A flesh-eating parasite that was eliminated from the United States decades ago has been detected in Texas, triggering a rapid response from federal and state officials. The U.S. Department of Agriculture (USDA) confirmed this week that a screwworm infestation was found in a three-week-old calf in Zavala County, marking the return of a pest that had been eradicated from the U.S. since the late 1960s.
According to Fox Weather, the discovery was made in the calf’s umbilical area, where New World screwworm larvae were feeding on living tissue. While officials say no additional cases have been identified so far, the detection has raised concerns because of the parasite’s ability to spread among livestock, wildlife, pets, and, in rare cases, humans.
State and federal agencies have already begun containment efforts, including quarantines, increased surveillance, and the release of sterile flies designed to disrupt the insect’s reproductive cycle.
Unlike common flies that feed on decaying material, the screwworm is a flesh-eating parasite. According to the CDC, female screwworm flies lay eggs in open wounds or body openings of warm-blooded animals and people.
Once the eggs hatch, the larvae burrow into healthy tissue and continue feeding for about a week before dropping into the soil to pupate and eventually emerge as adult flies. A single female can lay between 200 and 300 eggs at one time, allowing infestations to escalate quickly if left untreated.
The parasite gets its name from the screw-like ridges on its larvae and the way they burrow deeper into tissue while feeding. Adult New World screwworm flies are slightly larger than houseflies and are recognizable by their metallic blue-green bodies and reddish-orange eyes.
Symptoms in humans can include open sores, foul-smelling wounds, visible larvae, and bleeding at the site of infection. Untreated infestations can become severe and potentially fatal.
Texas officials are treating the situation as a major agricultural threat. “It is a direct and imminent threat to Texas, and we are treating it that way,” Texas Agriculture Commissioner Sid Miller said. “This is a high-stakes situation for our ranchers, our livestock industry and our food supply, and we are moving aggressively to stay ahead of it.”
The detection follows a growing number of screwworm cases across Central America and Mexico. In April, officials confirmed a case in Nuevo León, Mexico, roughly 60 miles from the Texas border. Since an outbreak was identified in Panama and Costa Rica in 2023, countries throughout the region have reported infections in both animals and humans.
Earlier this year, a Maryland resident contracted the parasite while traveling in El Salvador and later recovered without any reported spread inside the United States.
For now, the USDA is urging ranchers, pet owners, and veterinarians to closely monitor animals for unusual wounds, drainage, swelling, or visible larvae. Officials are relying heavily on the Sterile Insect Technique—a strategy that helped eradicate screwworms from the United States more than half a century ago.
