Scientists are warning that current eradication efforts may not be sufficient to contain the new world screwworm, a parasitic fly that has become endemic in wildlife across Central America and is pushing northward into the United States. The fly's spread has been accelerated by illegal cattle trafficking, which moves infected animals across borders without health checks.
Wildlife Infected Deep in Forests
Camera traps set up by conservationists in remote Central American forests, intended to monitor illegal cattle movement, have captured alarming images of wildlife—including jaguars, pumas, tapirs, deer, white-lipped peccaries, and even porcupines—covered in wounds caused by the screwworm. Jeremy Radachowsky, director of the Mesoamerica and Caribbean program at the Wildlife Conservation Society, noted that infestations are now occurring in the deepest parts of the forest, far from 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," he said.
Current US Detections and Eradication Efforts
In the United States, screwworm has been detected in 34 animals, mostly in Texas, with one case in New Mexico. So far, only livestock and pets have been affected, with no wildlife detections. To slow the northward spread, the US is dropping 100 million sterile flies in the southwest and Mexico. However, experts say eradicating the fly from the region would require about 500 million sterile flies. "What we lack are sufficient flies in order to start pushing the population back south," said Phillip Kaufman, professor and head of entomology at Texas A&M University.
Expanding Sterile Fly Production
Officials are rushing to expand sterile fly breeding capacity. A new facility opened in Mexico in late June, and another is planned for Texas in late 2027. The sterile flies are irradiated so that males mate with females without fertilizing eggs, causing the population to crash. Potential innovations include raising only sterile male flies and designing better bait traps, but these methods require time to develop. "We have to have things that work," Kaufman 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."
Root Cause: Illegal Cattle Trafficking
Radachowsky argues that current efforts focus on the fly itself rather than addressing the root cause: illegal cattle trafficking. "They’re either eradicating the fly, making a fly that can’t reproduce or trying to trap the fly. What they’re not doing is addressing the root cause of the cattle trafficking," he said. He noted that the livestock industry has changed significantly since the flies were eradicated in 1966. "There’s this expectation and this simple argument that the sterile fly technique worked once, so it will work again. But the problem is in the 60s, when we first eradicated screwworm, we didn’t have this high density of cattle like we do now."
Rapid Spread Along Trafficking Routes
The screwworm crossed the Darién Gap in 2022 and moved north through Central America, covering thousands of kilometers in just four to five months. "At the speed of a truck, and exactly along those illegal cattle-trafficking routes that we had already documented," Radachowsky said. Kaufman agreed that illegal animal movement is the primary driver. "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," he said, referring to transport of livestock or pets.
Challenges in the US and Mexico
Alarm bells began ringing 18 months ago when the screwworm re-entered Mexico after 35 years. Many experts who had dealt with the pest have retired or passed away. In the US, the screwworm is classified as a foreign animal disease pest, restricting research. "We were not permitted to have the fly in research facilities in the US for the last 50 years, because it’s such a dangerous organism," Kaufman said. Scientists are now starting work on bait traps, funded by USDA grants.
Broader Disease Risks
Conservationists are "extremely worried" about the potential for infectious disease transmission, not just screwworm but also other livestock-borne illnesses such as tuberculosis, brucellosis, hoof and mouth disease, and bird flu. "It frightens me that we’re not learning the main lesson here," Radachowsky said, calling for more disease monitoring in wildlife populations across the Americas. "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."
