What to know about malaria after rare cases are found in Florida, Texas
Health officials issued a warning after five cases of malaria were identified in Florida and Texas in people who had not recently traveled overseas, sparking concerns about local transmission.
The warning from the Centers for Disease Control and Prevention came as it said there was a risk of increased "imported malaria cases" amid a busy summer travel season.
Here's what to know about the risk of malaria and what to do if you become ill.
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What is malaria?
Malaria is a life-threatening but curable disease. Humans typically get it when they are bitten by mosquitoes that carry a malaria parasite.
Malaria can also be transmitted through infected blood during transfusions or organ transplants, for instance, or by a pregnant mother to her fetus.
The disease is most prevalent in warm countries, particularly in tropical climates. According to the World Health Organization, in 2021 there were about 247 million cases of malaria worldwide and 619,000 deaths.
The United States and 42 other countries and territories have been certified as malaria-free by the WHO.
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What are the symptoms of malaria?
Typical symptoms of malaria are similar to those of the flu. They include fever, chills, headaches, muscle aches, fatigue, nausea and vomiting. In severe cases, people can experience unusual bleeding, jaundice and difficulty breathing.
People typically begin to experience symptoms 10 to 15 days after they are infected with the malaria parasite, though symptoms can appear earlier or far later - even one year after infection, according to the CDC.
When malaria isn't treated properly, it can cause kidney failure, seizures, coma and death. Vulnerable people - children younger than 5, pregnant people, older adults and those with compromised immune systems - are more at risk.
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Where has malaria been detected in the United States recently?
Authorities in Florida have confirmed four locally transmitted cases of malaria since May in Sarasota County, while a further case has been identified in Cameron County, Tex.
All patients have received treatment and are improving, the CDC said.
The CDC says there is no evidence to suggest that the cases in Florida and Texas are linked, but they are the first cases of local transmission in the United States since 2003. There is also evidence that in at least two of the cases - one in Florida and the other in Texas - the individual had spent prolonged time outdoors.
According to the CDC, all five cases involve the P. vivax malaria strain. This is "good news," according to Jackie Cook, associate professor of malaria epidemiology at the London School of Hygiene & Tropical Medicine, because the strain is less likely to cause severe or fatal infections than other strains. However, P. vivax can cause relapsing malaria infections as the parasites hide in the liver and come back out months or even years later, she said. "So, you do need to get a bit of extra treatment if you have a vivax infection to make sure you don't get those relapsing episodes."
Malaria outbreaks were common in the United States into the 20th century: The disease ceased to be a major public health issue only in the late 1940s, before it was officially eradicated in 1970.
Since then, the vast majority of cases have been among travelers returning from abroad. Before the coronavirus pandemic, there were an estimated 2,000 cases of malaria every year among people who had recently traveled internationally, according to the CDC. Of those, five to 10 people died.
The CDC says it expects international travel to return to pre-pandemic levels this year - meaning the number of people infected while abroad could also increase compared with recent years.
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Which mosquito causes malaria?
Malaria is transmitted to humans when they are bitten by infected female Anopheles mosquitoes infected by the malaria parasite. The Anopheles genus is found across most of the continental United States, according to the CDC.
It's not clear how the recent local cases began, but authorities in Texas say that most locally transmitted cases in the United States begin when a mosquito bites an individual who has been infected abroad and then bites other people in the community.
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How can you prevent and treat malaria?
Health experts recommend that those living in countries with malaria install mosquito nets around their beds and screens on their doors or windows. They should also apply insect repellent and wear protective clothing - loosefitting clothes that cover the arms and legs and can be treated with insecticide.
There are also antimalarial medicines such as atovaquone or doxycycline you can take to reduce your risk of malaria infection.
In Sarasota County, where the four Florida cases were reported, authorities have also conducted pesticide treatments in affected areas to kill adult mosquitoes.
Once someone is infected, experts recommend that they immediately get medical help, because early detection and treatment usually lead to better outcomes. They will be prescribed medicine based on the type of malaria parasite they have, as some are more resistant to antimalarial drugs. Treatments include chloroquine and artemisinin-based combination therapies.
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Is there a vaccine for malaria?
There is no universal vaccine that can fully prevent malaria. Research to develop one is ongoing.
However, there is a vaccine recommended by the WHO for children living in countries with moderate to high transmission of malaria. The RTS,S vaccine is designed to act against P. falciparum, the malaria parasite most likely to cause severe infections. In trials conducted in Africa, the vaccine led to a 30 percent reduction in deadly severe malaria, according to the WHO.
The process to develop this vaccine was "laborious" and took about three decades, Cook said. Several other promising vaccine candidates are being tested globally, but "it has been such a slow process to get that first one out that none of us are holding our breath for there being a silver bullet in terms of a vaccine," she added.
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Is climate change related to the spread of malaria?
Scientists say climate change may have an effect on mosquito prevalence and behavior by making the world hotter and wetter on average.
In warmer weather, mosquitoes "need to feed more regularly," so they tend to bite more, Cook said. High temperatures also speed the development of malaria parasites inside mosquitoes, she said. And as climate change brings more intense storms and flooding to some parts of the world, the mosquito population could grow, because mosquitoes breed in stagnant water.
However, in other areas of the world, particularly in sub-Saharan Africa, malaria may become less prevalent as temperatures climb too high for mosquitoes, Cook said.
Felipe Colón-González, the technology lead at the London-based biomedical research charity Wellcome Trust, said in an interview that climate change can alter both the geographical area where diseases occur and the length of the transmission.
It's difficult to say why cases of local transmission have occurred in the United States now, he said, but "as we move into a warmer climate, the possibilities for this to happen are going to increase because there's going to be more suitable environments."
He also stressed the need for individuals to play their part in reducing the spread of disease. Mosquitoes that carry dengue, for example, thrive in plastic, man-made containers, so it's important to avoid littering and follow other health advice to avoid infection, he said.
Last week, European Union officials said climate change is putting the continent at greater risk of other mosquito-borne diseases - including dengue, chikungunya fever and West Nile fever - as invasive species responsible for their transmission spread farther into the continent.
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The Washington Post's Brittany Shammas contributed to this report.
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