U.S. officials say risk of locally acquired malaria remains low as nationwide cases jump to seven

U.S. public health officials say the risk of locally transmitted malaria in the country remains low as seven new cases in Florida and Texas raise questions. 

The Florida Department of Health on Friday said two cases of locally acquired malaria have been reported in Sarasota County, bringing the total in the state to six.

Last week, the Centers for Disease Control and Prevention issued a nationwide advisory over the four initial Florida cases and one in Texas to alert health-care providers, local health departments and the public about the possibility of local malaria transmission.

Those five patients “have received treatment and are improving,” according to the CDC. “Despite these cases, the risk of locally acquired malaria remains extremely low in the United States,” the agency added.

The seven are the first known cases of “locally acquired” malaria in the country since 2003. That means the new infections were not linked to foreign travel and appear to have been spread by U.S. mosquitoes carrying the parasite that causes the disease.

Malaria is a serious and potentially fatal disease typically transmitted through the bite of an infective female anopheline mosquito, according to the CDC. It was once endemic in the U.S., meaning it occurred regularly and required broad public health interventions.

The risk of the disease is higher in areas where warmer climate conditions allow those mosquitoes to survive during most of the year, the agency said.

The U.S. records roughly 2,000 malaria cases each year, nearly all of them in people who acquired the disease abroad, not within the country. 

Health experts say the new locally acquired cases shouldn’t warrant panic about widespread malaria transmission in the U.S.

But they also note that it’s important for the public to remain vigilant at a time when climate change and a rebound in international travel increasingly contribute to the spread of insect-borne diseases. 

U.S. public health authorities and health-care providers should also be prepared to ramp up their surveillance of malaria, experts added. 

Here’s what you need to know about the locally acquired malaria cases in the U.S. – and why the risk of transmission remains low right now.

Investigations by health departments in Texas and Florida confirmed that none of the five cases were directly linked to international travel, a CDC spokesperson told CNBC.

But it’s still unclear how mosquitoes in the U.S. came to carry malaria.

One possible explanation has to do with the nature of the malaria species identified in both states: P. vivax, the most common form of the disease. 

P. vivax isn’t the deadliest kind of malaria, but it’s more difficult to treat than other forms, according to Daniel Parker, associate professor of population health and disease prevention at UC Irvine.

The P. vivax parasite can cause symptoms – which range from fever to difficulty breathing – soon after infection, like other forms of malaria. 

But the parasite can also lie dormant in the liver for days, months or years before popping up in the bloodstream again and causing symptoms to reappear, Parker said. During that dormant period, P. vivax causes no symptoms and remains undetectable in blood tests.  

It’s possible a Florida or Texas resident was infected with P. vivax abroad and returned to the U.S. without realizing they had malaria due to a lack of symptoms, according to Sadie Ryan, a medical geography professor at the University of Florida and director of the Florida Climate Institute.

Local mosquitoes could have picked up malaria from an unknowing traveler after P. vivax became active in their bloodstream again, and those mosquitoes could have spread it to other people in the area.

“It might be that one malaria case came to the U.S. from somewhere else. Then local mosquitoes here picked it up and bit people locally,” Ryan said.

But without more details on the cases, experts say it’s difficult to offer definitive explanations. 

Experts told CNBC that it’s possible for locally acquired malaria cases to spread to other parts of Florida, Texas or potentially other states, but the probability is low.

That’s largely because public health authorities responded to the cases quickly and are mostly equipped to contain local malaria transmission, especially in areas known to be more suitable for mosquito-borne illnesses. 

Ryan said health authorities did a “really good job” alerting the public and health-care providers about the cases quickly.

The CDC and state-level warnings in Florida and Texas were also timely since they were issued ahead of the Fourth of July holiday, when more people typically expose themselves to mosquitoes outdoors, Ryan added.

“They got the message out and said, ‘Beware this is here. Here are the things you can do to protect yourselves from it,'” she told CNBC.

Local health authorities in Florida and Texas have also carried out aggressive “vector control” efforts in areas where the cases emerged, she added. That involves spraying insecticide from the ground or from a helicopter to kill off mosquitoes capable of carrying malaria. 

UC Irvine’s Parker also said the U.S.’s case-tracking protocols make the country more prepared to contain the spread.  

Health-care providers are required to report all cases of laboratory-confirmed malaria to their local or state health department, making it easier to track the potential spread of the disease. 

Parker said public health authorities also investigate cases after they’re identified to better understand their origins, which is in some ways “similar to contact tracing efforts that we’re now more used to because of Covid-19.”

“The CDC was partially born out of our malaria elimination efforts. While I would argue that we’ve neglected some of our public health infrastructure, there are systems in place…that can quickly be put into action when cases are identified,” Parker said, referring to the CDC’s inception in 1946.

The agency played a critical role in declaring the disease’s elimination in the U.S. in 1951.

But the U.S.’s toolkit for fighting local malaria transmission isn’t perfect. Not all areas of the country have the local public health infrastructure in place to track and combat the disease, putting them a step behind if locally acquired cases spread.

Several factors are also making the country increasingly vulnerable to malaria overall, regardless of whether they are local or imported infections. 

Climate change is causing a shift in weather patterns that can worsen malaria conditions, according to Dr. Rajiv Chowdhury, a global health expert from Florida International University. He said global warming could lead to “higher mosquito migration and abundance” in areas of the country that were previously uninhabitable by anopheles mosquitoes.

Existing evidence suggests warmer temperatures can increase the growth rate and transmissibility of the parasites responsible for malaria, Chowdhury added.

He also said climate change can lead to excess rainfall and sea level rise in the U.S., creating more open spaces with standing water that serve as “effective breeding grounds” for mosquitoes.

It’s unclear whether the new local cases in Texas and Florida are connected to rising temperatures. Ryan of the Florida Climate Institute noted those states were already warm enough for the disease to spread in the first place. 

Chowdhury agreed: “It’s really difficult to pinpoint causation for particular cases to the broader environmental changes that have been occurring. We need a bit more research to make that connection in the U.S.”

A CDC spokesperson told CNBC that “it is not clear that the recently reported cases are due to changes in climate,” even though shifting weather conditions do influence the distribution of diseases like malaria. 

But the agency said a rebound in foreign travel levels this year could also increase the number of imported cases of malaria in the country. The agency last week highlighted its “concern for a potential rise” in those cases associated with increased international summer travel that could return to pre-Covid levels.

Parker said increased international travel could potentially lead to more imported and local infections.

“It is possible that we’ll have more imported cases and since we already have the mosquitoes locally, it’s possible that they’ll get some and there could be more local transmission,” he said.

But he added: “I wouldn’t say I’m not too worried about it. As long as we remain vigilant.”

Experts noted there is more work that public health authorities, health-care providers and people can do to manage the nation’s rising risk of malaria. 

U.S. public health authorities should consider which areas of the country are becoming more suitable for malaria transmission and how those places can build or bolster the infrastructure needed to deal with the disease, according to Ryan. 

“That’s the sort of realm in which we need to be concerned – to think about where people should be anticipating this and what they can do to build that capacity needed to manage the disease with vector control, public health messaging and other pieces of the puzzle,” she said. 

Clinicians can also strengthen their surveillance of the disease by considering malaria diagnoses in any person with a fever of unknown origin, regardless of their travel history, according to the CDC.

“It’s possible for someone to come back with malaria and for their physician to have never seen a malaria case before. So they aren’t used to dealing with the disease,” Parker said. “But public health agencies are putting out reports on local cases, so physicians should have malaria on their radar.” 

There is no malaria vaccine available to the U.S. public yet, but travelers can prevent malaria infections during international travel using anti-malarial medicines. Those drugs appear to be underused: Only a quarter of travelers reported taking so-called malaria prophylaxis in 2018. 

It’s easy for people to mistake malaria for a common viral infection since the disease often causes flu-like symptoms. But the CDC says the “most important step” people can take is to see a doctor if they are sick and are presently – or have recently been – in an area with malaria. 

Getting a diagnosis early on can ensure that a malaria infection is treated before it becomes serious and life-threatening, the agency said. 

“Right now, we should not panic,” Chowdhury said. “But we definitely need to keep an eye on malaria and take those preparatory measures.”