First Wild Screwworm Fly Detected in Texas Since Outbreak Began
The US Department of Agriculture has caught its first wild screwworm fly since the parasite was detected in the country’s livestock nearly four months ago, signaling the potential for continued spread even as overall cases have remained muted.
The wild screwworm fly was detected in Brewster County, Texas, on Friday. It was caught in one of the USDA’s more than 100 screwworm-specific fly traps along the US border.
The spread of the parasitic fly, whose larvae burrow into the wounds of animals, has remained relatively contained, with just 50 cases in the US since June, according to the USDA. A horse in Presidio County — which is adjacent to Brewster — is one of only two active screwworm cases in the US; the second is a horse in New Mexico’s Grant County.
It has been unclear how the pest has spread across counties, as screwworm cases haven’t been found in wild animals, and wild flies have evaded government surveillance efforts until now.
“The limited number of detected cases is encouraging, but case numbers alone cannot tell us why spread has been limited or quantify the effect of any intervention,” Paolo Tizzani, the World Organization for Animal Health‘s senior veterinary epidemiologist, said Monday. He called for continued monitoring of wild animals and flies, noting that “surveillance results cannot by themselves establish that transmission has completely stopped.”
The detection of the wild fly could help inform the USDA’s dispersal of sterile male flies, which are meant to mate with wild female flies in order to curb the pest’s reproduction. The agency said in a Saturday post on X that it places traps “just outside of infested areas to avoid capturing sterile flies and better pinpoint where wild flies may be moving.”
🚨 USDA has confirmed the first wild New World screwworm fly in a trap in Brewster County, Texas.
Our surveillance system is working — using targeted fly traps, animal checks, drones, trail cams, boots‑on‑the‑ground monitoring, and a network of partners tracking this pest.
We…
— New World Screwworm Rapid Response (@Screwworm_RR) September 27, 2026
While no cases in wildlife have been confirmed, animal health experts say the upcoming hunting season in Texas could provide an opportunity for closer scrutiny.
The slow spread has nonetheless alleviated concerns for ranchers, while also allowing the US to ease a domestic cattle shortage by resuming livestock imports from Mexico.
The US had previously eradicated screwworm, pushing the pest south of Panama. But the fly began spreading north again after the pandemic, reaching Mexico in late 2024. The US halted livestock shipments from Mexico, and largely maintained that ban until late August when it began a phased reopening of ports.
Read More: US to Reopen Largest Port for Mexican Cattle Shipments
Still, a complete elimination of screwworm from the US likely won’t happen until at least next year, creating the potential for more bursts of cases. That’s as eradication will depend on a large sterile fly production facility in Texas that isn’t scheduled to open until April or May 2027. Currently, the US has been dispersing flies produced at smaller plants in Panama and Mexico.
“We are not out of the woods yet,” Agriculture Secretary Brooke Rollins said at a press conference earlier this month. “We’re still building the facilities for the the sterile flies, which we know is the one true way to eradicate. But we have made such amazing progress.”