The five cases of malaria diagnosed in Florida and Texas in recent weeks — the first cases of local transmission in the United States in 20 years — have worried some residents and led state and federal officials to issue public alerts about the cluster of infections.
But scientists and public health officials, while advising people in the affected areas to take precautions, say there is little cause for alarm, for several reasons: The number of cases is very low; there are easy, low-tech steps people can take to protect themselves; and the chances of any one mosquito transmitting malaria are infinitesimal.
“You are more likely to get struck by lightning,” said Philip Stoddard, a professor of biological sciences at Florida International University.
Still, climate change is shifting the landscape, Stoddard and other experts said, with West Nile virus, dengue fever, and other mosquito-borne illnesses creeping northward in the United States. As rising temperatures increase the length of mosquito season and help these insects thrive in previously inhospitable regions, the recent appearance of malaria could be a harbinger for worse disease outbreaks to come.
“Should we set our hair on fire over malaria? No, not with four cases” in Sarasota County, Stoddard said. “Will there be something new? Yeah, probably. The forests of West Africa have lots more mosquito-borne viruses that have not yet been well-studied — which haven’t broken out. They’re gonna break out.” One additional case of local-transmission has been diagnosed in Cameron County, Texas.
In 2018, about 1,800 cases of travel-related malaria were diagnosed in the U.S. — meaning that the cases were diagnosed in people who had been abroad — and seven people died from the illness. While all five malaria patients in Florida and Texas have received treatment and are improving, according to the CDC, the current uptick in cases is unusual because the patients all got the parasite from Anopheles mosquitoes inside the United States.
While malaria can technically be spread by blood transfusion, organ transplantation, or contaminated needles or syringes, it is almost always carried by mosquito vectors, according to Derrick Mathias, an assistant professor of entomology and nematology at the Florida Medical Entomology Laboratory at the University of Florida.
Mathias said that the recent appearance of cases is not necessarily surprising because Florida has 14 species of Anopheles mosquitoes, all of which are potentially capable of passing on malaria. “But for transmission to happen, you have to have three main parts: the host, the competent vector, and the parasite,” Mathias said. “Once you eliminate the parasite, the other two can just live, and the mosquito becomes a pest, and is not transmitting disease.” While more than 2,700 people in the United States died of malaria a century ago in 1923, locally transmitted malaria, for all intents and purposes, has been eliminated since 1951.
A growing problem
The recent cluster of cases does not mean that the disease will necessarily become a significant problem again. Mathias emphasized that malaria is the culmination of a series of chance events, any of which could be targeted to stop transmission.
To pass on malaria, the Anopheles mosquito must bite two separate people, once to get the parasite and the second to transmit it. And that becomes less likely given that the Anopheles mosquitoes in Florida and Texas “are not human specialists,” according to Mathias, meaning that they’ll bite all sorts of animals rather than exclusively humans. “That’s one of the things that’s in our favor.”
Another is the relatively long incubation period because, once the Anopheles mosquito is infected with malaria, the parasite has to mature and reproduce, a process that can take one or two weeks. During this period, the infected mosquito could dry out from lack of moisture, fly into a spiderweb, or die in a hundred other ways.
Climate change makes these chance events more likely to some extent. For example, the maturation timeline is highly temperature-dependent, speeding up when it gets warm and slowing down under cooler temperatures. “In Florida, the nights just don’t cool down in the summer,” Mathias said, “so the parasite is able to replicate a little bit faster.”
And as average temperatures rise across the world, there will be more months in the year when malaria and other mosquito-borne diseases could be a problem, according to Sadie Ryan, co-director of the Florida Climate Institute and an associate professor of medical geography at the University of Florida.
Stoddard noted that, from 2015 to 2016, the U.S. experienced an outbreak of the Zika virus, and in 2020, there were 51 cases of West Nile virus and 111 cases of dengue fever in Florida. “We didn’t need malaria to tell us this is a growing problem,” said Stoddard, who weathered these outbreaks as the mayor of South Miami from 2010 to 2020.
Ryan doesn’t think this problem will be limited to Texas and Florida, however, when disease-carrying mosquitoes move northward as temperatures rise. “My concern is places where we might see the climate becoming more suitable,” said Ryan, “places that aren’t expecting an outbreak — that don’t have regular surveillance in place.” Already, she points out we’re seeing Aedis mosquitoes, which carry Zika, dengue, and chikungunya viruses, in California, and West Nile outbreaks in Arizona, which saw more than 1,700 cases and 127 deaths from the virus in the summer and fall of 2021.
But Ryan also noted that the conditions in Florida have been ripe for malaria transmission for many years. Beyond the heat and humidity of the state, urbanization has increased interactions between humans and mosquitoes, and globalization has allowed malaria to travel in from around the world: 140 million tourists visited Florida in 2022 alone. “It’s not just temperature,” said Ryan. “It’s humans modifying the landscape.”
Early detection is important
For now, Americans probably don’t need to be worried about malaria. In fact, Stoddard noted that because the disease doesn’t spread from person to person like Covid-19 or the flu, the outbreak could simply extinguish itself.
Mathias also thinks that malaria is unlikely to become endemic in Florida because “there’s no animal reservoir here where this parasite can hang out,” he said. “We just have to focus on trying to stamp out these little transmission events.”
Because malaria is highly curable, early detection is important, with prompt diagnosis and treatment preventing progression to more severe disease. But vector-borne disease experts said the ideal situation is to stop malaria transmission from happening in the first place.
“We’re taking every case seriously, and we want to prevent it,” Jae Williams, the press secretary for the Florida Department of Health, said.
He noted that the best way for the public to protect against mosquitoes and mitigate transmission is the “drain and cover” method, where people drain small bodies of water like in bird baths, garbage cans, and buckets — and, in addition, wear long-sleeved shirts and pants. On top of that, Williams recommends that people wear insect repellant containing DEET and avoid leaving windows and doors open out of an abundance of caution. “We don’t expect people to lock up in their homes,” Williams said, “but we just want them to be equipped with resources to prevent, mitigate, and treat this illness.”
Window screens and air conditioners offer the best defense, Stoddard said, and if sitting outside, using two tower fans can also do the trick, especially if they are set to oscillate and have their wind fields cross. “That’s important because it creates a chaotic wind flow,” Stoddard said, “and then mosquitoes have a really hard time flying.” At this time, there’s no need to seek out malaria-prevention medications or try to get a malaria vaccine, Stoddard emphasized.
Mathias said that Floridians should remain more vigilant when going out in the evening because “most of the Anopheles that we have around here are night biters.” Residents in low-income and blue-collar communities may also be more vulnerable because they may be more likely to work outdoors and less likely to have air conditioning units in their homes.
As the country enters the July 4 weekend, these recommendations have become all the more critical. John Elliott, a college student living in Sarasota, said that he is still planning on going out in the evening with friends and watching the Bayfront Fireworks Spectacular, just off Florida’s Gulf Coast. Elliot already wears mosquito spray every day because, “if I’m outside for more than 10-15 minutes, it’s guaranteed that I’ll get swarmed by mosquitoes.” But having learned about the malaria cases in Sarasota County earlier this week, Elliott said that he’ll wear long sleeves and apply “lots of DEET” for the July 4 boat party.