Screwworm Fly Threat in South Texas: Response and Impact

The New World screwworm fly has recently been detected in south Texas, confirmed by the U.S. Department of Agriculture (USDA). This marks the first occurrence in decades that this flesh-eating larvae parasite poses a threat to the U.S. cattle industry. Agriculture Secretary Brooke Rollins revealed the detection in a 3-week-old calf in LaPryor, Texas, situated about 50 miles from the Mexico border.

Texas State Veterinarian Bud Dinges has established a quarantine zone of 12 miles, restricting the movement of any warm-blooded animals, including pets, without inspection. Rollins assured that this is the only detection in the U.S., emphasizing that although the larvae threaten livestock, they do not infest food. The infested calf should recover with proper treatment.

For over a year, U.S. and Texas agriculture officials have raised alarms about the screwworm fly’s progression across Mexico. The fly had previously caused substantial financial losses before its eradication in the 1970s. This is the first confirmed case in Texas since 1966.

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

Efforts to prevent the fly’s spread include deploying millions of sterile screwworm flies to mate with wild females, a strategy proven successful in the past. Rollins expressed confidence in the USDA’s preparations, believing there is no threat of mass infestation.

The announcement of the case came shortly after an online news conference by Rollins, highlighting the fly’s proximity to the border and USDA’s preventive measures. The screwworm fly is a tropical species that once thrived across the southern U.S. but was contained in Panama until late 2024.

The female fly deposits eggs in open wounds or mucous membranes, and larvae consume flesh. Infestations can affect livestock, wild mammals, pets, and humans, potentially leading to death if untreated. A case in a Maryland resident was confirmed in 2025 after a trip to El Salvador; the victim recovered without further transmission.

Previous outbreaks in the U.S., like in the Florida Keys in 2016, were contained swiftly. Female flies mate once, meaning mating with a sterile fly results in non-viable eggs, gradually dying out the population. Past eradication was so effective that only one sterile fly breeding facility remained in Panama, but efforts are now expanding.

The USDA allocated $21 million to convert a fruit-fly breeding facility in southern Mexico into a screwworm fly breeding site. Additionally, they opened a new center for dispersing sterile flies in southern Texas and commenced constructing a $750 million factory there. The breeding facility in Mexico is expected to begin operations soon.

Officials have implemented 8,000 fly traps along the U.S.-Mexico border and tested over 58,000 fly samples and 19,000 wild animals. Additionally, the USDA previously closed the border to livestock imports from Mexico, a decision supported by Rollins.

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