Sterile fly capacity insufficient to push parasite back

The new world screwworm has been detected in 34 animals in the United States — most in Texas, with one in New Mexico — though all cases have been limited to livestock and pets, with no wildlife detections in the country, according to officials. The parasite was declared eradicated from North America after a multi-decade mid-century campaign.

The recent spread follows a pattern that experts attribute directly to the movement of cattle across national borders without health inspections, a practice they said is not being addressed by current control measures.

Camera traps set by the Wildlife Conservation Society in remote Central American forests to monitor illegal cattle movement captured what the group described as widespread infection among wildlife, including jaguar, puma, tapir, deer, white-lipped peccary, and porcupines, Jeremy Radachowsky, director of the organization’s Mesoamerica and Caribbean program, said. The cameras recorded animals sharing water sources with illegally transported cattle.

“We see infestations in the deepest parts of the interiors of the forest, so now it’s become endemic in wildlife, far from the cattle infestations,” Radachowsky said.

The current U.S. response involves dropping 100 million sterile flies in the southwest and Mexico, a technique that uses radiation to render the flies unable to reproduce. Phillip Kaufman, professor and department head of entomology at Texas A&M University, said the country would need roughly 500 million sterile flies to begin pushing the population back south.

“What we lack are sufficient flies in order to start pushing the population back south,” Kaufman said.

Officials are rushing to expand breeding capacity. One facility opened in Mexico in late June, and another in Texas is expected to open in late 2027. Researchers are exploring innovations such as raising only sterile male flies and designing better bait traps, but Kaufman cautioned against abandoning proven methods.

“We have to have things that work,” he said. “We can’t stop doing things we know work in order to try things that don’t have any data to support. We are relying on science to solve this problem.”

Radachowsky argued that the sterile fly technique alone cannot address the underlying cause of the spread.

“They’re either eradicating the fly, making a fly that can’t reproduce or trying to trap the fly,” he said. “What they’re not doing is addressing the root cause of the cattle trafficking.”

The livestock industry has changed significantly since the mid-20th century, Radachowsky said. The human and cattle populations are far larger than when screwworm was first eradicated, and large-scale illegal cattle movements did not exist at that time.

“There’s this expectation and this simple argument that the sterile fly technique worked once, so it will work again,” he said. “But the problem is the human population and cattle population is just incredibly different from what it was back then, and the [illicit] cattle movements didn’t exist before.”

The screwworm crossed the Darién Gap in 2022, Radachowsky said, and moved northward through Nicaragua before accelerating rapidly through Central America. He described the spread as moving “at the speed of a truck, and exactly along those illegal cattle-trafficking routes that we had already documented.”

Kaufman corroborated that illegal animal movement is the primary driver.

“There’s not a lot of wildlife that go on long-distance migrations” in the region, he said. “When you see it jump 50 or 100 miles, that wasn’t an adult fly flying that far. They don’t do that. It was people” transporting livestock or pets.

The alarm began roughly 18 months ago when the screwworm re-entered Mexico after more than three decades, Kaufman said. He noted that institutional knowledge had been lost because Mexico had been free of the pest for about 35 years.

In the U.S., the screwworm is classified as a foreign animal disease pest, meaning scientists have been barred from studying it in research facilities for the past 50 years, Kaufman said. Researchers have lacked basic knowledge such as which odors attract the fly, information needed to create effective bait traps. The U.S. Department of Agriculture has started funding research in this area.

Radachowsky said conservationists are “extremely worried” that the current situation could leave the door open for the transmission of other livestock-borne illnesses.

“It frightens me that we’re not learning the main lesson here,” he said, adding that more disease monitoring is needed. “There’s a lot of wildlife that’s probably disappearing without anybody ever having any evidence of it. It’s guaranteed that we’re just scratching the surface.”