Flesh-Eating 'Screwworm' Parasites Are Headed to the U.S.

June 17, 2025

4 min read



Screwworm parasites primarily infect livestock, but human cases have risen in Central America after the pests escaped containment

By Stephanie Pappas edited by Jeanna Bryner

Officials in nine countries are trying to get a handle on the New World screwworm, a fly whose larvae eat the living flesh of livestock.

The pest is marching northward at an alarming rate and has now moved some 1,400 miles from southern Panama to southern Mexico in about two years. Screwworms are disastrous for ranchers, whose cattle can become infected when the flies lay eggs in cuts or wounds, after which their resulting larvae burrow, or screw, into that flesh. The northernmost sighting is currently about 700 miles south of the U.S. border.

Since the insect breached biological containment in Panama’s province of Darién in 2023, it has moved through Central America and is now found as far north as the Mexican state of Oaxaca. Thousands of animals have been infected, and officials have reported dozens of human cases in Costa Rica, Nicaragua, Honduras and Mexico this year.

As the fly spreads northward from the narrow Darién Gap in Panama and up the funnel of Central America, it becomes harder to control. Agricultural departments suppress fly populations by releasing millions of sterile male flies per week into the environment throughout Central America. These males are raised in a facility in Panama jointly run by that country's agricultural department and the U.S. Department of Agriculture. Because female screwworms mate only once in their lifetime, this population of infertile males reduces the size of the next generation of flies. Consistent application of this sterile insect technique eradicated the screwworm from the U.S. in 1966 and from regions north of the Darién Gap in 2006.

That invisible wall holding the screwworm back has crumbled, however. “I don’t know how it got away so quickly,” says Maxwell Scott, an entomologist at North Carolina State University, who studies genetic methods to control populations of the fly. “There had to be some movement of infested livestock, particularly through the middle [of Central America].... It just moved too fast,” Scott says about the swift speed of the screwworm spread.

More:
https://www.scientificamerican.com/article/flesh-eating-screwworm-parasites-are-headed-to-the-u-s/