The Screwworm Invasion: A Race Against Time to Contain the Flesh-Eating Parasites

As the New World screwworm continues its relentless march northward, officials in nine countries are working tirelessly to contain the spread of this devastating pest. The fly, whose larvae feed on the living flesh of livestock, has already traveled over 1,400 miles from southern Panama to southern Mexico in just two years. The northernmost sighting is currently about 700 miles south of the U.S. border, posing a significant threat to ranchers and their livestock.

The screwworm is particularly disastrous for ranchers as the flies lay eggs in cuts or wounds on cattle, which then hatch into larvae that burrow into the flesh. Thousands of animals have already been infected, and dozens of human cases have been reported in Costa Rica, Nicaragua, Honduras, and Mexico this year. The spread of the screwworm is particularly concerning because it has breached the biological containment in Panama’s province of Darién in 2023. Since then, it has moved through Central America and is now found as far north as the Mexican state of Oaxaca.

Agricultural departments have been using a sterile insect technique to suppress fly populations by releasing millions of sterile male flies per week into the environment throughout Central America. This technique has been successful in eradicating the screwworm from the U.S. in 1966 and from regions north of the Darién Gap in 2006. However, this invisible wall has now crumbled, and the screwworm is spreading rapidly.

On their own, the flies can usually fly no more than about 12 miles in their monthlong lifetime, but the screwworm larvae can travel great distances while developing inside 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. This rapid reproduction rate makes it difficult to control the spread of the pest.

While most people are not at risk of screwworm infections, which are rare compared with those in livestock, cases have appeared in Central America since the breach of the Darién Gap. People who work closely with livestock or sleep outdoors are most at risk for screwworm myiasis, a condition caused by fly larvae infection. Treatment involves removing the larvae, sometimes with surgery.

The U.S. Department of Agriculture has 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 wrote in a paper in Science that the screwworm is a good candidate for complete elimination with gene drive technology. This 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, and death by the insect is painful and slow. If successful, this technique could spell