Four things to know about malaria cases in the United States

The risk of local transmission in the United States remains extremely low

A close up photo of a mosquito resting on a person's finger.

Anopheles freeborni (pictured) is one of many mosquitoes in the United States capable of transmitting malaria from person to person.

James Gathany/CDC

For the first time in 20 years, five people have picked up malaria on U.S. soil.

On June 26, the U.S. Centers for Disease Control and Prevention issued a health advisory, announcing that over the last two months four people in Sarasota County, Fla, and one person in Cameron County, Texas, had developed the mosquito-borne illness. The new cases mark the first time since 2003 that U.S. residents have contracted malaria after being bitten by a mosquito close to home. All five people received treatment and are improving.

Malaria, which is caused by Plasmodium parasites and spread to humans by Anopheles mosquitoes, is not unheard of in the United States. The disease was once prevalent before widespread spraying of the insecticide DDT helped to purge the country of any parasite-infected mosquitoes. By 1951, malaria had been eliminated within U.S. borders.

But the disease still circulates in many countries around the world. Globally, there are more than 200 million cases of malaria each year and hundreds of thousands of deaths, most of which happen in Africa, according to the World Health Organization. In October 2021, the agency approved a malaria vaccine for children living in sub-Saharan Africa (SN: 12/22/21).   

Before the COVID-19 pandemic stifled travel, about 2,000 cases were diagnosed across the United States each year. Such cases were exclusively in people who traveled abroad to parts of the world where the disease is common and returned home with parasites in tow. But none of the five people in Texas and Florida had traveled, meaning they’d contracted the disease from local mosquitoes.

The risk of local transmission in the United States “remains extremely low,” the CDC noted in the health advisory. But the Anopheles mosquitoes, vectors capable of spreading the parasite from person to person, are found around the country, a grim reminder that malaria can still pose a threat.  

People tend to think of malaria as a tropical disease, says Christopher Vitek, a medical entomologist at the University of Texas Rio Grande Valley in Edinburg. “But the fact of the matter is, historically we’ve had a fair amount of malaria transmission here in the U.S.” And the risk of transmission now is not zero. Officials in Texas and Florida are keeping an eye out for more cases.

Here are four things to know about malaria and the latest cases.

Local transmission in the United States is rare, but not worrisome

At the moment, experts aren’t highly concerned about the five new cases. “We’ve gotten really good at understanding transmission,” says Johanna Daily, a parasitologist and infectious diseases physician at Albert Einstein College of Medicine in New York City. “And we’re very good at vector control, which is really going to be the win here.”

Spraying insecticide around homes and areas where mosquitoes breed can help bring the insects’ numbers down. Window screens keep the insects out of homes, and mosquito repellents or clothing treated with the insecticide permethrin can prevent bites. Getting rid of standing water can stop mosquitoes from multiplying. These tactics helped end a 2003 outbreak — when eight people in Palm Beach County, Fla., were infected by local mosquitoes — as well as 62 other outbreaks since 1957.

Because malaria is still a huge problem in many places, outbreaks in the United States, while rare, also aren’t unexpected. For this latest one, “it would be more surprising to me if it was far more cases or spread out across a lot of counties,” says Sadie Ryan, a medical geographer at the University of Florida in Gainesville.

One small plus is that the parasite behind the current outbreak is Plasmodium vivax, which is not the cause of the worst type of malaria. That title belongs to P. falciparum, the most common and deadliest of five species known to infect people. While some P. vivax infections are still fatal, it’s far less deadly, Daily says.

An image of red blood cells with dark spots in the middle on a white background.
Plasmodium vivax, seen as dark spots inside red blood cells, is one of five parasite species that can cause malaria in people. While less severe than the most common type of parasite, P. vivax infections can still be deadly.Ed Reschke/stone/Getty Images

The new cases will likely make health care workers hypervigilant, Daily says. But P. vivax can lie dormant inside the liver for weeks, months or years after the infection. “I do worry that as months go by and we forget about locally acquired malaria” that some people might go undiagnosed and spark new outbreaks.

It may seem odd that cases appeared in two separate states around the same time, but there’s no evidence to suggest that the cases are related, according to the CDC.

Certain events in other parts of the world — such as a spike in cases in a spot with lots of travel — can make it more likely that malaria might pop up in the United States, Vitek says. Increased travel to places where there are ongoing outbreaks can raise the chances a person or mosquito with malaria will arrive on U.S. soil. 

Malaria can be hard to identify by its early symptoms

People with malaria typically develop fever and flulike symptoms including chills, body aches, headache and tiredness. These symptoms are also typical of other diseases like mosquito-borne dengue fever, Daily says, which can make malaria hard to pick up if health care workers aren’t already considering it among the list of possibilities.

“You cannot clinically distinguish dengue from malaria from the flu from other things,” she says.   

Treatments are available, but if left untreated, malaria infections can become severe and cause seizures, coma and death.

There are some hallmarks, such as anemia, jaundice and fevers that come and go. Plasmodium parasites infect and reproduce inside red blood cells and liver cells, which burst and release offspring into the blood stream. Some people develop anemia or jaundice as infected cells die. And when the body detects these foreign invaders, it can spark a fever. As young parasites hide inside new cells, the alarm bells quiet and the fever fades. With P. vivax, such cyclic fevers might appear every 48 hours or so.

But sometimes a fever’s timing isn’t textbook, which can throw off health care workers, Daily says. So, it’s important for people involved in patient care to keep malaria in mind when considering the possibilities. “We never think about malaria unless it’s a traveler who comes, has a fever, has nonspecific symptoms and says ‘Well, I just got off a plane.’”

Now, health care workers working in and nearby the affected Florida and Texas counties need to consider malaria as a cause of illness.  

Infected travelers and mosquitoes can transport malaria across borders   

It remains unclear why the five people got infected without traveling. But there are two ways for malaria to return to places where it has been eliminated, Vitek says. Either infected travelers transport the parasite across borders, or infected mosquitoes do.

In the first scenario, a traveler might return home with malaria and just so happen to get bitten by an Anopheles mosquito. Around nine to 18 days later, after any parasites have multiplied inside the mosquito, the insect can transmit malaria to the next person it bites.

There are many Anopheles species capable of transmitting malaria, such as Anopheles freeborni, the western malaria mosquito, across the United States. Florida, for instance, hosts 14 different species. Two of those, A. quadrimaculatus and A. crucians, are responsible for most past cases. These insects are twilight biters that love to hang out in vegetation around swamps, Ryan says. Fortunately, not many people tend to go to these spots during Florida summers when mosquitoes are rampant.

But if a traveler with malaria happens to venture out and get bitten by A. crucians, that insect could become an unwitting vector of disease.   

Infected mosquitoes can also travel by land, air or sea — often by hitching a ride alongside people — and transport Plasmodium from place to place. “Airport malaria,” for example, can happen when the insects travel by plane and infect people who live nearby, Daily says. But Anopheles mosquitoes don’t travel very far, so such cases would typically pop up a mile or so away from airports. 

Warming temperatures and travel can help mosquito ranges spread

Climate change is expected to expand the range of many insect-borne diseases, including dengue and malaria (SN: 10/7/19). But whether it’s the reason malaria is circulating in the United States for the first time in decades is unclear.  

Independent of warming temperatures, Texas and Florida already “are in a very suitable part of the world for malaria,” Ryan says. While mosquito control pushed the disease out, such places aren’t going to stop being fitting spots for malaria transmission. “The possibility of introduction is always here.”

As the climate warms, however, the mosquitoes’ livable range could extend farther north, both Ryan and Vitek say.  

What’s more, an invasive, malaria-spreading mosquito that has an affinity for biting people is also on the move.

Anopheles stephensi thrives not in swamps or rural areas far from people, but in artificial containers in urban areas across India and the Middle East, Ryan says. Now, “it has left its original range, and it’s setting up shop across different countries in Africa.” If the species were to also spread to the United States, its tendency to bite people and live in urban areas could make it a riskier vector than current species to spread malaria among people.  

Experts have the tools to deal with these urban mosquitoes because Aedes aegypti, the species that spreads dengue and Zika, also thrives close to people, Ryan says. Still, “it just definitely makes me nervous that we need to be anticipating that and be ready for surveillance.”

For now, it’s unlikely malaria is going to spread all over the United States, Daily says. Even if more than five people develop malaria, she expects the current outbreak will end. “I hate for people to feel anxious going outside or getting mosquito bites. So, we can reassure the public: There’s ways to prevent [malaria], ways to treat it. It’s curable.”

More Stories from Science News on Health & Medicine