How to Protect Your Pets from the New World Screwworm Fly

Veterinarians and animal shelters are warning pet owners in Texas and New Mexico to watch their animals closely. A harmful parasite called the New World screwworm fly has been found in more than 30 cases in those two states, including two cases in dogs. The fly lays eggs in open wounds, and the hatched larvae eat live flesh — which can be deadly if not caught in time. Experts say knowing the warning signs and acting fast can save an animal's life.

The screwworm fly is not new to the United States, but it had been mostly wiped out for more than 50 years. It came back in cattle in June 2025, after spreading north through Panama and Mexico. The pest is actually the larvae — the worm-like young — of the screwworm fly. Unlike most fly larvae, which eat dead material, these eat the living flesh and fluids of animals.

Any mammal can be affected by this parasite. That includes cattle, wildlife, dogs, cats, and even people in rare cases. The female fly lays her eggs in open wounds or areas with mucus. After the eggs hatch, the larvae feed for about a week before they drop to the ground and grow into adult flies. Newborn animals and animals with fresh wounds or recent surgeries are at the greatest risk.

Pet owners in southern and southwestern Texas and southeastern New Mexico should check their animals often for cuts, wounds, and bites. One key warning sign is seeing maggots or movement inside a wound. A bad smell coming from an animal, or an animal acting restless and anxious, can also be signs of an infestation. Veterinarian Melissa Stansell said owners should also watch for animals that seem focused on chewing or looking at one spot on their body. Any one of these signs means it is time to visit a vet right away.

The condition is very painful for animals, and the pain alone can cause death from shock. The larvae can also move into vital organs or cause deadly infections if nothing is done. But the good news is that it can be treated. Stansell said treatment may include antibiotics and other care, and she stressed that the disease is only fatal if left untreated.

Animal shelters across Texas are already giving pets prescription flea and tick medications to prevent infestations. These medications work by killing the larvae when they try to feed on the animal's blood and tissue. Experts recommend that all pet owners in affected areas talk to their vet about using the same kind of medication. Catching a problem early — or stopping it before it starts — is the best way to keep pets safe.

The U.S. government is also working to wipe out the fly for good. Officials are using a method that worked decades ago: they breed male flies that cannot reproduce and release them into the wild. When a female mates with one of these sterile males, her eggs never hatch. Over time, this causes the fly population in an area to shrink and disappear. The USDA recently spent $21 million to set up a new breeding facility in Mexico and plans to spend $750 million on another facility in Texas, set to open next year.