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McALLEN — As the New World screwworm continues to infiltrate livestock and other animals in Texas, many have been asking how the parasitic fly landed here to begin with.
Last week, the Texas Animal Health Commission identified a potential cause: small wildlife and rodents like armadillos, opossums and rabbits.
Until now, the U.S. Department of Agriculture has pointed to multiple factors, including border policies under President Joe Biden to the illicit movement of cattle at the hands of drug cartels.
The new finding is based on conversations with entomologists, Lewis R. “Bud” Dinges, executive director of the Texas Animal Health Commission, told the Texas House Committee on Agriculture and Livestock.
However, the source of the first case of New World Screwworm remains under investigation, according to the Texas Parks and Wildlife Department. Other health experts say it’s still undetermined what allowed the invasive pest to finally breach the Texas-Mexico border.
Tracing the source
During last week’s committee hearing, Dinges said epidemiological investigators have found no evidence so far linking Texas cases to the illicit movement of cattle from Mexico.
The Texas Parks and Wildlife Department said it is still investigating to determine how screwworm spread.
“That’s very much an unknown, still, at this time. But wildlife is susceptible to New World Screwworm in the same manner that livestock and other warm-blooded animals are,” said a spokesperson for Texas Parks and Wildlife.
However, the spokesperson added that small mammals don’t travel long distances such as the distance between the last known detection in Mexico at the time to the location of the first case in Texas, which was detected in LaPryor on June 3.
Moving north
The USDA has repeatedly mentioned that models predicted that screwworm would inevitably arrive in the U.S. after the parasitic fly began moving north from South America in 2023.
The pest began trickling up through Panama after it broke through the Darien Gap, which had served as a barrier for screwworm for decades.
It then slowly moved through Costa Rica until it reached Nicaragua where it traveled quickly, said Jeremy Radachowsky, Mesoamerica and the Caribbean regional director for the Wildlife Conservation Society.
“Not only was it moving very quickly, but it was moving exactly along these paths of cattle trafficking and cattle contraband that we’ve been able to identify earlier,” Radachowsky said.
Screwworm detections followed those cattle-trafficking paths into Honduras and Guatemala. A few weeks later, Mexican officials detected their first case in November 2024.
Screwworm was predicted to have arrived in the U.S. last summer, USDA officials said, but efforts to stop it delayed it for a year.
“We’ve been actively and vocally warning that in order to stop screwworm, you have to stop this illegal and unregulated movement of cattle from south to north,” Radachowsky said. “That is definitely the driver.”
But how it crossed from Mexico into the U.S. remains unclear, he said.
The USDA closed all southern ports of entry to livestock imports from Mexico in May 2025 and have kept them closed since then, preventing cattle from legally crossing into the U.S. from Mexico.
The Wildlife Conservation Society does not have clear information on how screwworm crossed into Texas, Radachowsky said, but noted that it can infest and travel with other warm-blooded animals like pets and wildlife.
“At the Texas-Mexico border, you’ve got feral pigs, white tailed deer, other wildlife, basically moving back and forth as well,” he said.
The unknown
There are 13 active cases of New World Screwworm in Texas as of Tuesday. An average of 15 suspected cases are reported to the Texas Animal Health Commission every day, Dinges said during the committee hearing last week.
State Rep. Ryan Guillen, a Republican from Rio Grande City who chairs the committee, asked whether it would be logical to assume there are more cases between the Texas-Mexico border and the location of the confirmed infestations that just haven’t been reported.
Dinges replied that testing for screwworm has been ongoing for over a year and cases had not been detected until now.
“We’ve been submitting anywhere from two to six larvae samples a week since last May and we have not detected any New World Screwworm larvae until June 3,” Dinges said.
Despite those assurances, farmers and ranchers throughout Texas are operating under the assumption that screwworm is present in their area.
“There’s just so much country that’s unsurveilled,” said John Sewell, a rancher from Kinney and Uvalde County said during the hearing. “I’m in between two — one south of me and one north of me. Do I think I don’t have it? I would be a fool to think I didn’t have it.”
Reporting in the Rio Grande Valley is supported in part by the Methodist Healthcare Ministries of South Texas, Inc.