Screwworm Resurgence Threatens U.S. as USDA Battles to Hold Failing Biological Line in Mexico

The U.S. faces a renewed biological threat: the resurgence of New World Screwworms, parasitic flies that lay eggs in the wounds or orifices of warm-blooded animals. The resulting larvae burrow into living flesh, causing painful, potentially deadly infections known as myiasis. These parasites are not only horrific in behavior but also devastating to livestock, inflicting economic and ecological damage. To contain the spread, the U.S. Department of Agriculture (USDA) is currently conducting aerial drops of sterilized flies over Mexico to disrupt the screwworms’ breeding cycle.

Screwworms were once endemic across the southern U.S., Central America, and parts of South America. In the 1950s, the USDA initiated an aggressive eradication campaign using the Sterile Insect Technique, wherein gamma-irradiated, sterile male flies were released en masse. These sterile males competed with fertile ones, sharply reducing the population over time. By 1966, the U.S. was screwworm-free, and eradication efforts successfully extended down through Central America, culminating in Panama by 2006.

Biological Barrier Breached After Years of Success, Screwworms Advance Rapidly Through Mexico

Following eradication in Panama, a permanent defense was established along the Darién Gap, with monthly releases of sterile flies to prevent re-infestation from South America. This biological barrier held firm for years, helping farmers avoid an estimated $900 million in annual losses and sparing animals from suffering. The USDA and Panama even built a dedicated facility for sterile fly production to sustain this long-term effort, showcasing a rare and sustained victory in parasitic pest control.

Despite years of success, the barrier was breached in 2022. Screwworms have since moved north, reaching Costa Rica, Nicaragua, Honduras, and most recently, Mexico. In response, the USDA shifted its aerial fly releases from Panama to Mexico in an urgent attempt to halt their northward march. Agriculture Secretary Brook Rollins confirmed that efforts are now focused on preventing the screwworms from crossing Mexico’s Isthmus of Tehuantepec, but the complex geography and limited infrastructure make containment especially difficult.

Rare Human Infections Are Gruesome, Painful, and a Growing Public Health Concern

While animals are the primary victims, human cases—though rare—are gruesome. Infections involve hundreds of larvae that must be manually removed, often under surgery, followed by removal of dead tissue and intensive wound care. CDC officials stress the tenacity of the larvae and the immense pain involved. Past U.S. cases include infestations in ears, cheeks, and sinuses, often brought back from trips to Latin America. These cases underline the human health risk posed if screwworms spread unchecked.

The fight against screwworms is now at a pivotal moment. The USDA’s sterile fly campaign remains the best line of defense, but success depends on swift action, international cooperation, and overcoming logistical barriers. Experts warn that failure to contain the parasites in Mexico could see their return to U.S. soil—a nightmare scenario both medically and economically. The current campaign may determine whether a decades-long victory is preserved—or undone in just a few years.