Flesh-eating screwworm parasite confirmed in Texas calf, first US cattle case in decades

The US Department of Agriculture (USDA) confirmed on Wednesday evening that the New World screwworm (NWS) has been detected in a calf in La Pryor, Texas, approximately 48 km from the Mexican border. The parasite, Cochliomyia hominivorax, had been spreading through Central America and Mexico, with over 27,000 cases recorded in Mexico since November 2024. US officials said preparatory measures had delayed its arrival by a year.

The confirmed case

A three-week-old calf was found with larvae near its navel. Agriculture Secretary Brooke Rollins stated it is the only case detected so far. The last US cattle case was in 1976, though a deer tested positive in Florida in 2016. A 20-kilometer quarantine zone has been established around the site, and Texas State Veterinarian Bud Dinges imposed a 12-mile zone prohibiting movement of any warm-blooded animal without inspection.

USDA personnel have already arrived in South Texas to support operations.

How the parasite kills

Female screwworm flies lay eggs in open wounds and mucous membranes of warm-blooded animals. Hundreds of larvae hatch and burrow into living tissue with sharp mouth hooks, eventually killing the host if untreated. The flies are metallic green or blue with three dark longitudinal stripes. They do not travel far themselves but spread through the movement of infested animals, particularly by truck.

Human risk and regional spread

Officials assess the risk to humans as low, though cases do occur. In Guatemala, 80 human infestations were recorded by September 2025, approximately 70 in Costa Rica, and 44 in Mexico. An 86-year-old woman with pre-existing conditions died in Mexico after infestation. In August 2025, a Maryland resident who had traveled to El Salvador contracted the parasite but recovered without transmitting it.

Economic fears and control strategy

US cattle ranchers fear an outbreak could shrink herds, reduce beef production, and raise consumer prices. The 1976 outbreak caused $375 million in losses to the Texas economy. The USDA is accelerating the release of millions of sterile male flies, a technique that helped eradicate the pest in the 1960s. Female flies mate only once; mating with a sterile male prevents eggs from hatching.

Instead of using all available tools, the USDA has acted too slowly and relied solely on a partial solution that takes years to fully implement.

Texas Agriculture Commissioner Sid Miller criticized the federal response, while Rollins urged ranchers to remain vigilant. The USDA has kept the border closed to cattle imports from Mexico for over a year due to the parasite's spread.