Flesh-eating human parasite sweeping across Central America is raising concerns in US — what to know

A flesh-eating parasite that burrows into its host's skin through open wounds is making a comeback in Central America, U.S. officials warn.

New World screwworm (Cochliomyia hominivorax) primarily infects cattle and other livestock but is also known to infect humans. The worm can cause New World screwworm myiasis, a potentially fatal condition with no known treatment in humans..

For decades, countries across the Americas have invested billions of dollars to control the flesh-eating parasite, which is native to South America and the Caribbean. But since 2023, screwworm cases have been increasing and spreading north.

Between the 1930s and 1950s, screwworm was a major problem for livestock farmers in southern U.S. states, with producers losing up to $100 million every year due to infestations, according to the U.S. Department of Agriculture (USDA). By the mid-1960s, the parasite was all but eradicated thanks to sterilization efforts to stop the screwworm flies from breeding, and a New World screwworm barrier zone was set up along the U.S.-Mexico border.

By 1986, the parasite had been largely eradicated in Mexico, but it continued to circulate in South American and Caribbean countries where screwworms are endemic. Now, after a positive detection in Mexico on Nov. 22, 2024, the screwworm may be starting to make a comeback, the USDA said in a statement on Dec. 6.

"Since 2006, the United States and Panama have maintained a barrier zone in eastern Panama intended to prevent NWS [New World screwworm] from moving north from South America to screwworm-free areas in Central and North America," the department said in the statement. "However, since 2023, cases have been increasing in number and spreading north from Panama to Costa Rica, Nicaragua, Honduras, Guatemala, and now Mexico."

The outbreak — largely among livestock — has been particularly stark in Panama, with detections exploding from an average of 25 cases per year pre-2023 to 22,611 positive cases as of Dec. 4, according to the Panama and United States Commission for the Eradication and Prevention of Livestock Screwworm (COPEG).

Costa Rica has also seen an uptick in human cases, with one confirmed fatality earlier this year, according to the U.S. Embassy in Costa Rica.

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The parasite spreads when adult female screwworm flies lay their eggs in the wounds or open orifices of live, warm-blooded animals. According to the U.S. Centers for Disease Control and Prevention (CDC), these flies can lay up to 300 eggs in one go, potentially laying thousands during their 10- to 30-day lifespan.

Even a wound the size of a tick bite is big enough to attract the female flies to lay their eggs. These eggs then hatch into larvae, which burrow into the wound and feed on the surrounding flesh with their sharp, hooked mouths, essentially eating their hosts from the inside out. The wound becomes deeper and larger as more worm-like larvae hatch from their eggs..

These infestations are very painful. They also leave their hosts vulnerable to secondary infections.

Myiasis refers generally to infestations of live vertebrate animals with fly larvae, including screwworms. Mortality rates vary greatly between different species, but a large study by the World Health Organization found that mortality rates stood at around 3 percent for human cases.

Those living in rural areas who frequently work with livestock are most vulnerable to infestation, according to the CDC, although anyone with open sores or wounds, including from recent surgery, can be affected.

There is no approved treatment for screwworm infestations other than to physically remove the larvae from the infected tissue, according to the CDC. Instead, the agency notes that the best way to prevent screwworm myiasis is to avoid exposure. This includes cleaning and covering open wounds, especially when in contact with livestock and wild animals, and to use Environmental Protection Agency (EPA) registered insect repellents.

To prevent the spread of this parasite into the U.S., the USDA's Animal and Plant Health Inspection Service (APHIS) is restricting the import of livestock, including horses, from Mexico into the U.S., "pending further information from Mexican veterinary authorities on the size and scope of the infestation."

The USDA also advised pet owners to check for draining or enlarging wounds, as well as signs of screwworm eggs or larvae around open wounds and orifices.

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