3 people have acquired malaria in the US. They’re the first in 20 years.

In late May, Sarasota County, Florida, health officials confirmed they had identified a case of locally transmitted malaria. In mid-June, they confirmed the second. On June 23, Texas joined in: its state health department announced it had confirmed a case of local malaria transmission in Cameron County.

This is all highly unusual. The US hasn’t documented a locally acquired malaria case in 20 years.

Although about 2,000 people infected with malaria turn up in the US health care system every year, those cases are all linked to travel outside the US. Neither those involved in the Florida cases nor the Texas case had traveled. That means in both states, the infection was acquired within US borders.

Experts say the three cases shouldn’t warrant panic about widespread malaria transmission in the US. But it does warrant asking some questions, and being wary of the threat of more local transmission. Mosquitoes can infect multiple people before a full-on outbreak is even identified — so more cases could be out there.

Even if this turns out not to be widespread, it’s a good reminder: Malaria could make a comeback in the US, and we — and our public health infrastructure — ought to be prepared. This is especially true as a changing climate and shifting weather patterns increasingly drive mosquito migration into new places worldwide, allowing malaria to settle in where it hasn’t before.

These cases are not the worst form of the disease — but they’re not benign

Malaria comes in a variety of flavors, all in the genus Plasmodium, and all parasites that infect and kill red blood cells. The anemia that results is the major cause of most of the bodily havoc that follows.

The species that has been identified in both Florida and Texas is P. vivax. It’s not the worst of the malaria species: P. falciparum, the most severe form of malaria, is 10 times more deadly than vivax, according to a study of Americans diagnosed between 1985 and 2011. But vivax is no cakewalk. People with this infection can develop life-threatening brain swelling, lung congestion, and kidney failure.

The infection causes fevers that come and go, along with a wide range of symptoms that can be mistaken for flu, a stomach bug, or liver disease.

Vivax malaria is also sneakier than some of its counterparts: The parasite can hang out dormant in the liver for years after the initial infection, rearing its head long after an exposure. (Worldwide, most vivax malaria infections occur in East Africa, South Asia, and the northern parts of South America.)

Malaria was a huge problem in the US until it was eradicated in the 1950s, largely by spraying the insecticide DDT in homes and environmental areas to kill the mosquitoes that spread it. (All forms of malaria are spread by mosquitoes in the Anopheles genus, and the US still has lots of them.)

Also keeping malaria at bay: The mosquitoes that transmit the disease like to bite at night — and as American homes have increasingly incorporated window screens and air conditioning, the people inside them have been better protected from being food for this particular genus.

So malaria is not something you’d expect to catch in the US anymore. But when things line up just right, all the ingredients are there for malaria transmission to happen in the US.

What does it take for malaria transmission to happen in the US?

One big unanswered question right now is: Why are two geographically distant parts of the US seeing local transmission of malaria right now — especially after so many years without it?

Although both states see many travel-related malaria cases each year, those cases don’t usually lead to local spread. Is something different this year? Or is this just a coincidence?

To think through the possibilities, it’s helpful to understand how malaria spreads.

Malaria gets transmitted when a female Anopheles mosquito bites an infected person, then a week later, bites an uninfected person. In between bites, the mosquito does mosquito business in warm, stagnant water — but it’s fussy about the water it prefers. These malaria-spreading mosquitoes generally like to breed in bodies of water with vegetation growth along the banks, wrote Wade Brennan, a Sarasota County mosquito manager, in an email. In other words, it prefers the forest to puddles of water near human habitation, like the ones you might find in an empty bucket or garbage can outside your house.

To kick off local transmission, a person who’s acquired P. vivax malaria overseas needs to get close enough to an Anopheline mosquito’s habitat to get bitten. For the next few days, the vivax infection brews in the mosquito’s gut. About a week later, it’s ready to infect another person. When the mosquito bites its next victim, particles in its saliva mix with the person’s blood — and before the mosquito lets go, some of those particles get injected into the person.

A few weeks later, that second person gets sick. Meanwhile, the mosquito still has a few weeks to live — and in that time, it’s still feeding on other people, potentially infecting them, too. But even if it dies, its friends might be biting this newly infected person and transmitting the infection onward.

So: The basic elements of malaria transmission are a source (the infected returned traveler), a vector (the mosquito), an uninfected target (the newly identified cases), and an environment that allows them to come in contact. Increasing any of these elements could make local transmission more likely.

A robust public health system helps understand the causes in cases like this and is critical to the response

It’s not yet clear whether any changes in mosquito populations or in human behavior are the reason for this current spate of locally acquired cases. But public health investigations in both states have already identified risk factors for the people who’ve been infected — and environmental reservoirs of the infection.

Michael Drennon, a Sarasota County health department epidemiologist, couldn’t share demographic or location information about the two locally infected people. However, he noted both were adults who spend a significant amount of time outside at night, and they hadn’t traveled anywhere outside the US. A representative of the Texas Department of State Health Services said the case identified in that state was an adult who worked outdoors.

Brennan said his Sarasota County mosquito management team had found malaria-infected mosquitoes in an area swamp and had focused prevention efforts there, applying insecticides that kill both adult and juvenile forms of the mosquito. “We have been able to make sure the mosquito population in that area is extremely low,” he wrote.

It’s unclear whether more Anopheline mosquitoes than usual are circulating in either state. More mosquitoes would increase the chances that an infected person’s parasites could spread to an uninfected person — and would make the disease harder to eradicate.

Overall, the environment of the US is growing more conducive to growing populations of mosquitoes, which may raise the risk of malaria transmission. Climate change is “definitely playing a role in vector-borne disease” broadly throughout the United States, said Estelle Martin, an entomologist at the University of Florida in Gainesville who researches mosquito-borne diseases. Over the last two decades, increasing temperatures and extreme weather have favored mosquito replication — and they also favor the replication of malaria parasites. But it’s not clear exactly what role that dynamic is playing in these three cases.

A Centers for Disease Control and Prevention spokesperson wrote in an email to Vox on Friday, “Though we know in general that climate can be one of many factors that can impact vector-borne diseases, in this situation, there is no compelling reason to think so.” More likely at play, said the agency, were the forces of migration: “Today, global travel and trade allow vector-borne diseases to be moved around the world and transmitted by local mosquitoes or ticks, especially in places where those diseases may have once been common.”

It’s not time to panic, but it is time to protect yourself

Even if several people have been infected and there are some mosquitoes out there with the parasite in their bellies, it doesn’t mean the US is headed for an explosive malaria outbreak. “It’s always worrisome that you have local transmission in an area,” said Martin, but added that there’s no need to panic. And there’s a lot people can do to protect themselves.

It’s important for people to do their best to avoid mosquito bites, said Drennon. He advises people to wear long sleeves and cover their legs — which he acknowledges is challenging in the Florida heat — and to use mosquito repellent.

Eliminating mosquito hangout spots is also very important: Although Anopheline mosquitoes typically don’t congregate near houses, he advises people to drain any standing water and eliminate places where water can accumulate, as other mosquitoes that spread other infections can breed there. “It’s not uncommon to see dengue here,” he said; during the Zika outbreak in 2016-17, the state was also one of two US locations where that virus was transmitted locally.

It’s also important for local health care providers to be aware of the local risk for malaria. Mosquitoes can infect multiple people before the outbreak is even identified. So once one case is found, public health officials need to snap into action, alerting health care workers, educating the public on prevention, and coordinating mosquito seek-and-destroy missions.

Sarasota County’s first case was initially diagnosed while the patient was being evaluated for a fever; blood test abnormalities prompted further testing, said Drennon. Area health care providers were alerted to the case, and the second case was reported not long after.

Even if these outbreaks prove to be a blip, the threat of malaria taking a stronger hold in the US remains. “We know that we have individuals who travel all over the world who can wind up here with malaria,” Drennon said. “And we know that we have the vector species here that can transmit it.” It’s critical for the public “to take precautions to prevent not only malaria but other mosquito-borne diseases that we have in Florida.”

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