New World Screwworm Fly Returns to Texas, Poses Threat to Cattle Industry

The New World screwworm fly has been confirmed in south Texas, marking the first time in decades that this parasite has threatened the U.S. cattle industry. The U.S. Department of Agriculture (USDA) reported the case on Wednesday involving a 3-week-old calf in LaPryor, Texas, approximately 50 miles from the Mexico border.

Texas State Veterinarian Bud Dinges has implemented a 12-mile quarantine zone to control the situation. This zone restricts the movement of any warm-blooded animal, including pets, without inspection. Agriculture Secretary Brooke Rollins highlighted that no other cases have been detected in the U.S., and officials assured that while the larvae threaten livestock, they do not infest food. With proper treatment, the affected calf is expected to recover.

Rollins, along with other U.S. and Texas agriculture officials and cattle industry leaders, has raised public awareness about the fly’s movement across Mexico for the past year. The fly previously caused financial losses before its eradication in the 1970s. This recent case is the first in Texas since 1966.

Efforts to prevent the fly from entering the U.S. included deploying millions of sterile screwworm flies to mate with wild females, a proven method used before to eliminate the fly. Rollins expressed confidence in their preparations, stating there is no risk of a widespread infestation, and emphasized that the recent incursion would not establish the pest in the country.

This announcement came just a day after Rollins held an online news conference highlighting the threat’s proximity, with confirmed cases in Mexico as close as 25 miles from the border. The screwworm fly, a tropical species, infests cattle in warm climates, laying eggs in open wounds or mucous membranes. Its larvae consume flesh, unlike most fly species, potentially affecting livestock, wild mammals, pets, and humans. Untreated infestations can be fatal.

In August 2025, a Maryland resident contracted the parasite after traveling to El Salvador, but recovered without further transmission. Prior to this, the last U.S. outbreak was in the Florida Keys in 2016, primarily affecting wild deer and contained by early 2017.

Female flies, which mate once in their lifespan, if they mate with a sterile male, their eggs won’t hatch, eventually reducing the population. Due to past eradication success, the U.S. had closed several sterile fly breeding facilities, keeping only one operational in Panama for years. The USDA is now updating its approach, allocating $21 million to adapt a fruit-fly breeding facility in Mexico for screwworm flies and constructing a $750 million screwworm fly factory in southern Texas.

The new breeding facility in Mexico is expected to begin operations next month. Additionally, 8,000 fly traps have been placed along the U.S.-Mexico border. Rollins reported testing more than 58,000 fly samples along with 19,000 wild animals. Last year, the USDA closed the U.S.-Mexico border to livestock imports from Mexico to prevent the fly’s spread—a decision Rollins defended.

Rollins noted the fly can travel with humans, pets, and wild animals but does not migrate long distances independently. Dinges urged ranchers and pet owners to observe the quarantine zone to prevent further pest movement. “Please help us prevent any further movement of this pest by staying put,” he stated.