A Rare Domestic Resurgence of Malaria Is Circulating in the US

The mosquito-borne disease was eliminated here long ago. Now “revenge travel,” global migration, poor public funding—and maybe climate change—could help it come back.

AT LEAST FOUR people in Florida and one in Texas have been diagnosed with malaria that they must have caught near where they live—because, according to health officials, none of them traveled outside the US or their own states. The very unusual discovery has left infectious disease specialists wondering: Who else might be ill, and will local doctors recognize what’s wrong?

Malaria isn’t completely unprecedented in the US: About 2,000 residents contract it every year, but almost always because they traveled to a place where it’s endemic, were bitten by an infected mosquito there, and fell ill once they came home. Locally acquired malaria is extremely rare. It arises from a chain of transmission that probably starts with a US mosquito biting a tourist, migrant, or refugee who has been in an endemic country and is carrying the infection in their blood. Then the insect passes it along by biting someone else. That hasn’t occurred in the US since 2003.

There are roughly 247 million cases of malaria in the world each year, according to the World Health Organization, and in every one a mosquito is only the vector. Humans are the disease’s natural host; mosquitos transport it between people. (Unlike with Covid and some other diseases, animals don’t play a role.) So when experts learn someone has been diagnosed, and can be confident that person hasn’t traveled, their first impulse is to ask, who else was infected but has not been found?

“If there are five cases right now, that means there's got to be a lot more mosquitoes out there that are infected,” says Ross Boyce, a physician and assistant professor at the University of North Carolina at Chapel Hill School of Medicine, who runs a malaria research program in Uganda. “And there may even be more people that are infected than we know about at this point.”

The five people identified by health authorities, in Sarasota County on Florida’s west coast and Cameron County at the southernmost tip of Texas, have already received treatment and are recovering, according to the Centers for Disease Control and Prevention. In both areas, mosquitoes have been trapped and analyzed, and mosquito-control districts are spraying insecticide to knock down local populations.

Those are familiar actions in parts of the US, because they were key to defending against the epidemic of Zika that swept through North and South America in 2016 and landed in Brownsville, Texas, within Cameron County, late that year. Florida has been combating outbreaks of dengue, another mosquito-borne virus, for more than a decade, after it appeared in Key West and then moved north to Miami.

But malaria hasn’t been persistently present in the US since it was eliminated here in 1951, in a campaign that gave rise to the CDC. (The agency was originally called the Office of Malaria Control in War Areas, tasked with reducing the disease’s impact on soldiers headed to World War II and bases producing tanks and ships.) Thus malaria experts worry that doctors, especially primary care physicians, may not know it when they see it.

“Malaria often has very nondescript symptoms—you have fever, or feel like you have the flu,” says Amy Bei, an epidemiologist and assistant professor at the Yale School of Public Health, who spoke from a research collaboration site in Senegal. “And not all doctors are going to have that as their first thought. When you start to have local transmission, it’s an important thing for clinicians to be thinking about.”

If there’s any good news in these recent discoveries, it is in the species of parasite causing them. All five types of malaria parasites fall within the genus Plasmodium, and all of them destroy red blood cells. But the species in these new cases is P. vivax, which is more widely distributed geographically, but less frequently lethal, than the dominant form P. falciparum. Vivax still makes people gravely ill, though, and it possesses an evolutionary trick that falciparum does not. After the initial infection, it can lie dormant in the liver, causing no symptoms and not registering on blood tests until it reactivates—so a person who thinks they have recovered may not know they remain a danger to others.

The risks posed by dengue and Zika, and also West Nile virus, have caused Americans to start thinking about the impact of climate change on diseases spread by mosquitoes. The species that spread those viruses—Aedes aegypti for dengue and Zika, and several Culex species for West Nile—seem to be expanding their ranges, and the severe storms and warm nights caused by climate change may make areas friendlier to mosquitoes than they had been.

But malaria is transmitted by species in a different genus, Anopheles, and those are already present in much of the US. Climate change has expanded the regions where Anopheles circulate in Africa, and some modeling studies suggest that climate change will intensify risks in multiple countries where mosquitoes are already present, for instance by encouraging larger populations of insects or expanding the number of months in which mosquitoes survive.

In the US, however, where these new cases occurred are the same places where malaria would have infected people before it was locally eliminated; they already have Anopheles mosquitoes, and their weather is already warm enough to sustain them. So climate change may not have made these people more vulnerable than before.

That makes it even more important that municipalities in risky areas deploy robust mosquito control measures. It’s a big ask. Mosquito abatement is locally controlled in the US, and it’s very patchy: Some Florida cities can field the equivalent of a small Air Force of sprayer planes, but elsewhere in the South, funds are thin. “I hope this is a further wake-up call that there needs to be more investment in vector surveillance and vector control,” Boyce says. “So you spot it in the mosquitoes before you have a human case.”

In the end, keeping malaria from spreading in the US is simple but not easy. Simple, because it requires only that people not bring the infection into the country. But not easy, because any malaria prevention plan must take into account the enormous numbers of people entering the US from endemic zones, whether as tourists, economic migrants, or asylum seekers and refugees. Human movement spreads malaria and has done so since prehistory. That movement is unlikely to stop, but the risk of infection can be reduced.

Travelers can prevent malaria infection by taking a daily or weekly dose of drugs that kill the parasite as it circulates in the blood or reproduces in the liver. Almost all antimalarials have some side effects, however, from mild ones such as stomach upset and headaches to vivid dreams and flashes of psychosis, and so people often forgo them. With post-Covid “revenge travel” still booming, more Americans than ever are venturing to tropical endemic zones—and if they don’t protect themselves, they may bring malaria home as an unintended souvenir.

“We already have 2,000 cases a year, and a lot of those are because people did not take precautions,” says Johanna Daily, a longtime malaria researcher who is a professor at Albert Einstein College of Medicine and a physician at Montefiore Health System. “If people are traveling to endemic areas, please go to a travel clinic, please take antimalarials. It could prevent malaria here in the US.”

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