New World screwworm, or Cochliomyia hominivorax, is a parasitic blow fly. As an adult, the fly causes little harm but becomes a hazard because it lays eggs in open wounds. The eggs develop into screw-shaped larvae (maggots). Unlike more common species of maggots that eat dead material, screwworm larvae eat living tissue. They cause a devastating impact on livestock when the blow fly lays eggs in open wounds. It gets its name from the habit of burrowing (or screwing itself) into the wound.

The parasite also targets wildlife, pets or rarely humans. By 2016, efforts eradicated the pest in the United States. This was done through the mass release of sterile male flies (sterile insect technique). Since wild female flies mate just once, if that encounter is with a sterile male, outbreaks can eventually be halted as the flies die out. However, the pests continued to victimize animal hosts in Central America and migrated north into Mexico by 2024. But now, the screwworm has crossed into the United States.

New World Screwworms Back in the U.S.

According to Jeffrey Collins (AP) reporting on PBS, as of June 8, there are five confirmed cases: three calves and a goat in Texas and a dog from neighboring Lea County, New Mexico. The dog, which the USDA initially reported as a Texas case, lives in New Mexico and was reclassified as the first in that state. The animal’s travel history is being investigated.

On June 8, 2026, Texas Agriculture Commissioner Sid Miller responded to three new New World screwworm (NWS) detections. These include those identified in Central and West Texas and the first confirmed case in New Mexico. He warns that experts anticipate the parasite will continue to spread into Texas. Commissioner Miller renewed his call for USDA to immediately deploy the Screwworm Adult Suppression System (SWASS), a proven USDA-developed program that uses targeted bait to kill fertile adult screwworm flies before they can reproduce. This protocol proved effective in the initial eradication of the pest from the United States. On June 11, 2026, the seventh and eighth cases of NWS were confirmed- one in a Zavala County bovine and one in an Edwards County goat.

As a result of the reported cases, Canada has temporarily stopped the importation of cattle and horses. Winter kills most of the flies each year, so one long-term goal includes breeding enough sterile flies to release them in 2027 and stop the pests from reproducing.

“The cost of prevention is measured in millions. The cost of an outbreak in Texas would be measured in billions. The science is settled. The tools are available,” Miller added. Read his full response here.

How Do Screwworms Affect Pets

Dr. Guilherme Verocai, an associate professor and director of the Parasitology Diagnostic Laboratory at the Texas A&M College of Veterinary Medicine & Biomedical Sciences, says the female fly zeroes in on tiny wounds like scratches, or even a healing surgery wound. The fly lays a couple hundred eggs along the edge of the lesion. They hatch within 12 hours and begin eating the living tissue. Dr. Verocai says this species requires living animals to reproduce and survive. The hundreds of maggots grow as they feed, and can quickly cause massive damage to the tissue.

Stray dogs and cats remain at highest risk for infestation. Pay particular attention to pets with histories of squabbles such as cats with abscesses or doggy athletes with a skinned-up paw or leg. You won’t see these blow flies lighting on your pet, either. They instead hang out in vegetation until ready to lay eggs. Because the maggots quickly cause life-threatening extensive damage, stay vigilant and seek veterinary care immediately. Prompt removal of the worms usually means your pet should quickly recover with supportive care.

Watch For These Signs

• Check for wounds daily, including sores from tick bites or seeping/weepy sores
• Alert to bad smells of tissue decay
• Review sores for tissue damage or signs of maggots (worms)
• Prevent fighting or other injury risks
• Be vigilant keeping wounds clean (such as spay/neuter, or other surgeries)
• Watch for lethargy, loss of appetite, or unusual irritability or aggression–or wound licking behaviors.

Screwworm Treatment

Should you discover evidence of this parasite, see your vet asap. Treatment to remove the maggots and clean out the wound may require sedation, so never try to DIY treatment. In addition, your veterinarian will need to collect samples and report the issue. The presence of maggots on a wound could instead be the more common type that feeds on decayed material rather than live tissue. Of course, that’s still distressing and needs attention, but leave it to your vet to diagnose the issue.

Texas A & M says that up to 10 larvae samples may be stored in 70 percent ethanol and submitted to the National Veterinary Services Laboratory for identification. All maggots that are not submitted must be killed, either by freezing or in alcohol.

Protecting Pets

Currently, there are no treatments or preventives FDA-tested and approved for pets for this parasite. However, the FDA has provided a list of approved/conditionally approved, and emergency use authorizations for animals dealing with potential or actual screwworm infestations. Your veterinarian can review and make decisions based on the information on this page.

According to Dr. Kate Elden, DVM and Chief Medical Officer at Dutch, the 24/7 online vet, the single best defense against screwworm is keeping pets on routine, year-round prescription flea and tick prevention like Credelio Quattro, Simparica Trio, and NexGard Plus. If your pet has an isoxazoline on board, they are not going to get screwworm. These products clear screw worm infestations and reduce the bites and the scratching that create the small wounds these flies need to lay their eggs. Learn more about parasite prevention here.