What is New World screwworm and what should Texas horse owners know about it?

New World screwworm (NWS), scientifically known as Cochliomyia hominivorax, is one of the most destructive parasites of livestock in the Western Hemisphere. Unlike ordinary blowflies that feed on dead tissue, the female screwworm fly lays her eggs at the edge of an open wound on a living animal. The larvae then burrow into healthy flesh, which is why early awareness matters so much for horse owners (USDA APHIS). Horses are susceptible. Any break in the skin - a fly bite, a scratch from fencing, a castration site, a foal's umbilical stump, or the vulva of a mare that has just foaled - can attract an egg-laying female. This article is for general educational awareness and is not veterinary advice. If you suspect screwworm or have concerns about a horse's wound, contact your veterinarian promptly. ## A brief history of screwworm in U.S. livestock Before eradication, screwworm was an annual catastrophe for American ranching. The sterile insect technique, pioneered by USDA scientists Edward Knipling and Raymond Bushland, eliminated the pest from the continental United States by 1966 and pushed it south of the Darien Gap in Panama, where sterile flies are still released to hold a biological barrier (USDA APHIS / COPEG). That barrier was breached in 2023, and the pest moved north through Central America into Mexico. USDA suspended live cattle, bison, and horse imports from Mexico in November 2024, and after more than a year of joint eradication work announced a phased reopening of southern ports of entry beginning in August 2026, contingent on continued progress and risk assessment. In 2026 the Texas Animal Health Commission established a New World Screwworm Infested Zone covering parts of South and West Texas, with movement restrictions on warm-blooded animals leaving the zone. The situation is evolving - for the current status, see the USDA's official tracker at screwworm.gov and the Texas Animal Health Commission's screwworm page, both linked below. ## Signs that warrant prompt veterinary evaluation Only a veterinarian - with laboratory confirmation - can diagnose screwworm. No photograph, odor, or wound appearance proves it. That said, Texas Animal Health Commission guidance describes general warning signs in livestock and horses that deserve prompt professional attention: - A wound that appears to be enlarging or deepening rather than healing - Blood-tinged discharge or a foul odor from a wound - Visible larvae in or around a wound - Restlessness, decreased appetite, isolation from the herd, or signs of pain Sites worth checking during your daily routine include any fresh wound (especially on the lower legs), the navel of newborn foals, the vulva and perineum of recently foaled mares, and recent castration or branding sites. If you find larvae in a wound, do not assume they are harmless - and do not try to treat it as screwworm yourself. Contact your veterinarian promptly. Screwworm is a federally reportable disease, and Texas owners can also report a suspected case to the Texas Animal Health Commission's state veterinarian on call at 1-800-550-8242. Your veterinarian and TAHC will advise on specimen collection and any next steps. ## Why prompt veterinary attention matters Because screwworm larvae feed on living tissue, an untreated infestation can become life-threatening quickly, particularly in foals. Treatment decisions - including any medications, wound care, and laboratory confirmation - belong entirely to your veterinarian, who will follow current USDA APHIS and state animal-health guidance. The single most important thing an owner can do is call the vet early rather than attempt home treatment. ## Prevention: general awareness for the barn There is no vaccine for screwworm. Prevention is management-based, and the same habits that protect horses from ordinary wound complications are what authorities recommend: - Inspect every horse daily and know what normal healing looks like. - Address every wound promptly - even small ones - and keep wounds clean and protected from flies. Ask your veterinarian which wound-care products they recommend for your situation. - Practice good foaling hygiene and check the foal's navel and the mare's hindquarters daily after foaling; your veterinarian can advise on navel care. - Maintain sensible fly control: regular manure removal, and fly-management methods your veterinarian or extension agent recommends for your barn. - Ask your veterinarian about quarantine and inspection practices for new arrivals, especially horses traveling from Mexico, Central America, or affected areas. - Report suspicious wounds promptly. Early reporting is what allowed the United States to eradicate screwworm the first time. At Circle Lake Ranch we inspect our horses daily and work closely with our equine veterinarian on wound care and fly management. We are horse people, not veterinarians - for anything beyond general awareness, your vet is the right call. Questions about our lessons or camps are always welcome at (281) 395-4311. ## Official resources - USDA APHIS - New World Screwworm current status: https://www.aphis.usda.gov/livestock-poultry-disease/cattle/screwworm (and screwworm.gov) - Texas Animal Health Commission - New World Screwworm information and infested-zone updates: https://www.tahc.texas.gov/emergency/nws.html - Texas Department of Agriculture - New World Screwworm FAQ (including how to report): https://texasagriculture.gov/New-World-Screwworm-FAQ - Merck Veterinary Manual - Screwworm Myiasis in Animals (clinical reference for professionals) ## Citations - USDA Animal and Plant Health Inspection Service. New World Screwworm. https://www.aphis.usda.gov/livestock-poultry-disease/cattle/screwworm - Texas Animal Health Commission. New World Screwworm Infested Zone. 2026. https://www.tahc.texas.gov/emergency/nws.html - Texas Department of Agriculture. New World Screwworm FAQ. 2026. https://texasagriculture.gov/New-World-Screwworm-FAQ - Congressional Research Service. New World Screwworm: Background and Issues for Congress. August 2026. - Merck Veterinary Manual. Screwworm Myiasis in Animals. 11th ed. - American Veterinary Medical Association. News coverage of the 2026 phased port reopening.