The United States spent about two decades and released more than 94 billion sterile male flies to wipe out the New World screwworm, declaring the flesh-eating livestock parasite eradicated in 1982. Since 2023, the parasite has pushed north from Panama through Mexico, crossing into Texas on June 3, 2026, in the first confirmed U.S. animal case in more than four decades. Researchers at the University of California San Diego say the pest is moving faster than expected and have built a county-by-county risk dashboard meant to catch new detections before laboratory confirmation, which can take days or weeks.
Ninety-Four Billion Sterile Flies and a 1982 Victory
New World screwworm larvae hatch from eggs a female fly lays in an open wound or a mucous membrane, then burrow into living tissue rather than dead flesh, deepening the wound each time it is disturbed, a feeding motion that gave the insect its common name. A single female mates only once and carries that one mate’s sperm for the rest of her life, a reproductive quirk that made the species unusually easy to erase from a landscape once enough sterile males were competing with fertile ones for mates.
Between 1962 and 1975, the United States and Mexico bred and released more than 94 billion sterile flies across the two countries, a campaign that built on smaller trials on the island of Curacao in 1954 and cleared the pest from the southeastern and southwestern United States by 1960. Screwworm was declared eradicated from North America by 1982, and the effort is still taught as one of the more successful biological pest-control campaigns ever run.
The Line That Broke in Panama and Kept Moving
That eradication held for more than four decades until the parasite breached the biological barrier long maintained near the Panama-Colombia border, spreading through Panama and Costa Rica in 2023 and then up through the rest of Central America and Mexico. Regional agriculture and health agencies have logged more than 209,400 animal cases across that outbreak, and the first confirmed U.S. detection followed on June 3, 2026, in Texas.
New Mexico reported confirmed cases soon after, and by September the U.S. Department of Agriculture’s own tracking dashboard was logging detections by county, species and confirmation date across both states, treating wildlife and pet cases the same as livestock ones. The dashboard shows active detections concentrated in Texas and New Mexico, the same two states where the insect regained its foothold decades after eradication.
A Dashboard Built Because Lab Results Take Weeks
San Diego County’s public health department asked University of California San Diego researchers for a faster way to see the parasite coming, after confirmed lab results kept arriving well behind informal word of a likely case. Seema Shah, the county’s medical director for epidemiology and immunization services, said traditional reporting mechanisms for public health are often delayed, and lab confirmation can take days or weeks.
The tool the team built, called the New World Screwworm Risk Dashboard, folds confirmed detections together with weather and vegetation data and even news coverage, on the theory that a local news report of a suspicious livestock wound can flag a county days before a lab confirms it. It is currently limited to public health officials and the county and university partners who built it, with a public version still in development.
Mark Beatty, the county’s assistant medical director for epidemiology, said the team had expected the parasite’s advance for years but not its pace, describing the insect’s return to Texas this way: “It’s in a place where it used to live, so it’s not surprising it’s taking hold again.” Natasha Martin, an infectious-disease epidemiologist on the project, and Eliah Aronoff-Spencer, the physician who leads the design lab behind the dashboard, said the same surveillance approach could extend to other emerging threats county health departments are asked to track with shrinking lab capacity.
Sterile Flies Again, and a Genetically Modified Backup Plan
On the ground, the U.S. Department of Agriculture is running the same core strategy that worked the first time: breeding and releasing sterile male flies so wild females mate with a partner that cannot produce offspring, alongside quarantines on animal movement out of infested counties. Announcing a new case in La Salle County, Texas, in June, the agency said it was releasing sterile insects and adjusting these operations as needed for the most effective coverage.
The Department of Agriculture has also sought federal approval for a genetically modified all-male strain of the fly, already in field testing as a faster-to-produce backup to the original sterile-fly technique, while the Centers for Disease Control and Prevention classifies the current risk to the public as very low. The single human case tied to the outbreak was a traveler returning from El Salvador in August 2025, not a domestic infection.
For the ranchers and county health officers watching the dashboard update in real time, the open question is how much ground the parasite covers before the sterile-fly campaign catches up to it again, the way it did the first time.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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