What to know about flesh-eating screwworms

The New World screwworm is a dangerous parasitic fly that has reemerged as a growing concern for animal health across parts of North America. Unlike common maggots that feed on dead tissue, screwworm larvae burrow into and consume the living flesh of warm-blooded animals, including cattle, horses, pets, wildlife, and, in rare cases, humans.

The pest was once widespread across the southern United States but was declared eradicated in the country in 1966 after decades of control efforts using sterile flies. In recent years, however, the parasite has spread northward from Central America and Mexico, prompting increased surveillance and response efforts.

Ranchers, veterinarians, pet owners, and wildlife officials have been urged to remain vigilant and report any suspected cases involving animals with unusual, non-healing wounds.

Everything to know about screwworms and whether they are in Arizona.

Are screwworms in Arizona?

Though cases have been detected in New Mexico and Texas, there were no confirmed cases of the New World screwworm in Arizona as of July 8. The Arizona Department of Agriculture and the Arizona Game & Fish Department were actively enforcing import bans, setting up fly traps and distributing sampling kits to prevent their spread into the state.

How many cases of screwworm are there in the US?

As of July 2026, the United States has 32 confirmed cases of New World screwworm in animals, according to the latest updates from the United States Department of Agriculture. Most cases have been reported in Texas, with a smaller number in New Mexico.

For people, there have been no locally acquired human cases in the current U.S. outbreak. The only recent U.S. human case was a travel-associated infection in 2025 involving a person who had returned from El Salvador.

How do you protect pets from New World screwworm?

Protecting pets from New World screwworms start with preventing and caring for wounds. The flies are attracted to open cuts, scrapes, surgical incisions and other skin injuries where they can lay their eggs. According to the USDA, pet owners should inspect their animals regularly, especially after spending time outdoors, and clean and treat any wounds promptly. Keeping injuries covered when possible and following a veterinarian's care instructions can help reduce the risk.

Keeping your pet's coat clean and free of mats makes it easier to spot cuts, skin irritation or other problems before they become serious. Pay close attention to areas around the ears, tail, paws and hindquarters, where wounds or infections can sometimes go unnoticed.

What kills screwworms in dogs?

If a dog has a New World screwworm infestation, it requires immediate veterinary treatment. The larvae feed on living tissue, and delaying care can allow the wound to become much more severe.

Treatment typically involves physically removing all of the larvae, thoroughly cleaning and disinfecting the wound and treating any damaged or infected tissue. Veterinarians may also prescribe antibiotics if there is a secondary bacterial infection, along with pain medication and supportive care to help the dog recover.

In some cases, veterinarians use antiparasitic medications such as Ivermectin or Doramectin to help kill or eliminate remaining larvae, depending on the dog's condition and veterinary guidance.

If you suspect screwworm, don't try to treat it at home. Contact a veterinarian as soon as possible so the wound can be properly examined and all larvae removed, according to the FDA.

What is the mortality rate for New World screwworm?

Untreated infestations can be fatal, with animals sometimes dying within 7-14 days due to tissue destruction and infection, according to the National Cattlemen's Beef Association.

Newborn livestock are especially vulnerable. In newborn calves, mortality can be as high as 100% if they become infested, particularly through the healing umbilical cord.

So, while there isn't one overall mortality percentage, the parasite is considered potentially deadly if left untreated, which is why veterinarians and animal health officials emphasize rapid detection and treatment.

What animals does New World screwworm affect?

New World screwworm can affect any warm-blooded animal, including pets, livestock, wildlife, birds and even people in rare cases.

Animals that can be affected include: Cattle, horses, dogs, cats, sheep, goats, pigs, deer, elk, bison, bears and rabbits, birds with open wounds, zoo animals and other captive wildlife.

How to tell if a dog has screwworm?

The earliest sign of a New World screwworm infestation in a dog is often a wound that doesn't heal and instead becomes larger and more painful. Because the larvae feed on living tissue, the affected area may become swollen, red and produce a foul-smelling discharge, according to the American Kennel Club. Dogs may also constantly lick, bite or scratch at the wound because it is painful and irritating.

As the infestation progresses, pet owners may notice small, cream-colored larvae moving inside the wound. Other symptoms can include bleeding, tissue damage, lethargy, loss of appetite, fever and signs of pain. In severe cases, the wound can deepen rapidly as the larvae continue feeding on healthy tissue.