Screwworm Is Back in the U.S. — 60 Years After We Eradicated It

That barrier held until it didn't.

Why did the parasite come back?

Several forces converged. Climate change expanded the warm, humid conditions screwworm needs, reducing seasonal die-off and adding generations per year. Political instability in Panama, drug-trafficking activity and heavy migration through the Darién Gap weakened the barrier. U.S. budget cuts — including early-2025 reductions by the Trump administration's U.S. DOGE Service to USDA's Animal and Plant Health Inspection Service and the end of a USAID monitoring program in Central America — thinned surveillance. Mexico-U.S. livestock import rules relaxed and tightened repeatedly, opening windows for infected animals, and illegal crossings continued regardless.

Researchers also worry that wild flies may be evolving to reject mass-reared sterile males, a behavior called assortative mating that would undercut SIT itself.

A 2016 outbreak in Florida's Key deer — the first U.S. cases in decades — foreshadowed the return and exposed the same weaknesses: slow early detection and wildlife populations with no natural resistance.

The warning signs then accelerated. In November 2024, screwworm turned up in a cow at a checkpoint in Chiapas, Mexico. By late 2025, cases appeared in northern Mexico within about 100 miles of Texas. The USDA moved sterile fly dispersal into Texas in January 2026 and opened a dispersal facility at Moore Air Base in February — the first domestic installation of its kind. It was not fast enough.

What is being done now?

The core response remains sterile fly release. Moore Air Base has released 100 million flies weekly since February 2026, and a second facility there is projected to start in 2027, scaling to 300 million flies per week. More flies are going to Mexico.

On June 16, the USDA announced roughly $105 million for 40 projects. Only a few tools are near-term usable:

• NovoFly, a genetically engineered male-only strain. Conventional programs must rear and sterilize both sexes; a male-only strain sharply cuts waste. The EPA granted an emergency exemption in June, but Mexico has not approved it, and as of November it was not yet deployed.
• eBeam sterilization, which uses high-energy electrons instead of the radioactive isotope cobalt 60. Lab results look promising, but retrofitting production facilities would take years.
• Swormlure-5, a synthetic wound scent that lures adult flies into traps; paired with insecticide it has cut local fly populations by up to 90 percent and now runs in border surveillance traps.
• CRISPR-based sterile males remain many years away.

Surveillance has gone modern. Drones with thermal sensors and high-resolution cameras surveyed livestock and wildlife across Texas ranches — more than 2,000 flights and nearly 40,000 animals observed between June 4 and October 22. AI tools in development will flag suspicious animals in imagery, while predictive models combine weather, habitat and outbreak data to forecast spread. Trail cameras and thousands of insect traps add layers.

What counts as success?

After hunting season began in September, the USDA confirmed the first wildlife detection — an elk killed on a game ranch in Pecos County, Texas, on October 21 — followed by a second elk case and a dog in Arizona. Nearly four dozen new domestic detections landed in November alone, and veterinarians fear a foothold in deer, elk or feral hogs sharing range with livestock. Many U.S. vets have never seen a case, which makes rancher education and rapid diagnosis essential.

Progress will look like sustained declines in new infestations and wild-fly detections. Historically, programs treat six months without detection as a technical benchmark and 12 months as grounds to declare an area free. The tools exist; the challenge is deploying them at scale long enough to matter.