USDA expands NWS response as threat intensifies near Southern Border
Federal and state animal health officials are ramping up defense efforts against the New World screwworm (NWS) as cases continue to spread across northeastern Mexico.
In an effort to create a buffer zone against the pest’s northward spread, USDA’s Animal and Plant Health Inspection Service (APHIS) announced on Jan. 30 it will shift its sterile NWS fly dispersal program to include areas inside the state of Texas, in which approximately 100 million sterile flies will be released each week.
“At U.S. Secretary of Agriculture Brooke Rollins’ direction, our highest priority is protecting the U.S. from NWS,” says USDA Under Secretary for Marketing and Regulatory Programs Dudley Hoskins. “The northernmost active case of NWS in Mexico is still about 200 miles away from the border, but we’ve seen cases continue to spread in Tamaulipas, Mexico and further south in Mexico, so we are proactively shifting our polygon as we make every effort to prevent NWS from reaching our border.”
Expanding efforts
According to a Jan. 16 Southwest Farm Press article by Senior Editor Shelley Huguley, NWS cases have increased rapidly in Tamaulipas, Mexico, a northeastern state bordering Texas, where eight new cases were confirmed in January.
This brings the total number of cases in Tamaulipas, Mexico to 11 since Dec. 30, 2025.
In its Jan. 30 press release, USDA notes the sterile insect technique (SIT), first used against NWS in the 1930s, remains the cornerstone of eradication efforts today. The department’s recent announcement notes its dispersal program will expand nearly 50 miles into Texas along the border it shares with Tamaulipas, Mexico.
According to USDA, the SIT process involves mass-producing male flies and sterilizing them using ionizing radiation before release.
Because female screwworm flies only mate once in their lifetime, mating with a sterile male results in unfertilized eggs which do not hatch, sharply reducing the population over time.
The agency explains releasing sterile flies north of known outbreak areas helps ensure flies moving into new regions will likely only encounter sterile mates, preventing reproduction and establishment.
In addition to sterile fly releases, USDA says it will continue to lead extensive surveillance and monitoring along the U.S.-Mexico border.
The department reports more than 120 NWS specific traps are placed in high-risk areas across border states, supplemented by thousands of additional insect traps already in place for other pests.
Through the end of January, more than 42,000 flies captured in these traps were submitted for identification, with no NWS detections. APHIS’s Wildlife Services also inspected more than 9,300 wild animals representing 39 species across 131 U.S. counties, with no signs of infestation.
Texas has further expanded its own surveillance efforts, activating a comprehensive trapping plan to target ports of entry, livestock export facilities and other high-risk locations. Traps are monitored daily by the Texas Department of Agriculture, with samples assessed quickly to ensure rapid response if needed.
“Because it is important to continue ongoing surveillance efforts while releasing sterile insects, it is possible sterile NWS flies could be caught and/or reported within Texas,” USDA notes. “To ensure officials can tell the difference between sterile and wild NWS flies, USDA will dye sterile pupae, and the dye will transfer to the sterile flies when they hatch.”
“The fluorescent dye will glow under ultraviolet light and may also be visible to the naked eye,” USDA adds. “If a sterile fly is captured in a trap, this dye will allow animal health officials to quickly rule the fly out as a threat.”
Finding success
In its seemingly endless fight against the ongoing threat of NWS, USDA reports it has seen some success.
In its Jan. 30 press release, USDA cites a recent detection at an equine import quarantine facility in Florida as evidence its existing import safeguards are working as intended.
“Earlier this week, a horse from Argentina was presented for routine importation at an equine import quarantine facility in Florida. Upon examination, APHIS identified an open wound with larvae on the animal and promptly collected and shipped samples to the National Veterinary Services Laboratories (NVSL) in Ames, Iowa,” USDA explains.
NVSL was able to confirm the larvae were NWS, and the animal was placed in quarantine where it will remain until it has been reexamined and officials confirm it is free of the pest.
“This is an example of long-standing import protocols working as designed,” USDA states. “While this situation does not appear to be associated with the NWS outbreak in Mexico USDA is currently fighting, it underscores the need for vigilance in all of USDA’s coordinated efforts to fight NWS.”
Additionally, the department has received praise for its expanded effort to eradicate NWS, especially from livestock organizations.
The National Cattlemen’s Beef Association and R-CALF USA thanked the agency for taking swift, science-based action to protect animal health and prevent economic harm.
“This is great news for U.S. livestock and wildlife,” R-CALF USA Chief Executive Officer Bill Bullard tells DTN in a Feb. 2 article by Senior Livestock Editor Jennifer Carrico. “Rollins is demonstrating her absolute commitment to preventing this devastating pest from entering our country.”
Staying ahead
While no NWS cases have been confirmed in the U.S., USDA emphasizes early detection and aggressive prevention are critical, particularly given the pest’s history and destructive potential.
Federal and state officials continue urging livestock producers nationwide to remain vigilant.
“Now NWS may be moving closer on its own, with no apparent link to commercial animal movement,” says Texas Agriculture Commissioner Sid Miller. “Producers must act now – stay informed, stay vigilant and prepare immediately. We cannot drop our guard for even a moment.”
USDA reminds producers NWS larvae infest living tissue and typically enter animals through open wounds, surgical sites, navels of newborns or natural body openings. Signs of infestation may include irritated behavior, head shaking, draining or enlarging wounds, foul odors or visible larvae.
“Adult screwworm flies are about the size of a common housefly or slightly larger, with a metallic green or blue body, orange eyes and three dark stripes down the back,” USDA explains. “NWS maggots can infest livestock and other warm-blooded animals, including people, and they most often enter an animal through an open wound and feed on the animal’s living flesh.”
“While NWS is not common in people, if a person notices a suspicious lesion on their body or suspect they may have contracted screwworm, they should seek immediate medical attention,” the department continues.
Although NWS remains hundreds of miles south of the U.S. border, officials say current response efforts reflect lessons learned from past outbreaks and the high cost of complacency.
“Early detection and aggressive surveillance are how we protect livestock,” Miller said. “With preparation and rapid action, we will stop NWS.”
Hannah Bugas is the managing editor of the Wyoming Livestock Roundup. Send comments on this article to roundup@wylr.net.
