TEXAS — Veterinarians and humane societies issued warnings on Wednesday, July 8, 2026, regarding New World screwworm infestations in Texas and New Mexico. The parasite reappeared in U.S. cattle in June 2026.

There are more than 30 confirmed instances of New World screwworm in Texas and New Mexico, with two cases identified in dogs. The infestation is present in southern and southwestern Texas and southeastern New Mexico. New World screwworm larvae can hatch and breed in any mammal, including wildlife, dogs, cats, and occasionally humans.

Shelters across Texas are administering prescription flea and tick medications to animals to prevent screwworm infestations. Veterinarians and humane society officials recommend pet owners use similar medications for prevention. Melissa Stansell, a veterinarian at Austin Pets Alive!, said, "The chemical compositions of those products are what kill the actual larval stages of these flies." She added, "It will kill the larvae as they ingest the blood and tissue."

Aaron Grady, executive director of the Houston Humane Society shelter, said, "Even a tick bite can host an infestation." Stansell said other signs include a foul smell and restlessness or anxiety in an animal, or an animal "hyper-fixating on looking or chewing in a certain area of the body." Screwworm infestation can cause death from shock due to pain, or from larvae moving into vital organs or causing infections. Stansell said, "It is only fatal if left untreated." Veterinarians can treat infestations with methods that may include antibiotics.

State and U.S. Department of Agriculture (USDA) officials are breeding sterile male New World screwworm flies and releasing them into the wild to eradicate the pest. A female New World screwworm fly mates once in her monthslong life, and if she mates with a sterile male, her eggs will not hatch. The New World screwworm was largely eradicated from the U.S. more than 50 years prior to its reappearance in June 2026. The U.S. cattle industry is valued at $113 billion.

The New World screwworm fly is a tropical species that historically disappeared annually with colder fall or winter weather. The fly migrated north from Panama starting in 2024, moving through Mexico in 2025. For years, the only factory breeding sterile flies in the Western Hemisphere was located in Panama. The USDA invested $21 million to convert a site in southern Mexico from breeding fruit flies to breeding screwworm flies. The USDA plans to spend $750 million on a new fly factory in Texas, which is set to open in 2027.