Screwworm Fly Detected In Texas Decades After Cattle Threat Was Largely Eradicated In U.S.

The New World screwworm fly has been detected in south Texas, marking the first time in decades this parasite has posed a threat to the U.S. cattle industry. The U.S. Department of Agriculture confirmed this development on Wednesday, noting that this is only the third occurrence of the fly in the U.S. during this period.

Agriculture Secretary Brooke Rollins reported that the fly was found in a 3-week-old calf located in LaPryor, Texas, approximately 50 miles from the Mexican border. Texas State Veterinarian Bud Dinges has established a 12-mile quarantine zone around the area, restricting the movement of any warm-blooded animals, including pets, without inspection.

Rollins stated that no other instances of the fly have been detected in the U.S. She emphasized that while the fly’s larvae pose a risk to livestock, they do not contaminate food. Rollins noted that, with appropriate treatment, the affected calf is expected to recover.

Public warnings about the fly’s spread across Mexico have been issued by Rollins, U.S. and Texas agriculture officials, and cattle industry leaders for over a year. They recall the significant economic losses caused by the fly before its eradication in the 1970s, which were estimated in the tens of millions of dollars, potentially equating to billions today.

Rollins mentioned 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, a method that previously led to the fly’s eradication. Rollins expressed confidence in the USDA’s preparations, stating there is no perceived threat of widespread infestation.

Rollins asserted, “There is no reason to believe this incursion will result in establishment of the pest in our country.”

The announcement of the suspected case followed Rollins’ online news conference, where she outlined the threat’s proximity, with cases confirmed in Mexico just 25 miles from the border, and detailed the USDA’s response strategies.

The New World Screwworm fly, a tropical species, previously infested cattle in warm southern U.S. regions but was contained in Panama until late 2024.

The female screwworm fly lays eggs in open wounds or mucous membranes, and the emerging larvae consume flesh, which is a unique characteristic among fly species. These larvae can infest livestock, wild mammals, pets, and even 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 no further transmission was reported. Prior to this, the last outbreak occurred in the Florida Keys in September 2016, predominantly affecting wild deer, and was swiftly contained by early the following year.

The female flies mate once in their lives, and if they mate with sterile males, the eggs will not hatch, gradually leading to population decline. Previous eradication efforts were so effective that the U.S. ceased operations at sterile fly-breeding facilities, leaving just one in Panama for many years.

This situation is evolving. The USDA has allocated $21 million to repurpose a fruit-fly breeding facility in southern Mexico for screwworm flies, established a new center for dispersing sterile flies bred in southern Texas, and begun building a $750 million screwworm fly factory there. Rollins indicated that the Mexican facility should be operational next month.

Additionally, 8,000 fly traps have been set along the U.S.-Mexico border, and the USDA has examined over 58,000 fly samples and 19,000 wild animals, according to Rollins.

Rollins also noted that the U.S.-Mexico border was closed last year to livestock imports from Mexico, a decision she defended during a news conference. Although the fly can travel with people, pets, and wild animals, Rollins emphasized that it does not travel long distances on its own.

Dinges stressed the importance of respecting the quarantine zone for ranchers and pet owners.

“Please help us prevent any further movement of this pest by staying put,” he said.