The New World screwworm — a fly whose larvae feed on the tissue of livestock, wildlife and pets — has been detected in a 3-week-old calf in La Pryor, Texas, the Agriculture Department confirmed on Wednesday night. It is the first case found in cattle in the United States since the insect was eradicated from the country in the 1960s.
“We are taking immediate action this afternoon and evening already to deploy, to contain and to eradicate this case of the New World screwworm in South Texas,” Brooke Rollins, the agriculture secretary, said at a news conference.
If more screwworms are found beyond the single case, the infections, which can kill if left untreated, could devastate the American cattle industry.
The Agriculture Department has been preparing for the screwworm to re-emerge in the United States for months. By the mid-2000s, the screwworm had been eradicated from all of North and Central America, but the parasite has been moving north and was detected on Mexico’s border with Guatemala in November 2024.
Screwworm flies do not move very far, but they spread with the movement of unknowingly infected animals, especially if trucked to a new location. More than 20,000 cases have been detected in Mexico, according to the Agriculture Department, and Mexican cattle have been barred from entering the United States for more than a year.
Screwworm flies do not infest fruit or vegetables, and the Agriculture Department says its Food Safety and Inspection Service will ensure that all beef, poultry and eggs are safe to eat.
The Agriculture Department has declared a 20-kilometer “infested zone” quarantine where the infected calf was found, increasing surveillance and limiting the movement of all warm-blooded animals out of that zone. The department has also deployed veterinarians and animal health officials to the area, as has the Texas Animal Health Commission.
Officials are also dispersing millions of sterile New World screwworm flies, aerially and by truck, in the area. Releasing sterile flies, which mate with wild female flies and drive the population toward eradication, is the only way currently to control the spread of the fly.
Only one facility, in Panama, produces sterile flies — around 100 million a week. The U.S. Army Corps of Engineers has recently broken ground for a $750 million production facility at Moore Air Base in Edinburg, Texas. The facility is expected to begin producing 100 million sterile flies per week in late 2027, and later up to 300 million per week.
