For the first time in 20 years, the United States has recorded home-grown malaria cases.
The country sees about 2,000-2,500 malaria cases each year linked to travel to malaria-endemic areas such as sub-Saharan Africa and parts of South America and Southeast Asia — but the nine locally transmitted cases seen so far this year were contracted by people who had not recently traveled to those areas. Seven cases were recorded in Florida, and one each in Texas and Maryland.
Malaria experts say this handful of cases is no cause for panic — catching malaria in the United States is still highly unlikely. But they also emphasize that if malaria and other diseases reappear, or appear in places where they have not been before, it is a cause for concern.
History of Malaria in North America
In the early 20th century, “malaria was common even as far north as Cleveland,” says Prakash Srinivasan, PhD, MS, an assistant professor in Molecular microbiology and immunology and know Johns Hopkins Malaria Research Institute.
The disease was endemic in the United States until the 1950s. In 1951 was malaria considered eliminated from the country.
Many factors contributed to achieving this status. Industrialization, the clearing of wetlands where mosquitoes breed, the use of insecticides and window screens — on top of public health measures such as antimalarial drugs and improved diagnostics — were “game changers” in the United States and most other Western countries in terms of eradicating the disease, Srinivasan says.
Anopheles mosquitoes—the genus that carries malaria—are still present in the U.S. “However, because malaria transmission in the U.S. has not been a major problem, there is no monitoring of Anopheles populations,” explains Photini Sinnis, MDprofessor at MMI and at the Johns Hopkins Malaria Research Institute.
The vectors and the parasites
Transmission of malaria “is a relationship between a mosquito, a parasite and a person,” says Sinnis. A woman Anopheles mosquitoes must be infected with the malaria parasite to pass on an infection. But there are many variables.
The lifetime of a Anopheles mosquitoes is typically a few weeks to a month, and during that time, female mosquitoes feed on blood, which they need as an energy source to mature eggs. A mosquito will typically take a few blood meals during its lifetime. The malaria parasite can remain in the mosquito for weeks, so it has the potential to transmit to multiple people – but “the chances are quite low,” explains Srinivasan.
And that is why we see isolated cases, and not clusters or larger outbreaks.
Another factor: Not everyone Anopheles species transmit malaria in the same way.
“It’s about how anthropophilic the mosquito is – how much it prefers to bite humans. Malaria parasites are very species-specific,” explains Sinnis. The different species of Anopheles in the US will bite people “maybe 30% to 50% of the time.” If a human is not available, they may instead bite a dog or other mammal, breaking the cycle of human infection.
The Anopheles mosquitoes found in sub-Saharan Africa, on the other hand, bite humans 98% of the time—making it far easier for the cycle of human infections to continue.
Another reason not all malaria infections are the same: There are several malaria-causing parasites. The most lethal is Plasmodium falciparumsays Sinnis, which is most common in sub-Saharan Africa, where malaria kills over 600,000 people each year — 95% of them children under 5.
“It is estimated that a child dies of malaria every minute,” says Srinivasan.
Another malaria parasite, Plasmodium vivaxis more widespread in South America and Southeast Asia and generally causes less severe disease – and is therefore less fatal.
How malaria probably (re)emerged in the United States
Anopheles mosquitoes capable of carrying malaria are still very present in the US – they’ve just had very few opportunities to transmit the parasite because there are so few infected people to feed on.
Experts believe this new wave of locally transmitted cases likely occurred because a specific set of circumstances came together: A person infected with malaria traveled to the United States from a malaria-endemic area and was bitten by a local Anopheles mosquito which picked up the parasite and then bit another and passed the parasite on.
While the case in Maryland is confirmed as attached to P. falciparumthose in Florida and Texas are believed to be linked P. vivaxmore likely originating in travelers from South America.
Symptoms and treatments
Malaria causes flu-like symptoms such as fever, body aches, vomiting and chills. The disease can be treated with effective antimalarial drugs and IV fluids, but it is important to catch it early.
A telltale sign of infection is fever cycles every few days that coincide with the life cycle of the parasite in the blood.
People in the United States need not panic or even avoid spending time outdoors in hot and humid areas where mosquitoes thrive. But it’s worth knowing the symptoms and risks, advises Srinivasan: “If you have a fever and they can’t figure out why you have a fever, [and] if you spend a lot of time outside where there is a lot of heat and humidity, [malaria] should be considered, especially because when you catch it early, you can treat it.”
Without treatment, the disease can develop into very severe or cerebral malaria. “When you reach the cerebral malaria stage, even after anti-malarial treatment, [the] the mortality rate can be as high as 20% to 25%,” says Srinivasan.
Malaria’s expanding territory and what we can expect in the future
The US currently does not have the conditions for a major outbreak, Sinnis explains. The country does not have the species of mosquito that makes malaria so widespread in sub-Saharan Africa—Anopheles gambiae. Plus, the cold of winter wipes out mosquito populations every year.
But the conditions are becoming more favorable for the transmission of malaria. Warmer winters provide Anopheles mosquitoes an opportunity to start breeding earlier – meaning their populations grow to the point where they are more likely to bite an infected person who has been in a malaria-endemic area.
“These changes in the environment could be more conducive for these mosquitoes to establish a niche,” says Srinivasan. Plus, the increase in travel since the downturn of COVID — and globalization in general — makes it easier for malaria to move around.
The post-COVID surge in travel has increased the reservoir for infection. Mosquitoes can reach new places by “hitting a trip on cargo planes or ships or airliners,” he says.
It remains to be seen whether more homegrown cases can be expected in the United States. Sinnis says the next five years will be critical to understanding how much of a threat malaria really poses to the United States. Either way, she hopes this year’s case will encourage more monitoring Anopheles mosquitoes in the United States
“Is this just a random occurrence? Or is this a harbinger of things to come? It could be that sporadic cases are here to stay. We don’t know yet. Time will tell,” says Sinnis.
Annalies Winny is a producer and writer at the Johns Hopkins Bloomberg School of Public Health
