First US horse case of New World screwworm in 60 years confirmed; fourth dog case also reported

The first equine case of New World screwworm (NWS) in the US since the parasite was last eradicated 6 decades ago has been confirmed in Presidio County, Texas, marking a new development in the pest's 2026 incursion into the country.

NWS larvae were detected on the hind limb of a working ranch horse on September 9, 2026, according to US Equestrian.¹ It is the first equine case associated with the ongoing outbreak. Before this equine detection, the most recent known case of NWS in a domestic animal was confirmed in a dog in Crockett County, Texas, on August 31.² To date, 4 dogs have been confirmed with NWS in the US amid the current outbreak.

NWS is a parasite whose larvae feed on the living tissue of warm-blooded, usually healthy animals, burrowing into wounds and causing serious damage. “Horse owners should inspect their horses every day for wounds, swelling, drainage, foul odor, or maggots. Pay close attention to small cuts, surgical sites, the umbilical area of foals, and moist areas such as the lips, nostrils, eyes, and genital areas,” US Equestrian wrote in an announcement, urging owners to contact their veterinarian immediately if they find maggots or anything unusual in a wound.¹ “Early reporting helps animal health officials respond quickly with eradication tools like sterile fly release in your area.”

The sterile insect technique, in which sterile male flies are released to mate with wild females and suppress reproduction, is the primary method being used to eradicate the pest. NWS was eliminated from the US in the 1960s through a similar campaign before its 2026 return, which began with a bovine detection in Zavala County, Texas, in June.

US Equestrian said it has protocols in place for licensed competition organizers and participants to reduce opportunities for the fly to spread. Required measures may include inspecting horses for wounds upon arrival and monitoring them throughout the competition. Finding and reporting suspected cases early helps protect horses and competition grounds from infestation and the quarantines that can follow, the organization said.¹

Because state animal health officials set policy for animals within their state, as well as for animals entering from states or zones with confirmed screwworm cases, owners should remain aware of both state and federal requirements. Protocols and links to state and federal resources are available at usef.org/screwworm.

“We encourage every horse owner to help protect their horses and the broader equine community by practicing good wound care, fly control, and biosecurity,” US Equestrian said. “Check horses daily, keep wounds clean and covered as directed by your veterinarian, use veterinarian-recommended fly control products, and limit fly access with physical barriers such as fly masks, sheets, and clean stabling practices when appropriate.”

Additional information for horse owners and competition managers is available at usef.org/screwworm. Questions may be directed to the US Equestrian Federation Equine Health team at equinehealth@usef.org or to Katie Flynn, DVM, at 859-225-6991.

As of September 15, the US Department of Agriculture has detected 49 cases of NWS in domestic animals in Texas, including cattle, sheep, goats, dogs, and, most recently, a horse.²

References

1. Equine case of screwworm confirmed in the United States. News release. US Equestrian. September 10, 2026. Accessed September 15, 2026. https://www.usef.org/media/press-releases/equine-case-of-screwworm-confirmed-in-united
2. Confirmed detections of New World screwworm. US Department of Agriculture. Updated August 21, 2026. Accessed September 15, 2026. https://tinyurl.com/56dp55d2