A flesh-eating parasite known as the screwworm has made its way back into the U.S. Southwest for the first time in roughly five decades, raising serious concerns for the livestock industry, wildlife, and household pets alike.
What exactly is a screwworm?
The screwworm is a parasitic fly that targets warm-blooded animals. Female flies deposit their eggs inside open wounds, and once those eggs hatch, hundreds of larvae use sharp mouthparts to burrow through living tissue. Without treatment, the infestation can be fatal to the host animal. The parasite primarily spreads when infested animals are moved from one location to another. Experts note it poses no risk to food safety and rarely affects humans. Animals that receive prompt care — including maggot removal, wound cleaning, and antibiotics — typically survive.
In June, a dog in New Mexico was confirmed to have a screwworm infestation, highlighting the real danger to pets as the pest has spread northward from endemic areas in Central America and into Mexico.
What warning signs should pet owners watch for?
Veterinarians say owners should be on alert for wounds that swell, ooze pus, or refuse to heal. Other red flags include signs of pain such as excessive licking or chewing at a wound, unusual tiredness, and loss of appetite. One of the most distinctive indicators is the odor of rotting flesh coming from a wound.
Experts advise taking a pet to the veterinarian at the first sign of any of these symptoms — even if maggots or eggs are not visibly present.
Stray cats and dogs may be especially vulnerable to screwworm and could potentially pass the parasite on to household pets. Residents are encouraged to monitor strays closely and notify local animal control if they observe any suspicious wounds.
Dr. Pancho Hubert, a veterinarian in Corpus Christi, Texas, and president of the Texas Veterinary Medical Association, emphasized the importance of acting quickly. “The consequences of missing one or two cases is so great that if the public sees anything suspicious, take it into your vet,” he said. “It might not be anything, but these wounds carry so much significance.”
Can pet owners prevent an infestation?
Monthly parasite prevention medications already used for ticks and fleas may also offer protection against screwworm. Chewable treatments like Credelio and Simparica work by entering the animal’s bloodstream and killing parasites that feed on it, and experts say they may work similarly against screwworm.
How is screwworm treated?
A veterinarian will examine the wound, remove larvae and eggs, and send samples to animal health authorities for identification. The vet will then clean and disinfect the wound, administer an oral insecticide, and likely prescribe pain medication and antibiotics. The U.S. Department of Agriculture has approved approximately a dozen drugs for treating screwworm infestations.
Experts strongly caution against pet owners attempting to treat screwworm on their own. DIY treatment risks missing larvae that will continue burrowing deeper, and accidentally dropping maggots on the ground could allow them to mature into flies and spread the infestation further.
Can pets recover?
With early detection and proper veterinary treatment, animals typically make a full recovery. However, screwworm infestations are deadly if left untreated.
What are authorities doing to combat the outbreak?
The U.S. Department of Agriculture has ramped up surveillance, put movement restrictions in place, and invested in the production of sterile flies, which are used to interrupt screwworm reproduction. Two new sterile fly production facilities are currently under construction, but experts say current output falls well short of what is needed to get the outbreak under control.
Reuters previously reported that hundreds of veterinarians, support staff, and lab workers at the USDA’s animal health division departed after the Trump administration sought resignations, leaving fewer disease response specialists on hand. Agriculture Secretary Brooke Rollins stated that screwworm had been projected to enter the U.S. last year, and credited the Trump administration’s actions with delaying that entry and giving the USDA time to mount a rapid response.
