New World screwworm, a parasitic fly that feeds on living tissue, has reemerged in the U.S. after nearly 60 years, with the first cases confirmed in Texas in June 2026. This parasite poses a critical risk to horses and other livestock, as it can turn even minor wounds into severe infestations. Veterinarians are advising horse owners to be vigilant about inspecting wounds and to report any potential signs of screwworms to prevent further spread.

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The U.S. Department of Agriculture's Animal and Plant Health Inspection Service (USDA-APHIS) confirmed the outbreak first on June 3, 2026, in a three-week-old calf in Zavala County, Texas, where larvae were found in the umbilical area. Subsequent cases have been documented in Texas and New Mexico, leading to increased surveillance and movement restrictions in affected regions.

New World screwworm, scientifically known as Cochliomyia hominivorax, is particularly dangerous because its larvae feed on healthy tissue rather than decaying matter. Female flies lay hundreds of eggs in wounds or natural openings, with larvae emerging within 12 to 24 hours to begin feeding on the host. This can lead to serious tissue damage, which may not be immediately apparent during initial examinations.

Dr. Kyle C. Johnson, a professor at Texas A&M University, discussed the situation on a September 27, 2026 episode of the Stable Management Podcast, highlighting the need for horse owners to closely monitor their animals for any wounds or suspicious lesions. At that time, approximately 50 cases had been reported, including two horses. He noted that small injuries, often overlooked, can provide a perfect breeding ground for screwworms.

Initial signs of infestation include changes in behavior, dove to notice discomfort, reduced appetite, and wounds that may produce an unpleasant odor or visible larvae. These can occur not only on typical injury sites but also around body openings like the eyes and nose.

Veterinarians stress that if larvae are discovered in a horse, the animal should not be transported without consulting a veterinarian. Moving infested horses can facilitate the spread of larvae into new environments. Instead, owners should aim to keep the horse at its current location and ask for an on-site veterinary assessment.

Diagnosis involves careful examination for visible larvae and tissue damage, along with a history of animal movements. Laboratory confirmation is essential, as not all myiasis cases result from New World screwworm.

To combat the parasite, routine wound care is paramount, including regular inspections for cuts and insect bites. Horse owners are also encouraged to manage their pastures effectively to prevent injuries that could attract flies. In addition, previous success in eradicating the parasite through techniques such as releasing sterile flies emphasizes the need for ongoing surveillance and prompt reporting of any infestations.

In October 2026, over 209,400 infected animals and 2,700 human cases were reported in Central America and Mexico, showcasing the potential threat posed by the parasite to animal agriculture and public health. As the U.S. strengthens monitoring efforts against New World screwworm, veterinarians and horse owners must maintain high vigilance and implement preventive measures to safeguard their livestock.