A highly destructive parasite, the New World screwworm fly, has been confirmed in south Texas, marking its first appearance in the state since 1966 and only the third time in the U.S. in decades.
The U.S. Department of Agriculture (USDA) announced Wednesday that the parasite, known for its flesh-eating larvae, poses a significant threat to the nation’s cattle industry.
The confirmed case involves a three-week-old calf in LaPryor, Texas, located approximately 50 miles from the Mexican border, according to Agriculture Secretary Brooke Rollins.
In response, Texas State Veterinarian Bud Dinges has established a 12-mile quarantine zone, prohibiting the movement of any warm-blooded animal, including household pets, without prior inspection.
While officials have not detected the fly elsewhere in the U.S., they emphasized that the larvae are a danger to livestock but do not infest food. Secretary Rollins noted that even the affected calf is expected to recover with proper treatment.
However, the re-emergence of the screwworm fly has prompted alarms from U.S. and Texas agriculture officials and cattle industry leaders, who have been monitoring its movement across Mexico for over a year.
Memories of the parasite causing tens of millions of dollars in losses in the 1970s, potentially billions in today’s currency, underscore the gravity of the situation.
The New World screwworm fly is a tropical species that historically infested cattle across the southern United States before being contained in Panama, where it persisted until late 2024.
Female flies lay eggs in open wounds or mucous membranes, which then hatch into larvae that consume living flesh – a characteristic that distinguishes them from most other fly species. Infestations can affect livestock, wild mammals, pets, and even humans, potentially leading to death if left untreated.
In August 2025, a Maryland resident who had traveled to El Salvador was confirmed to have contracted the virus, but they recovered without transmitting it. The last significant outbreak in the U.S. prior to this was in the Florida Keys in 2016, primarily among wild deer, which was contained early the following year.
To combat the threat, the USDA has been employing a strategy of releasing millions of sterile screwworm flies, a method that has been successfully used in previous eradication efforts. Female screwworm flies mate only once in their month-long lives; mating with a sterile male prevents eggs from hatching, leading to a population decline over time.
Secretary Rollins expressed confidence in these preparations, stating, “There is no threat of mass infestation,” and adding, “There is no reason to believe this incursion will result in establishment of the pest in our country.”
The U.S. had previously shut down its sterile fly breeding facilities, leaving only one in Panama for decades.
However, this is changing rapidly. The USDA has allocated $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 in the region.
The Mexico facility is expected to be operational next month.
Further preventive measures include deploying 8,000 fly traps along the U.S.-Mexico border.
The USDA has also tested over 58,000 fly samples and 19,000 wild animals. Last year, Secretary Rollins closed the U.S.-Mexico border to livestock imports from Mexico, a decision she defended recently. While the fly can travel with people, pets, and wild animals, Rollins emphasized that it “doesn’t fly great distances on its own.”
Texas State Veterinarian Bud Dinges urged the public to cooperate with the quarantine measures. “Please help us prevent any further movement of this pest by staying put,” he appealed to ranchers and pet owners.
