Federal and state animal health officials have confirmed the first equine case of New World screwworm (Cochliomyia hominivorax) in the United States this century. The detection, officially verified by the USDA Animal and Plant Health Inspection Service (APHIS) on September 9, 2026, involved a working ranch horse in Presidio County, Texas, marking a significant escalation in the ongoing domestic incursion.
The Presidio County Horse Detection
The screwworm larvae were discovered burrowing into an open wound on the hind limb of a working ranch horse in far West Texas, near the Mexican border. Unlike common blowfly maggots that feed solely on dead, necrotic flesh, New World screwworm larvae feed aggressively on living tissue.
For horses, limb wounds present severe clinical risks: screwworm larvae can quickly tunnel past skin and muscle layers into critical synovial structures, including tendon sheaths and joint capsules, threatening permanent lameness or death if not caught early. The infested horse in Presidio County remains classified as an active animal case on the USDA dashboard, with veterinarians carrying out targeted wound management and larvicide treatment until the animal is fully clear of myiasis.
In response to the equine confirmation, the Texas Animal Health Commission (TAHC) and USDA established an incident response zone covering portions of Presidio and Jeff Davis counties. Under these protocols, animal movement restrictions and mandatory veterinary inspections are in place to prevent infested stock or working horses from hauling out of the quarantine boundary.
Calving Season Vigilance for Cattle Producers
With fall calving underway across the region, cattle producers face an elevated threat because newborn calves represent the highest-risk target for New World screwworm flies. The moist, unhealed umbilical cord is a primary attractant for female screwworms to deposit eggs, with past outbreaks documenting navel strike rates as high as 50% in unprotected newborn calves. In addition to navels, fresh maternal birth trauma on dams—including vulvar tears and postpartum discharge—creates prime egg-laying sites. Cow-calf operators should thoroughly inspect cows and pairs daily, dip or spray every newborn calf’s navel stump immediately with a veterinary-approved antiseptic and fly barrier, and consult their herd veterinarian regarding preventive antiparasitic protocols authorized for newborn calves. Elective calf-working procedures that create open wounds, such as knife castration, dehorning, and branding, should be delayed until cooler late-fall temperatures significantly suppress adult fly activity, or paired with immediate larvicide wound dressings and approved systemic protection.
Current Outbreak Status from the USDA Dashboard
According to the latest USDA APHIS national tracking data, the current outbreak landscape includes:
- Active vs. Inactive Animal Cases: Across the country, only two animal cases remain in active treatment status: the Presidio County ranch horse and a working dog detected on August 31 in Crockett County, Texas. All previous detections have been reclassified as inactive, meaning the individual animals either recovered and finished myiasis treatment or were handled under strict biosecurity carcass protocols.
- Zone Restrictions Remain: Individual cases shifting to inactive status does not lift regional quarantine lines. State animal health authorities continue enforcing active infested and surveillance zones across affected Texas counties until strict biological and fly-free surveillance benchmarks are satisfied.
- Wide Host Range: While the summer outbreak predominantly hit beef cattle, sheep, and goats across Southwest Texas (including Crockett, Val Verde, Terrell, Pecos, Brewster, Sutton, and Edwards counties) along with a single canine case in Lea County, New Mexico, the horse detection underscores that screwworms do not discriminate among warm-blooded hosts.
- Fly Trapping and Sterile Releases: USDA APHIS continues operating fly trap lines along border transit corridors to track wild screwworm flies independently of livestock wounds. Ground release chambers and aerial drops delivering millions of sterile male flies per week remain active to break the pest’s reproductive cycle.
Critical Action Steps for Oklahoma Producers and Horse Owners
Because working ranch horses, stock dogs, and cow-calf pairs share the same pasture environment, Oklahoma producers must maintain intense biosecurity and daily observation:
- Examine Stock Daily: Inspect cattle, saddle horses, and dogs for cuts, wire scrapes, girth sores, tick bites, or navel infections. In horses, screwworm flies are especially attracted to leg scrapes, surgical incisions, and thin, moist skin around the lips, nostrils, sheath, and udders.
- Aggressive Wound Care: Clean and dress every fresh scratch or surgical wound immediately. Apply veterinarian-approved topical wound barriers and fly repellents to deny adult flies an egg-laying site.
- Look for Key Warning Signs: Watch for wounds that expand rapidly, weep foul-smelling dark fluid, or house visible masses of white larvae. Behavioral indicators include tail-swishing, limb stomping, lethargy, head shaking, and horses isolating themselves from the herd.
- Do Not Drop Maggots on the Ground: If you suspect screwworm larvae in a wound, do not wash or scrape them onto pasture dirt, where they can burrow into the soil, pupate, and emerge as breeding adult flies. Keep the animal on concrete or inside an enclosed stall, immediately contact your local veterinarian, and notify the Oklahoma Department of Agriculture, Food and Forestry (ODAFF) or USDA APHIS for official sampling.
To see more visit Screwworm.gov
