What to know about protecting pets from the New World screwworm fly

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Two New World screwworm cases in dogs are among more than 30 confirmed instances in Texas and New Mexico, prompting warnings Wednesday from veterinarians and humane societies that pet owners need to remain vigilant to protect their animals.

The parasite reappeared in cattle in the U.S. in June, more than 50 years after it had been largely eradicated from the country. The pest is actually the larvae of the New World screwworm fly. It eats live flesh and fluids rather than dead material, as the larvae of most fly species do.

Here is what to know about the parasite, the threat it poses to pets and how to protect them:

The fly's migration north from Panama starting in 2024, and through Mexico in 2025, has agriculture officials warning that it poses a threat to the $113 billion U.S. cattle industry, but the larvae can hatch and breed in any mammal, including wildlife, dogs, cats and occasionally humans.

The problem develops when a female fly lays its eggs in open wounds and mucus. After the eggs hatch, the larvae feed for about a week before maturing, dropping to the ground and continuing to develop into an adult fly.

The American Veterinary Medical Association says newborn animals and animals with open wounds or who have undergone surgery or other medical procedures recently are especially vulnerable. Even a tick bite can host an infestation, Aaron Grady, executive director of the Houston Humane Society shelter, said during a webinar on the screwworm.

Animal health experts say pet owners in areas where the screwworm is present — southern and southwestern Texas and southeastern New Mexico so far — should watch their animals closely and examine them for wounds, cuts and bites regularly.

Pet owners should look for any maggots or movement in a wound. 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," said Melissa Stansell, a veterinarian at the shelter Austin Pets Alive!

Any one of those is reason enough to go to a veterinarian. The affected animal is likely in a great deal of pain, and that can cause death from shock. The larvae also can cause death if they move into vital organs or by causing infections that turn deadly.

Humane society officials and veterinarians said shelters across Texas are trying to prevent infestations in animals by giving them prescription flea and tick medications. They recommend that pet owners do the same.

“It will kill the larvae as they ingest the blood and tissue,” Stansell said. “The chemical compositions of those products are what kill the actual larval stages of these flies.”

Veterinarians also can treat infestations and animals can recover if pet owners contact them quickly. Stansell said the treatment could include antibiotics.

“It is only fatal if left untreated,” she said.

The New World screwworm fly is a tropical species and decades ago would disappear each year when colder weather arrived with the fall or winter.

But state and U.S. Department of Agriculture officials aren't waiting for the weather to turn. They've returned to an eradication method that worked decades ago, breeding sterile male flies and releasing them into the wild. The female New World screwworm fly mates once in her monthslong life, and if her partner is sterile, her eggs won't hatch — causing the population in an area to drop and then disappear.

For years, the only factory breeding sterile flies in the Western Hemisphere was in Panama, but the USDA invested $21 million to convert a site in southern Mexico from breeding fruit flies to recently start breeding screwworm flies. The agency also plans to spend $750 million on a new fly factory in Texas, set to open next year.