The mosquito-borne illness malaria has sickened five U.S. residents with no recent travel history, meaning they caught the disease locally, the Centers for Disease Control and Prevention (CDC) warned Monday (June 26).
Before these cases, locally acquired mosquito-borne malaria had not been reported in the U.S. for 20 years, since 2003 when eight people in Palm Beach County, Florida caught the disease, according to the CDC health advisory.
The five recent cases took place within the past two months. Four happened in "close geographic proximity" in Sarasota County, Florida. The remaining case was in Cameron County, Texas, the Texas Department of State Health Services reported. The CDC advisory notes that the cases were "mosquito-transmitted," meaning those affected got malaria from mosquito bites, which is the most common way of catching the disease.
Each year, about 2,000 people test positive for malaria in the U.S., but the vast majority have recently traveled internationally to regions where the disease is endemic, meaning it regularly spreads there, the CDC notes. However, because mosquito species that can carry malaria live in the U.S., there's a potential risk for local mosquitoes to pick up malaria parasites from an infected person and thus reintroduce the disease in the area.
(Malaria was eliminated in the U.S. in the 1940s and 1950s, thanks to a campaign largely aimed at eliminating mosquito breeding grounds and spraying mosquito-killing pesticides.)
Since malaria was eliminated in the U.S., outbreaks of locally acquired disease have been "small and relatively isolated," the CDC states. When outbreaks do occur, they're considered public health emergencies and the CDC works with local health departments to identify additional human malaria cases and coordinate with ongoing mosquito control and surveillance programs, as necessary.
Mosquitoes in the genus Anopheles carry malaria-causing parasites and spread them to humans through their bites. The pests initially pick up the parasites by biting a person already infected with malaria. (Very rarely, people can get malaria via blood transfusions, organ transplantation or unsafe needle-sharing practices, the CDC notes. If picked up in pregnancy, malaria parasites can also pass through the placenta to fetuses or infect babies during birth.)
Five species of Plasmodium parasites can cause malaria in humans. The parasite identified in the recent U.S. cases is called P. vivax, according to the CDC, and it's less likely to cause fatal disease than another, more dangerous species called P. falciparum.
The five people recently infected with P. vivax have "received treatment and are improving," the CDC advisory notes. Treatment for malaria depends on several factors, including the parasite species and the severity of the patient's illness.
Typically, within a week to a month of being bitten by a mosquito carrying the malaria-causing parasite, people experience an initial "attack" that lasts hours and has several stages of symptoms, including cold and shivering; then fever, headaches and vomiting; and finally, sweating and fatigue. Additional signs of malaria that doctors look for include an enlarged spleen, mild jaundice (yellowing of the skin and eyes), enlargement of the liver and increased breathing rate.
Severe malaria can cause kidney injury, inflammation in the lungs that hinders oxygen exchange, the destruction of oxygen-carrying red blood cells, and serious neurological symptoms, including seizures and coma, among other serious symptoms.
Some species of malaria parasite, including P. vivax, can go dormant, lurk in the liver and then reactivate months or years down the line. Treatments can lower the likelihood of this reactivation, the CDC notes.
"Malaria is a medical emergency and should be treated accordingly," the CDC advised in its notice. "Patients suspected of having malaria should be urgently evaluated in a facility that is able to provide rapid diagnosis and treatment, within 24 hours of presentation."
To limit their risk of malaria, iIndividuals can take steps to prevent mosquito bites and control mosquitoes at home, the CDC advises. People traveling to a place where malaria is endemic should consult a health care provider about taking preventive medicine while abroad. (Again, malaria is not endemic anywhere in the U.S.)
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Nicoletta Lanese is the health channel editor at Live Science and was previously a news editor and staff writer at the site. She holds a graduate certificate in science communication from UC Santa Cruz and degrees in neuroscience and dance from the University of Florida. Her work has appeared in The Scientist, Science News, the Mercury News, Mongabay and Stanford Medicine Magazine, among other outlets. Based in NYC, she also remains heavily involved in dance and performs in local choreographers' work.