise said, "it's more efficient to ensure we're not missing any cases" in people. 

Malaria can be deadly, especially when it's caused by a particular species of the parasite, called Plasmodium falciparum. That was the case for the Maryland patient, who was sick enough to be hospitalized. The earlier cases this year were caused by species linked to less severe illness.

Early detection of malaria is key, Sinnis said, for proper treatment.

"It's very important that doctors be suspicious of malaria and test for it so that treatment can be timely. If you have a patient with a fever in the summer in one of the states where we've had transmission," think of ordering a malaria test, Sinnis said.

"We can treat malaria before somebody dies," she said. 

Why now?

No one knows for sure why the U.S. has had an uptick in malaria cases this year, but experts say one factor is a combination of increasingly hot and humid weather and international travel.

"The climate is changing such that it's making mosquitoes and mosquito-borne diseases more common. That's a fact," Markowski said. "When you combine that with increased globalization and traveling, there's not a disease on the planet that's not about a four- to five-hour plane ride away."

Sinnis said it is likely that the cases detected this year occurred when a person was infected in a place where malaria is endemic — sub-Saharan Africa or India, for example — then traveled to the U.S. Once here, a local Anopheles mosquito bit that person, then bit those who became ill.

Weather also plays a role: It takes two weeks for a mosquito to develop the ability to transmit malaria after it takes that first bite, Sinnis said. Mosquitoes usually don't live that long to infect anyone else, but they thrive in heat and humidity, living up to three weeks.

"Humidity is very important for mosquito longevity," she said.

How to protect yourself

Typical insect repellents, such as DEET and lemon eucalyptus oil, work well against Anopheles mosquitoes, experts said. The bugs tend to bite at dusk and dawn.

Those most at risk for becoming sick after a bite from an infected mosquito are those who have never been exposed to malaria — the vast majority of Americans. Still, the risks remain extraordinarily low.

"People should be aware of what's out there and take whatever precautions they feel are appropriate," Markowski said. "With that said, I don't think we should all hit the panic button and just stop sitting out in our patios and having a good time on Friday night."

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