The recent cases of malaria identified in Florida and Texas may have you wondering if the U.S. is turning into a paradise for tropical diseases typically associated with foreign travel. These cases—four in Sarasota County, Florida, and one in Cameron County, Texas—were in people who had not recently traveled outside the U.S. These are the first locally acquired ones since eight cases occurred in Palm Beach County, Florida, in 2003.
The truth is that the potential for malaria in the U.S. has never gone away, experts say. These cases are the latest reminder that vigilance and precautions are necessary to ensure that mosquito-borne diseases eliminated from the U.S. don’t return and illnesses from the ones already here don’t grow worse.
Though an estimated 2,000 cases of malaria occur in the U.S. each year, nearly all of them occur in individuals who traveled from sub-Saharan Africa or South Asia. The Anopheles mosquitoes that carry malaria are still in the U.S. and just as capable of transmitting the parasite that causes the disease as they were for centuries in the past, before malaria was eliminated from the U.S.
“The United States was once a place where there was a lot of malaria and mosquito-borne illnesses, including yellow fever,” Amesh Adalja, an infectious disease, pandemic, and biosecurity scientist at Johns Hopkins University, says. “There’s not some insurmountable barrier for those mosquito diseases to be able to appear in the United States. What really drove back those diseases was very effective vector control and making areas inhospitable for mosquitoes in terms of standing water.”
That’s why these new cases were not a surprise to infectious disease experts. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston, calls these cases “predicted and predictable.”
“The reason we’re seeing these rises in not only the Southern U.S. but other hotspots of the world is because of a confluence of four or five factors, which include poverty, urbanization, human migrations, and climate change,” Hotez says. “Those are all converging, and because of this constellation of forces, we’re seeing not only malaria but also dengue fever, Zika, chikungunya, West Nile, Chagas disease, typhus, and worm infections.”
Ian Cheeseman, an infectious disease researcher at the Texas Biomedical Research Institute in San Antonio, agrees, adding that the areas where these cases occurred certainly have the conditions that are conducive to allowing malaria to spread.
“With increased travel, the fact that we’re a much more interconnected world and other climate-driven features that are making transmission a little easier for diseases like malaria, it’s not a massive surprise that we’re seeing a very small number of locally acquired cases.”
To help people make sense of these new cases, Adalja, Hotez, and Cheeseman spoke with National Geographic about what we should know about malaria in the U.S.
What is malaria and how serious is it?
Malaria is a disease caused by a parasite that’s transmitted by the Anopheles species of mosquito. Malaria symptoms include a fever and flu-like illness, such as shaking chills, headache, muscle aches, fatigue, nausea, vomiting, and diarrhea. It can also cause anemia (low blood cell count) and jaundice, where the skin and eyes become yellow due to fewer red blood cells. Symptoms usually begin 7-10 days after infection but could occur as much as a year later.
Of the five types of malaria that infect humans, the two responsible for nearly all of the estimated 247 million cases that occurred globally in 2021 are P. falciparum and P. vivax. P. falciparum is far more dangerous, responsible for the majority of the 619,000 global deaths from malaria in 2021. Nearly all those deaths (96 percent) occurred in Africa, and eight in ten deaths were in children under 5 years old.
The recent U.S. cases, however, were P. vivax, the malaria type found throughout the world outside of Africa and a far less severe — though still serious — illness that is less likely to cause death. “The vivax malaria parasite tends only to infect very young blood cells, and the percentage of those cells in your system usually doesn’t exceed a couple percentage points,” Hotez says. P. vivax is not benign, he adds, but it’s not likely to kill someone.
How did malaria get to the U.S.? Was it climate change?
The environmental conditions for malaria and the mosquitoes that carry it have always existed in the U.S.— but a concentrated effort to eliminate the disease succeeded by the early 1950s.
“We were able to break the chain of transmission by doing things like draining swamps, better housing, the advent of air conditioning and a lot of people being inside more and out of the way of mosquitoes during peak times [of mosquito activity],” Cheeseman says.
Cleaning up places where mosquitoes breed played the biggest role, Adalja adds. Further, most Americans have air conditioning, “especially in the areas where mosquitoes flourish, and mosquitoes don’t tend to like air conditioning,” he says.
Although none of the recently infected individuals had a history of travel, it is possible someone else in the area had come recently from a country where malaria is endemic, Adalja said. The most likely scenario is that someone else in the county had an infection from recent travel, a local Anopheles mosquito bit them and contracted the parasite while feeding on their blood, and then that mosquito infected an individual nearby. It’s impossible to be certain that’s what occurred, but no other cases have been reported so far, suggesting there is not a large population of malaria-carrying mosquitoes in those areas.
Is there a vaccine against malaria?
Yes, but it is not available in the U.S., and it does not protect against P. vivax malaria. The RTS,S malaria vaccine targets the P. falciparum parasite and reduces the risk of malaria by 50 to 75 percent based on trials extending from 2009 through 2020. After a successful pilot program in sub-Saharan Africa in 2019, the World Health Organization recommended the vaccine in 2021 for children throughout Africa in areas with endemic P. falciparum. In fact, the WHO, UNICEF, and Gavi, an international vaccine nonprofit, announced July 5 that 12 African countries hardest hit by malaria will receive 18 million doses of the vaccine over the next two years.
Should you worry about catching malaria in the U.S.?
The risk of anybody catching malaria in the U.S. has not dramatically increased from a few weeks ago, even in the areas where these cases occurred, Cheeseman says. The far greater risk from mosquitoes is West Nile virus, which arrived in the U.S. in 1999 and has since spread throughout the country and become the leading cause of mosquito-borne disease in the continental U.S.
“West Nile probably won’t get nearly as many headlines as the five malaria cases, but it kills more people in the U.S. every year,” Adalja said. Nearly 3,000 people contracted West Nile virus in 2021, resulting in 227 deaths. Meanwhile, Hotez says, more cases of other mosquito-borne diseases will likely grow over time, including dengue fever, Zika, and chikungunya.
How can you protect yourself against these diseases?
Follow the same precautions you normally would to prevent mosquito bites, starting with using mosquito repellent when you’ll be around mosquitoes, all three experts say.
Cheeseman also advised wearing longer clothing to cover up as much as possible, though that can be difficult given the heat in areas where malaria thrives. He also advises staying out of areas with mosquitoes at dusk and dawn—when mosquitoes are most active.
“Try to keep your own property maintained as best as possible, not leaving pools of water or filled drains,” Cheeseman says. “Mosquitoes love to thrive in warm, wet places like puddles or leftover tires.”
How can people slow the spread of these diseases, in general?
Vector control is the biggest way to stop the spread of mosquito-borne illnesses, all three experts agreed. “One of the reasons the Gulf Coast remains vulnerable is because when you go into poor neighborhoods in the South, you see houses without screens and dilapidated housing with a lot of drainage ditches filled with water,” Hotez says. Another problem, he says, is lack of surveillance for these diseases and lack of funding for public health departments to be on the lookout for these diseases and the conditions that help them spread.
People can also help by picking up trash, Adalja says. “Don’t make your yard a place where mosquitoes can breed, and if you see trash in the street, pick it up,” he says. “All you need is a bottle cap [filled with water] for mosquitoes to breed.”