The re-emergence of the New World screwworm (NWS) has triggered alarm throughout the U.S. livestock sector, prompting swift action from industry leaders and federal agencies to prevent a resurgence of a parasite once eradicated from the mainland in 1966.
The NWS, or Cochliomyia hominivorax, is a parasitic fly whose larvae feed on the living tissue of warm-blooded animals, including cattle, deer, and, in rare cases, humans. Infestations begin when female flies lay eggs on open wounds. The resulting maggots burrow into flesh, causing severe pain, tissue damage, and often death if untreated. The economic and animal welfare implications for the beef industry are potentially catastrophic.
Originally eradicated from North America through the sterile insect technique—a process involving the mass release of sterilized male flies—NWS is now making a concerning return. The pest has advanced north from Central America into southern Mexico, with larvae recently detected near Oaxaca and Veracruz, just 700 miles from the U.S. border.
In response, the U.S. Department of Agriculture (USDA) suspended imports of live cattle, horses, and bison through southern border ports as of May 12. The National Cattlemen’s Beef Association (NCBA) and Texas Farm Bureau (TFB) have both expressed strong support for the move. “The NWS is a devastating pest that could cripple the U.S. livestock industry,” said Texas Farm Bureau President Russell Boening. “It’s critical to be proactive and provide resources for swift response.”
The resurgence of NWS has also reignited calls for domestic sterile fly production. The STOP Screwworms Act, introduced by Rep. Tony Gonzales (R-TX) and Sen. John Cornyn (R-TX), would fund construction of a new sterile fly facility within the U.S. Currently, the only operational facility is in Panama, run by the Panama-U.S. Commission for the Eradication & Prevention of Screwworm (COPEG), but it is struggling to meet demand.
“When I was growing up, I heard the stories of how damaging the New World screwworm was to our cattle before it was eradicated in the 1960s,” said NCBA President Buck Wehrbein. “I never want to see that kind of devastation return to our country.”
The sterile insect technique has proven highly effective. NWS flies only mate once, making sterile male releases a powerful tool in collapsing populations. It was used successfully in 2016 to contain an outbreak in the Florida Keys deer population.
However, industry leaders warn that without shared responsibility from neighboring countries, the strategy may fall short. “The Mexican government’s failure to knock down senseless obstacles has left America with no alternative but a closure of the U.S. border,” said NCBA CEO Colin Woodall, citing delays in allowing U.S. aircraft to release sterile flies over Mexican territory.
Daniel Manzanares, director of the Santa Teresa Livestock Crossing in New Mexico, echoed those concerns. “This is a multinational problem,” he said, noting that cross-border trade, auction markets, and rural economies are already feeling the impact. “The ripple effect of the border closures is getting really ugly.”
Beyond economic disruption, the threat to animal health is top of mind. Livestock, wildlife, and even pets are vulnerable if the parasite crosses into U.S. territory. The potential for secondary infections and permanent damage from infestations has both veterinarians and public health officials on alert.
With bipartisan support building behind the STOP Screwworms Act and increased collaboration between USDA and livestock groups, the hope is to halt NWS before it breaches the U.S. border. As Texas & Southwestern Cattle Raisers Association President Carl Ray Polk Jr. put it, “Texas will be among the first impacted by the New World screwworm, making domestic sterile fly infrastructure critical.”
What’s Next:
Industry stakeholders await Congressional approval of the STOP Screwworms Act and further USDA action. In the meantime, ranchers and producers are encouraged to monitor livestock for unusual wounds, report suspected infestations, and remain vigilant in biosecurity practices.
More Information:
For clinical details or to report suspected cases in animals, contact your state animal health official or APHIS office. Healthcare providers should follow CDC protocols for human cases.
