A working horse in a Texas border county has tested positive for the New World screwworm, the first confirmed equine case in the United States this century, as the parasite's northward spread from Mexico shows no sign of slowing.
The U.S. Equestrian Federation confirmed the infestation on the hind limb of the horse in Presidio County, a remote stretch of West Texas that sits directly on the Mexican border. The case marks a grim milestone in an outbreak that began earlier this year when an infected cow turned up in Zavala County, roughly 120 miles southwest of San Antonio. Since then, 48 screwworm cases have been confirmed across Texas and one in New Mexico, that one on a dog, The Guardian reported.
The New World screwworm is not a worm at all. It is the larval stage of a fly that lays eggs in open wounds on warm-blooded animals. The larvae burrow into living flesh and feed, widening the wound and inviting secondary infection. Left untreated, infestations can kill. The USDA has warned that all mammals are at risk, livestock, wildlife, pets, and in rare cases, people and birds.
Texas Agriculture Commissioner Sid Miller sounded the alarm months before the first domestic case was confirmed. Speaking to San Jacinto County Republicans in February, Miller warned that swarms were already active deep inside Mexico during what should have been their dormant season.
"We've got swarms of screwworm flies 187 miles south of the Rio Grande, and it's winter. We shouldn't have any active flies right now, but it's been a mild winter. If they're active in the middle of winter, look out when spring or early summer gets here."
Spring and early summer arrived. So did the screwworm. By June, an infected calf was detected near the border in Zavala County, marking the start of the domestic incursion. The count has climbed steadily since.
In an exclusive interview with Breitbart Texas, Miller put the threat in blunt terms.
"This will impact everybody in some form. Livestock, wildlife, pets, even people. Ranchers, deer hunters, pet owners, get ready."
That warning now reads less like caution and more like prophecy. The parasite has moved from cattle to dogs to horses, crossing county lines and at least one state border along the way.
Federal officials have not been idle. The USDA recently announced that two zones in Texas had been cleared of screwworm infestation, the first such clearances since the outbreak began in June. Zone 2 covers portions of La Salle and Webb counties in South Texas. Zone 3 spans parts of Gillespie, Kerr, and Kimble counties in the Hill Country, well north of the border.
USDA Under Secretary for Marketing and Regulatory Programs Dudley Hoskins credited state cooperation for the progress.
"The Texas Animal Health Commission has been an outstanding partner throughout our response to New World screwworm. Their vigilance, on‑the‑ground expertise, and unwavering commitment to protecting livestock and wildlife have been essential to our shared success."
Two cleared zones is a start. But the USDA established those zones in early September, and the Presidio County horse case surfaced after that announcement, in a county hundreds of miles west of the previously affected areas. The parasite is not retreating in a straight line. It is popping up in new territory even as old ground gets cleared.
The U.S. Equestrian Federation issued detailed guidance in the wake of the Presidio County case, urging horse owners across the region to treat every open wound as a potential entry point.
"Horse owners should inspect their horses every day for wounds, swelling, drainage, foul odor, or maggots. Pay close attention to small cuts, surgical sites, the umbilical area of foals, and moist areas such as the lips, nostrils, eyes, and genital areas. If you see maggots or anything unusual in a wound, contact your veterinarian right away."
The federation also called on owners to practice strict biosecurity: keep wounds clean and covered, use veterinarian-recommended fly control products, and limit fly access with physical barriers like fly masks, sheets, and clean stabling.
"We encourage every horse owner to help protect their horses and the broader equine community by practicing good wound care, fly control, and biosecurity."
That guidance applies well beyond the show-horse circuit. Presidio County is ranch country. Working horses there are exposed to brush, barbed wire, and rough terrain every day, exactly the conditions that produce the small wounds screwworm flies seek out.
The numbers tell a story of steady escalation. The incursion started with a single infected cow in Zavala County. It spread to 48 confirmed cases across Texas and crossed into New Mexico. It jumped from cattle to a dog, and now to a horse. Each new species and each new county represents a failure to contain the parasite at the border, where it entered from Mexico.
Several basic questions remain unanswered. No one has publicly identified the owner of the Presidio County horse or disclosed the animal's current condition. The exact date the infestation was discovered or confirmed has not been released. And it is unclear whether the 48 confirmed Texas cases include the Presidio County horse or whether that number will climb again when updated.
Equally unclear is whether any human or bird cases have emerged during this incursion. The USDA has acknowledged both are possible, if rare. No confirmed cases in either category have been reported in the material available.
The screwworm was effectively eradicated from the United States decades ago through a massive sterile-fly release program, one of the great success stories of American agriculture. Its return, spreading northward from Mexico into Texas ranch land, is a reminder that borders matter for biosecurity just as much as they matter for national security. When threats move freely across the line, Americans on this side pay the price.
