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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 overpowered local containment efforts 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. 

On their own, the flies can usually fly no more than about 12 miles in their monthlong lifetime, says Sonja Swiger, an entomologist at Texas A&M University. But the screwworm larvae can travel great distances while developing inside (and gnawing on the flesh of) their hosts. A new generation reaches sexual maturity every week to two weeks, and females can lay up to 2,800 eggs over the course of their lifespan, according to the California Department of Food & Agriculture.

Most people aren’t at risk of screwworm infections, which are rare compared with those in livestock. But cases have appeared in Central America since the breach of the Darién Gap. Nicaragua first detected the parasite in livestock in March 2024; by February 2025, health officials there confirmed 30 human cases. Costa Rica saw 42 confirmed cases between January and May 2025 and at least two deaths, according to the country’s health ministry. Honduras has reported 40 human cases and three deaths, according to the public health network EpiCore, while Guatemala reported its first human case in May. The Mexican Ministry of Health has confirmed eight human cases.

In humans, infection with fly larvae is known as myiasis. Those who are most at risk for screwworm myiasis are people who work closely with livestock or who sleep outdoors. Treatment involves removing the larvae, sometimes with surgery. 

Screwworms haven’t made it back into the U.S. yet. How quickly this might happen depends on whether agricultural officials can hold the line in Mexico or push the fly southward. On May 27, U.S. Department of Agriculture officials announced $21 million in funding to retrofit a fruit fly production plant in Metapa, Mexico, to produce sterile screwworm flies. When operational, the plant will churn out between 60 million and 100 million additional flies a week to help suppress the breeding population in Mexico.

While the sterile insect technique is likely to remain the key tool in the arsenal against screwworms for years to come, new genetic methods of insect control could eventually come to bear against the problem. In May ethicists and entomologists, including Scott, wrote in a paper in Science that the screwworm is a good candidate for complete elimination with gene drive technology, which involves genetic engineering to ensure that a deadly mutation will be included in an animal’s sperm and egg cells and thus will be passed on to the next generation. The loss of screwworms does not seem to substantially affect the ecosystem, the researchers wrote, and death by the insect is painful and slow.

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