What could cause a malaria comeback in the US — and what could stop it

Over the last few months, nine people in the US (seven in Florida, one in Texas, and, in late August, one in Maryland) have acquired malaria within the country’s borders. That’s pretty uncommon — at least, in this century; until the 1950s, malaria was a persistent plague in the US, especially in the Southeast.

Many of the conditions that favor malaria’s spread haven’t changed much since then. The Anopheles mosquitoes that spread malaria still thrive in many parts of the country, and states that receive high numbers of travelers from countries where malaria is endemic still have warm, wet weather that favors mosquito reproduction.

Nevertheless, it’s extraordinarily rare for American mosquitoes to be infected with malaria. Since the turn of the last century, there have been only about a dozen cases of local malaria transmission in the US. But the disease remains a major force of destruction elsewhere in the world: In 85 countries across Africa and parts of Asia and South America, malaria caused 240 million illnesses and 627,000 deaths in 2020 alone.

The last spate of local malaria transmission in the US took place 20 years ago. Now circumstances are different: These cases are happening amid rising rates of other insect-borne infections nationwide, and smack in the middle of a heat and wildfire wave that together make climate change’s health risks undeniable. It’s reasonable to wonder whether the US is at risk for becoming a malaria hot spot again.

“Something would have to go seriously wrong for malaria to become endemic in the United States,” said 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.

It’s perhaps the understatement of the year to say the nation is not immune to “things going seriously wrong.” Recent history, ahem, has shown that the country’s public health infrastructure, which Americans rely on to catch and contain invasive infectious diseases, is far more fragile than many realized.

But how vulnerable is the nation, really, to a malaria comeback?

Here’s what the US has going for it — and against it — when it comes to future malaria risk.

The US has some things in its favor in the fight against a malaria resurgence

One key factor the US has going for it is that it’s already eliminated malaria. “Our centralized focus” on getting rid of both malaria parasites and their mosquito hosts’ breeding grounds “really kicked it in the butt,” said Kyndall Dye-Baumuller, a post-doctoral student in vector-borne disease epidemiology at the University of South Carolina’s public health school.

Counterintuitively, it’s also helpful that when people in the US get exposed to malaria, they tend to get sick. When infections are obvious, outbreaks of a disease are easier to spot, treat, and contain.

Conversely, in countries where malaria is more common, there’s more asymptomatic infection due to more immunity to the disease, which makes outbreaks harder to spot. “We don’t have immunity like that as a population,” said Kelly Searle, a malaria epidemiologist at the University of Minnesota School of Public Health. So when a handful of cases pop up, we see them.

Containing a handful of malaria cases — and eliminating a handful of malaria-infected mosquitoes — is much easier than battling back an infection that’s already entrenched.

That’s also made easier by the fact that most malaria only causes disease in humans and not in any other animal, said Dye-Baumuller. She compared the infection with West Nile virus, another infection spread by mosquitoes that leads to illness in humans and in a variety of wild birds. Because it’s so hard to contain this virus in the bird population, there’s a persistent reservoir of West Nile virus in many parts of the US — and the persistent risk of some crossover to humans. That’s not a risk with malaria.

The US health care and public health systems are plagued with problems that don’t affect other developed nations. But compared with poorer nations, these US systems have more capacity to mobilize against malaria transmission when a case occurs, said Carlson. That’s particularly important now because in the case of malaria, “you want to sort of take people off the grid before onward transmission happens,” he said. “And we’re capable of doing that here.”

The US also has good (if uneven) capacity for controlling mosquito populations — a key element of reducing human risk for infections they transmit.

Sadie Ryan, a medical geographer at the University of Florida who studies the ecology of emerging pathogens, remembers what happened when, as a graduate student, she returned to her northern California home from a trip to Ghana with a malaria infection. The local health department “started hanging traps in my trees in my yard where I rented at the time,” she said. The goal was to ensure no mosquitoes near her home had gotten infected, enabling them to spread the disease to others.

Mosquito control — which experts call “vector control” — is also extremely robust in Ryan’s new home of Florida, a state that has been an entry point for more than one invasive mosquito-borne disease (including the last outbreak of locally spread malaria, in 2003). “We have fairly effective vector control response in places we’re expecting the vectors to be,” she said.

The Anopheles mosquitoes that spread malaria are nighttime biters, so the broad prevalence of window screens and air conditioning in the US offer an additional measure of security against the broad spread of malaria. “Here, mosquitoes mostly stay outside the home,” said Carlson. So even if we deal with a major outbreak, “is it going to be something that every single household is worrying about? Probably not.”

But climate change, travel, and social vulnerability create some undeniable risk

Climate change is one of the key factors that’s making the US increasingly vulnerable to malaria transmission, in part by making more of the country warmer and thus more hospitable to the malaria parasite and its Anopheles mosquito vector.

This summer in particular, the warming climate has also driven record levels of rain and flooding in the US. When high rainfall happens near sea level, the result is a lot of water close to the ground’s surface — perfect conditions for mosquito breeding, said Searle.

Rising air temperatures could mean an expansion of the malaria risk range well beyond the Southeastern US and into other parts of the US, said Dye-Baumuller. In a recent analysis she led, 32 states had Anopheles mosquitoes capable of spreading malaria.

There is such a thing as weather that’s too hot for most malaria transmission — “A mosquito is not a mosquito is not a mosquito,” said Carlson. When an area’s temperatures don’t dip below 80 degrees F, American Anopheles mosquitoes don’t fly or reproduce as well, and the malaria parasite itself doesn’t thrive.

But that actually means a new, invasive mosquito species could be particularly dangerous in the US: Anopheles stephensi, a type of mosquito that until recently lived only in South Asia and the Middle East, has been on the move. Unlike the American Anopheles species, this pest thrives in hotter temperatures. Also unlike the American species — which prefer to make their families in forested swamps — it loves reproducing in the cleaner water that gathers near human habitation, especially in urban environments.

All of that means the extreme heat that would normally reduce one malaria threat now sets the stage for another, even worse version, should this particular mosquito get entrenched in the US, said Ryan.

“Being prepared for something like that is really essential,” she said — but not all states are prepared. A 2020 report by the National Association of County and City Health Officials judged that only 24 percent of mosquito programs nationwide were capable of seeking out and destroying dangerous mosquitoes in the event of elevated outbreak risk.

“There are large-scale vector control districts in many other states than Florida and Texas,” said Ryan. “But they’re not necessarily anticipating the specific vectors that may show up as climate shifts them around, or as travel moves them around.”

That’s partly because there’s no central source for the data that would help authorities anticipate changes. Although local public health authorities and academic institutions routinely trap mosquitos to help them estimate the insects’ numbers in various parts of the US, there’s no one resource that gathers those estimates in one place and in real time. That data vacuum makes it challenging to understand how exactly heat and precipitation impact mosquito populations, said Searle — which in turn makes it harder to predict and respond to big explosions in mosquito numbers.

Climate change is also increasing US malaria risk in another way: by increasing infection rates in other parts of the world. While widespread prevention and treatment initiatives have greatly reduced malaria in many endemic countries over the past two decades, a lot of that progress has been undone in some areas of political instability — for example, along the Colombia-Venezuela border, where rising malaria prevalence has raised the specter of spread throughout South America, especially as that continent’s temperatures rise.

In the wake of the Covid-19 pandemic, it shouldn’t be news to anyone that infectious diseases outside the US pose a risk inside the US. The same is true for malaria.

Anyone who enters the US with malaria risks serving as a source of infection to local mosquitoes, and eventually to other people. Most of the malaria cases identified in the US are among Americans returning from foreign travel. That’s largely preventable: While medicines are available to prevent malaria infection during travel, only a quarter of travelers reported taking so-called malaria prophylaxis in 2018.

A variety of persistent social vulnerabilities in the US also help create an environment that favors malaria transmission. Because malaria-spreading mosquitoes bite mostly at night, people who sleep outdoors are at higher risk for infection in the event the disease is introduced — and homelessness is on the rise in the US.

Fundamentally, there’s still enough poverty and poor sanitation in the US to sustain many diseases that should long since have been eliminated, says Carlson. He points at hookworm, a disease transmitted when people walk barefoot on soil contaminated with infected feces, that causes anemia in millions of children and adults worldwide. The parasite’s persistence was recently identified in the poorest part of the Southeastern US — to him, proof that there’s enough neglect and vulnerability in the country to allow almost anything to re-entrench.

To Carlson, hookworm’s persistence sends a warning about malaria risk in the United States. “It will be shocking and sort of impossible to reconcile with how we think of our country if it happens,” he said.

“And also, things happen.”

Update, August 30, 2 pm ET: This story was originally published on July 4 and has been updated to include newly identified cases and additional information on the lack of US population malaria immunity and mosquito surveillance.