8 people have been infected with malaria in Florida and Texas. What’s going on?

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 26, after an additional two cases were confirmed, Florida health officials issued a statewide mosquito-borne illness advisory. So far in July, three more cases have been confirmed.

Meanwhile, Texas has reported one case of local malaria transmission: On June 23, its state health department announced the case was detected in Cameron County.

That’s a total of eight cases so far this summer. This is all highly unusual: Until now, the US hadn’t documented a locally acquired malaria case in 20 years. On Monday, the Centers for Disease Control and Prevention (CDC) issued a nationwide advisory to alert health care providers and public health authorities about the possibility of locally acquired malaria in people with fevers of unclear origin.

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 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 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 eliminated 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 and 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.

In an interview on June 23, Michael Drennon, a Sarasota County health department epidemiologist, couldn’t share demographic or location information about the two locally infected people who had been identified at that time. 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.

The Texas case occurred in a southern Texas county that borders Mexico and the person has been identified as a National Guardsman who was working on a border security assignment shortly before his symptoms started. Although vivax malaria is present in parts of Mexico, the infection is not typically found in the northern part of the country that abuts Texas.

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.

Colin Carlson, a global change biologist at Georgetown University’s Center for Global Health Science and Security who has led research on the rapidly expanding reach of malaria-spreading mosquitoes in Africa, tweeted about the cases over the weekend. Climate change may contribute to malaria risk by increasing the size of mosquito populations, he noted — however, “we know very little, climate evidence is weak, and epidemiological impacts would be tricky to guess.”

A CDC 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.”