Carefully monitor horse wounds for signs of screwworm larvae. | Erica Larson/The HorseNew World screwworm (NWS) remains a significant animal health concern because the fly larvae feed on living tissue and can quickly turn a minor wound into a painful and potentially life-threatening problem. During a June 18 webinar hosted by the United States Equestrian Federation (USEF) and the American Association of Equine Practitioners, Katie Flynn, DVM, USEFs senior veterinarian of equine health and biosecurity and Neil Gray, DVM, with the Equitarian Initiative, reviewed how NWS spreads, what horse owners should watch for, and the steps facilities can take to reduce risk.New World Screwworm Flies vs. Other BlowfliesWhile New World screwworm flies and other blowflies might appear similar at first glance, they behave very differently. Unlike typical blowfly larvae, which feed on dead or decaying tissue, NWS larvae feed on living tissue and require a live host to complete their development. Because NWS can infest any warm-blooded mammal and cause severe illness or death if left untreated, it poses a far greater animal (and human) health threat than the nuisance blowflies or bot flies horse owners commonly encounter.New World Screwworm Life CycleThe NWS fly is attracted to wounds. The adult fly smells the break in skin, which attracts the female to lay her eggs around the wound edge, said Flynn. These eggs hatch within 12-24 hours, and the larvae then eat the living flesh, burrowing deeper into the tissue. The larvae feed and grow over five to seven days, through multiple stages, and as they mature around seven days, they drop to the ground and pupate in the soil, said Flynn. The adult fly emerges from the soil seven to 54 days later, depending on the temperature and humidity.The adult female fly will only mate once, said Flynn. Once she mates, she will wait until she senses a wound to lay her eggs on. That wound can be any size, even as small as a tick bite.New World Screwworm Appearance and Clinical SignsScrewworm larvae differs from other blowfly larvae in appearance. They literally look like a screw, said Gray. They have a conical shape with bands around that allow them to screw their way into the tissue as they burrow in. The flies themselves can be difficult to differentiate from other flies. There are many flies that are green, metallic flies with red eyes, said Gray.When identifying affected wounds, Gray said, youre looking for a wound that is more irritated than youd expect with a foul smelling odor. He added that owners should be suspicious of wounds with maggots or a large presence of flies. If left untreated, cases can be fatal.Screwworm fly | USDAHistory of New World Screwworm in the U.S.Since its eradication from the U.S. in 1966, three distinct incursions of NWS have occurred, said Flynn. In 1976, there was an isolated outbreak in an animal in Texas, which was contained and eliminated. In 2016 to 2017, an outbreak happened in the Florida Keys, which was confirmed in Key deer in late September 2016 and affected some pets. Officials eradicated the pest by March 2017. In August 2025 the Centers for Disease Control and Prevention reported one confirmed case in a human returning to the U.S. after travel to El Salvador. Flynn said this case was not evidence of local establishment in the U.S. because it was caught and addressed before the screwworm could go through its life cycle and become a fly.New World Screwworm Management for Horse FarmsFlynn recommended a six-step procedure for protecting your animals from NWS infestation:Check wounds, body openings, and recent surgical sites daily for pain, odor, swelling, or larvae.Promptly clean, cover, and monitor cuts, including surgical sites, tick bites, and foal umbilical areas.Reduce fly breeding areas by removing manure regularly, improving drainage on the property, and cleaning wet organic debris.Report suspicious cases immediately to your veterinarian and local animal health officials.Protect the facilitys environment by maintaining a clean and safe property to reduce injury and fly exposure.Use targeted fly control products labeled for use on horses and horse properties.Screwworm infestation can progress rapidly and can kill an animal if not promptly discovered and treated, said Flynn. Most animals recover fully when they are diagnosed and treated early.Regulatory Response and Horse Movement RestrictionsFlynn said general regulatory responses to NWS cases include:Identifying suspected casesReporting cases to state animal health officials and the U.S. Department of Agriculture Animal and Plant Health Inspection Service (USDA APHIS) immediatelyConfirming the diagnosis through sample (in this case, larvae) collection and lab testingResponding to the case with quarantine and movement controls, surveillance and treatment of cases, wildlife coordination (monitoring because wildlife can also be affected), and sterile fly releaseThe only way to eradicate NWS is through sterile fly release to knock down generations, she said. It typically takes three generations to knock the numbers down to zero. The NWS sterile fly release is dependent upon identification of cases, which highlights the importance of reporting.Flynn said a common question among horse owners is How can I move my horses? She emphasized that because the situation is constantly changing, owners planning to transport horses should keep in contact with both the state of origin and the state of destination for their animal. Movement requirements can change rapidly as a new case of screwworm is confirmed, a quarantine issued and an infested zone with movement restrictions implemented by state animal health officials.Different states will have different requirements for moving horses who have been in infested zones, said Flynn. Infested zones are defined as a radius of 20 kilometers or about 12 miles around a confirmed case. Horses moving from these zones to another local area or to another state will likely be required to undergo inspections and certification that they do not appear to be carrying screwworm larvae. Depending upon the state of destination and state of origin, those inspections might need to take place before they leave the infested zone or after their arrival to their destination, said Flynn.Horse owners should expect that equine event and travel plans might be disrupted, and cross-border movement could be affected, said Flynn. As of the time of the webinar, horses couldnt travel to Mexico, and horses could only fly into the U.S. from Mexico. Flynn added that Canada had also placed restrictions on equine movement, allowing only horses that have been outside of an infested state for at least 21 days to cross the border. These restrictions are still applicable as of the time of publication.NWS Mitigation for Equine EventsFlynn said the current plan to mitigate the risk of NWS spread at USEF equine events includes requiring a health declaration at entry that discloses the animals recent travel and notes any wounds. On arrival at a USEF licensed event, competition management will inspect a horse coming from an infested state for wounds, odor, irritation, or maggots. If inspectors find a wound, a veterinarian will need to check the horse for larvae. If the veterinarian finds larvae on the horse, they will immediately isolate the animal and report a suspect case. Flynn said competition venues are also encouraged to proactively manage manure and enhance sanitation to improve insect control.Flynn said the USEF has been contacted by equine industry stakeholders outside of licensed competitions, and she has shared her suggested protocols with them in hopes of reducing the flys spread. While the fly can expand its geographic range under its own power, the concern for Flynn and other animal health officials is also that an animal could be transported well outside the flys current range while carrying larvae, leading to a new population center for the screwworm fly. Take-Home MessageNew World screwworm infestations can develop rapidly, making early detection and prompt reporting critical for protecting horses and preventing spread. As response efforts continue, horse owners should expect movement requirements to change as officials work to limit further spread.