The New World screwworm fly has reached south Texas, the U.S. Department of Agriculture confirmed June 3, the first time in decades that the parasite with flesh-eating larvae has threatened the nation's cattle industry and only the third time it's appeared in the U.S. in that time.

Agriculture Secretary Brooke Rollins said the case was in a 3-week-old calf in La Pryor, Texas, about 50 miles from the Mexico border. Texas State Veterinarian Bud Dinges said he has established a 12-mile quarantine zone, prohibiting the movement of any warm-blooded animal — including pets — outside that zone without an inspection.

Rollins said there have been no other detections of the fly in the U.S., and officials were quick to say that while the fly’s larvae are a threat to livestock production, they don’t infest food. Properly treated, even the infested calf should recover, Rollins said.

Rollins, U.S. and Texas agriculture officials, and cattle industry leaders have been sounding public alarms about the fly’s movement across Mexico for more than a year, spurred on by memories of it causing tens of millions of dollars of losses — potentially billions in today’s dollars — before its eradication in the 1970s.

It is the first case confirmed in Texas since 1966, Rollins said.

The months of effort to keep the fly out of the U.S. have included dropping millions of sterile screwworm flies in the area to mate with wild females — the same method used successfully before the fly was eradicated. Rollins said the USDA is confident enough in its preparations that it believes “there is no threat of mass infestation.”

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

The announcement of the suspected case comes only a day after Rollins had an online news conference to highlight the nearness of the threat, with cases being confirmed in Mexico as close as 25 miles from the border — and to outline the USDA's efforts to combat it.

The New World Screwworm fly is a tropical species that decades ago infested cattle in warm weather across the southern United States, but it was contained in Panama until late in 2024.

The female fly lays its eggs in open wounds or mucous membranes and they hatch into larvae that eat flesh — making them unlike most fly species — and can infest livestock, wild mammals, household pets and even humans. Infestations can lead to death if left untreated.

In August 2025, federal health officials confirmed a case in a Maryland resident who had traveled to El Salvador, but the victim recovered and officials found no transmission of the parasite. Before that, the last outbreak was in the Florida Keys in September 2016, mostly among wild deer, and it was contained early the next year without spreading further.

The female flies mate once in their monthslong lives, and if they do so with a sterile fly, their eggs would not hatch — and the population would die out over time. Past eradication efforts were so successful that the U.S. shut down facilities for breeding sterile flies, leaving only one in Panama for decades.

That is changing. The USDA dedicated $21 million to convert a fruit-fly breeding facility in southern Mexico into one for breeding screwworm flies, opened a new center for dispersing sterile flies bred elsewhere in southern Texas and has started construction on a $750 million screwworm fly factory there. The breeding facility in Mexico should be operating next month, Rollins said.

Officials also deployed 8,000 fly traps along the U.S.-Mexico border, and Rollins said the USDA has tested more than 58,000 fly samples, along with 19,000 wild animals.

Rollins also closed the U.S.-Mexico border last year to livestock imports from Mexico, a decision she defended during her news conference Tuesday. The fly also can travel with people and their pets and with wild animals, officials noted, but Rollins stressed Wednesday evening that it doesn't fly great distances on its own.

Dinges said ranchers and pet owners need to understand that it's important to respect the quarantine zone.

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