The New World screwworm fly poses a significant threat to the U.S. cattle industry, with the first confirmed infestation in south Texas in over half a century. This worrying development threatens an industry valued at $113 billion.
The infestation was identified in a three-week-old calf in La Pryor, Texas, situated around 100 miles southwest of San Antonio and near the U.S.-Mexico border. Both federal and state authorities had been vigilant to prevent the parasite’s arrival in Texas, a state home to $17 billion worth of cattle.
The Fly’s Unique Threat
The New World screwworm fly is a unique threat due to its larvae, which consume live flesh rather than dead tissue. The females lay eggs in open wounds or mucous membranes and only mate once in their lifetime. This makes livestock, especially cattle, vulnerable as common practices like shearing and de-horning can break the skin, increasing infection risk. Even small wounds, such as those from tick bites, can pose serious risks. If untreated, infestation can lead to the death of affected animals. Although treatments exist for various species, past infestations have caused significant financial losses.
U.S. Agriculture Secretary Brooke Rollins has assured that the pest’s presence is unlikely to impact beef production, which is a relief amidst already high consumer prices. The USDA has not linked the fly to food infestation.
Tracking the Fly’s Spread
Concerns about the screwworm fly’s northward spread from Mexico to the U.S. have been escalating since late 2024. Officials noted that the fly was considered eradicated from Central and North America until an outbreak in Panama in early 2023. This prompted emergency measures and the fly’s spread to Costa Rica and Nicaragua.
Entomologist Edward Burgess explains that the fly reproduces swiftly and can travel across large areas, mainly through wild animals such as deer. Control programs outside Panama have struggled, leading to difficulties in managing the fly’s rapid movement.
Human and Animal Impact
The U.S. Centers for Disease Control and Prevention (CDC) reported that the parasite has affected over 171,700 animals and 2,000 people across Central America and Mexico, resulting in 10 human fatalities. Secretary Rollins took action by closing livestock border entries in May 2025, slowing the fly’s progress into Texas. She mentioned the Mexican government’s insufficient measures in animal movement control. However, Lee Haines, a researcher at the University of Notre Dame, pointed to climate change as a contributor to the fly’s spread due to extended warm weather.
State Response in Texas
Texas State Veterinarian Bud Dinges has enforced a 12-mile quarantine zone in Zavala County and parts of Uvalde County. Animals within the zone cannot be moved without inspection, and local ranchers fear the fly may spread among wildlife, referencing a similar outbreak in the Florida Keys in 2016.
Preventive Measures and Future Plans
Texas ranchers are taking preventive measures, such as administering injections to ward off screwworm infestation and closely monitoring cattle for symptoms. The USDA has been releasing sterile flies in south Texas since February, aiming to curtail the fly population. A new fly-breeding operation in southern Mexico is set to begin soon, producing up to 300 million sterile flies weekly, complementing another facility under construction in southern Texas. This investment supports the long-standing strategy of releasing sterile male flies to reduce the overall population effectively.