Return of the New World Screwworm Fly in South Texas

The New World screwworm fly has returned to South Texas. The U.S. Department of Agriculture confirmed this on Wednesday. It marks the first instance in decades where this flesh-eating parasite has posed a threat to the U.S. cattle industry. This is only the third appearance of the fly in the country during this period.

According to Agriculture Secretary Brooke Rollins, the case was found in a 3-week-old calf in LaPryor, Texas. This location is approximately 50 miles from the Mexican border. Texas State Veterinarian Bud Dinges has imposed a 12-mile quarantine zone around the affected area. This measure prohibits moving any warm-blooded animals, including pets, without an inspection.

Rollins emphasized that there have been no other detections of the fly within the U.S. While the larvae of these flies are hazardous to livestock, they don’t contaminate food. Rollins assured that with proper treatment, even the infested calf should recover.

For over a year, U.S. and Texas agriculture officials and cattle industry leaders have been alerting the public about the screwworm fly’s movement from Mexico. The fly previously caused tens of millions of dollars in losses before its eradication in the United States during the 1970s. Rollins noted that this is the first confirmed case in Texas since 1966.

Efforts to prevent the fly’s entry into the U.S. have involved releasing millions of sterile screwworm flies to mate with wild females. This approach successfully eradicated the fly in the past. Rollins expressed confidence in current preparations, asserting that they see no threat of a mass infestation. She stated, “There is no reason to believe this incursion will result in establishment of the pest in our country.”

The suspect case was announced a day after Rollins highlighted the proximity of the threat during an online news conference. Cases were confirmed as close as 25 miles from the border in Mexico. The USDA is actively working to counter this threat.

The New World screwworm fly is a tropical species. Decades ago, it infested cattle in warm areas across the southern U.S. It was contained in Panama until late 2024. The female fly lays eggs in open wounds or mucous membranes. These hatch into larvae that consume flesh, distinguishing them from most other fly species. Infestations can affect livestock, wild animals, pets, and humans, potentially leading to death if untreated.

In August 2025, a case was confirmed in a Maryland resident who had traveled to El Salvador. The individual recovered, and officials found no transmission of the parasite. The previous outbreak was in Florida in September 2016 among wild deer and was contained by early the next year. Female screwworm flies mate once during their lifespan. If they mate with sterile flies, their eggs won’t hatch, leading to a gradual die-off of the population.

Past eradication efforts were highly successful, resulting in the closure of U.S. facilities for breeding sterile flies except in Panama. This situation is changing. The USDA has allocated $21 million to convert a fruit fly breeding facility in southern Mexico for screwworm flies. They opened a center in southern Texas for dispersing sterile flies bred elsewhere and are constructing a $750 million screwworm fly factory. The Mexican facility is expected to start operating next month, according to Rollins.

Officials have deployed 8,000 traps along the U.S.-Mexico border. The USDA has tested over 58,000 fly samples and 19,000 wild animals. Rollins also closed the border to livestock imports from Mexico last year, a measure she defended publicly.

The screwworm fly can travel with humans, their pets, and wild animals, but it doesn’t fly long distances on its own. Dinges urged livestock and pet owners to comply with the quarantine zone regulations. “Please help us prevent any further movement of this pest by staying put,” he advised.