Experts Quell Worries About Locally Acquired Malaria in the U.S.

Earlier this week, the CDC issued an alert on the first locally acquired malaria cases in the U.S. in 20 years. MedPage Today spoke with experts and physicians to determine how rare the cases are, whether there may be more, and what may have contributed to them.

Within the last 2 months, four locally acquired malaria cases were identified in Florida, and another was detected in Texas, the CDC reported, noting that there is no evidence the cases in the two states are related. In addition, all patients have received treatment and are improving, and the risk of locally acquired malaria remains "extremely low" in the U.S., the agency said.

"It's not the next COVID," Brian Grimberg, PhD, of the Center for Global Health and Diseases at Case Western Reserve University School of Medicine in Cleveland, told MedPage Today.

For one thing, transmission is a lot trickier. Malaria is transmitted through the bite of an infected female mosquito, the CDC said, with rare instances of transmission congenitally from mother to fetus at birth, through blood transfusion or organ transplantation, or through unsafe needle-sharing practices.

Accordingly, Grimberg shared a hypothetical anecdote of how difficult transmission would be among family members within the same home. An infected mosquito would have to come through a screened window, bite one person, fly away for a couple of weeks, and then come back, find, and bite another person.

Malaria is not transmitted by sneezing or shaking hands, he pointed out.

Grimberg and other experts agreed upon a likely scenario of how the locally acquired cases could have occurred -- beginning with an infected traveler from a malaria-endemic area coming into the affected areas.

David Sullivan Jr., MD, of the Johns Hopkins Malaria Research Institute in Baltimore, explained that "more than a couple" of mosquitoes would have likely bitten the infected traveler, with a very small percentage of those mosquitoes surviving for a week or so and going on to bite another person, thereby causing transmission.

In mosquitoes' short life span, they usually take just two blood meals and stay within a limited area spanning roughly a mile, he added.

"It's a fantastic chance that the transmission stops here," Sullivan said of the locally acquired cases in Florida and Texas, while acknowledging that nothing is 100% certain.

He noted that there have been sporadic episodes of locally acquired malaria in the U.S., pointing to cases in Palm Beach County, Florida, in 2003, Loudoun County, Virginia, in 2002, and Suffolk County, New York, in 1999, among others.

"We've seen this before," Sullivan said. "I'm taking a longer view that this is newsworthy, but it's not totally new."

Fortunately, he added, the death rate for the type of cases identified in Florida and Texas is very low. "Usually, it's completely curable," he said.

Indeed, the locally acquired cases have been identified as Plasmodium vivax, or P. vivax, the CDC reported, noting that malaria is caused by any of five species of protozoan parasite. P. vivax infections are among those less likely to cause severe disease.

Symptoms of P. vivax infection can include fevers that occur every other day, as well as low platelet counts, Sullivan said. The initial treatment course is typically chloroquine.

For severe malaria, the CDC stated that intravenous artesunate is the only drug available for treatment in the U.S., and that hospitals "should have a plan for rapidly diagnosing and treating malaria within 24 hours of presentation."

Additionally, if P. vivax remains dormant in the liver, it requires additional treatment to ward off chronic infection and relapsing episodes that may occur after months or years without symptoms, the agency added.

"Malaria can cause people to be quite sick," said Erin Reed, MD, a hospitalist at AdventHealth Central Texas in Killeen. It's important for physicians to be aware of the locally acquired cases, especially if they have a patient experiencing unexplained symptoms, she told MedPage Today.

"It's been a long time since we've seen cases that appear to have been caught locally," she said. It's important to keep in mind that the occurrence is "rare, but to be aware."

"You don't want to miss a case," she added. It's also important to not cause panic over what appears to be low-level spread, she said.

Any potentially contributing factors that may have led to locally acquired cases in the U.S. remain unclear and speculative, experts said, acknowledging that COVID affected many aspects of life worldwide.

"If anything," Sullivan said, "there was negligible travel to malarious areas amongst U.S. citizens for a couple of years."

Grimberg also pointed to the fact that public health efforts needed to be diverted to the pandemic, which may have had an effect on other prevention efforts, such as indoor spraying to reduce the number of mosquitoes.

As for climate change, Grimberg said that it would be "difficult to say whether it's responsible" for the locally acquired cases in any way. "We won't find out until next year if the mosquito population was bigger this year."

As the world warms, there may be an increase in the areas in which mosquitoes are found, he noted. However, the "evidence isn't there yet."

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