The Centers for Disease Control and Prevention is warning doctors to be on the lookout for malaria cases after five people who had not traveled outside of the country fell ill with locally acquired infections in Texas and Florida. In a health alert Monday evening, the agency expressed concern that the country could see a rise in imported cases due to increased international travel this summer.

The Florida and Texas cases are the first locally acquired malaria infections in the US since 2003, the agency highlighted.

So far, there have been four cases in Florida's Sarasota County and one case in Texas' Cameron County, which sits at the state's far eastern border with Mexico, where malaria is endemic. There is no indication that the Florida cases and the Texas case are linked in any way. The four infected people in Florida, who have all since recovered, were in close geographic proximity to each other, and the Texas case occurred in a person who spent time working outdoors.

Prior to the COVID-19 pandemic, the US typically saw around 2,000 imported malaria cases each year, with around 300 severe cases and five to 10 deaths. The cases are mostly among people who return from travel to malaria-endemic regions. The occasional small clusters of locally acquired cases, like the recent cases in Florida and Texas, are typically thought to be spread from an imported case, usually in a tight geographic area.

Malaria is caused by an infection with one of five protozoan parasites of the Plasmodium genus. The parasites are transmitted by the bites of female mosquitoes of the Anopheles genus. There are multiple species of Anopheles mosquitoes in the US capable of picking up and spreading malaria, and 32 states have reported the presence of at least one potential malaria vector.

Once a person is infected, malaria symptoms begin between 10 days to four weeks later, with non-specific signs such as fever, chills, headache, myalgias, and fatigue. It can also involve nausea, vomiting, and diarrhea. If left to progress, it can develop into a severe life-threatening disease, with symptoms including mental status changes, seizures, renal failure, acute respiratory distress syndrome, and coma.

Persistent parasites

Two species of malaria parasites can also go dormant in the liver, causing chronic and relapsing disease that requires additional treatment. The two species capable of doing this are P. ovale, a rare cause of malaria that's primarily found in sub-Saharan Africa, and P. vivax, one of the two most common malaria parasites, found mostly in Asia and Latin America. The recent locally acquired cases in Florida and Texas were caused by P. vivax.

The other top malaria parasite is P. falciparum, which is the deadliest cause of malaria. P. falciparum is found in dozens of countries but is most concentrated in Africa.

In the last outbreak of locally acquired malaria in the US, which occurred in 2003, at least seven people in Palm Beach, Florida, fell ill from malaria with a P. vivax infection. The source of the infection was not identified, but investigators noted that it could have been from international travelers, people who immigrated, or migrant farm laborers from Mexico or Central or South America.

In 2002, another cluster of locally acquired malaria cases was reported among two teenagers in Loudon County, Virginia. The cause was again P. vivax. The source of the infection was not determined, but investigators assumed that a malaria-infected person must have been in the vicinity. They noted a nearby international airport (Dulles) that receives travelers from P. vivax-endemic areas, though the teen cases were more than 10 miles from the airport. Interestingly, anopheline mosquitoes collected and pooled from the area tested positive for P. vivax, marking the first time since 1957 that US mosquitoes linked to locally acquired cases had tested positive for a malaria parasite.

While the CDC still deems the overall threat of locally acquired malaria in the US to be "extremely low," these occasional cases are a reminder that reemergence is possible and vigilance is required. Global travel and climate change will continue to create opportunities for malaria transmission to increase in endemic and controlled areas, like the US, and even in places that have traditionally been malaria-free.

Currently, there are more than 240 million malaria cases each year worldwide, and there were over 600,000 deaths in 2021. Ninety-five percent of the case and 96 percent of the deaths are in the African region, according to the World Health Organization. Eighty percent of the malaria deaths in Africa are in children under age 5.

Malaria used to be endemic in temperate areas of the US but was considered eliminated from the country in 1951 after the establishment of the Office of Malaria Control in War Areas—a precursor to the CDC. The elimination was achieved by improved sanitation, medical care, drainage of mosquito breeding sites (mosquitoes breed in standing water), and insecticide use.