AP – The New World screwworm fly is posing a threat to the $113 billion US cattle industry for the first time in six decades, as its flesh-eating larvae have been confirmed in South Texas.
The USDA reported that this is the first case in Texas since 1966.
The infestation was identified in a 3-week-old calf in La Pryor, Texas, located approximately 161 kilometers (100 miles) southwest of San Antonio and 80 kilometers from the US-Mexico border.
These dangerous flies were detected in Mexico in late 2024, following years of containment to the southern tip of Panama.
Historically, the fly was a seasonal issue for cattle ranchers from the 1930s through the 1960s until eradication efforts in the US, which involved breeding sterile male flies and dispersing them from planes to mate with wild females.
The New World screwworm fly in the Americas and its Old World counterpart in Africa and Asia are unique among flies because their larvae, or maggots, consume live flesh and fluids rather than dead tissue.
After mating once in their lifespan, females lay eggs in open wounds and mucous membranes.
All warm-blooded animals, including wildlife, pets, and sometimes humans, are susceptible to infestation.
According to Lee Haines, an associate research professor at the University of Notre Dame, livestock are particularly at risk.
Cattle practices such as shearing, de-horning, and moving animals in and out of corrals can cause skin breaks, making them vulnerable, Haines noted.
Even birth can expose a mother and calf to risk, added Haines.
Stephen Diebel, a Texas rancher and president of the Texas & Southwestern Cattle Raisers Association, emphasized that even small wounds, like a tick bite, can endanger cattle.
Untreated infestations can be fatal, though numerous treatments have been approved for different species.
Agriculture officials stress that the fly does not infest food. US Agriculture Secretary Brooke Rollins noted that it is unlikely to affect beef production, a relief for consumers already facing record prices.
Federal and state authorities, along with cattle industry leaders, have been alerting the public about the fly’s progress through Mexico toward the US since its presence was confirmed in southern Mexico in November 2024.
The pest was considered eradicated in Central and North America nearly 20 years before an outbreak in Panama led to a state of emergency in early 2023, according to a joint US-Panama initiative.
Subsequent cases emerged in Costa Rica and Nicaragua later that year.
Edward Burgess, an entomologist at the University of Florida, stated that the fly reproduces rapidly and spreads widely through hosts like wild deer.
“It’s hard to stay ahead of it because of how fast that fly is able to move and regenerate,” Burgess said.
Outside the US, thousands of animals and hundreds of humans sickened.
As of June 2, the parasite had infected more than 171,700 animals and 2,000 people in Central America and Mexico, according to the US Centers for Disease Control and Prevention.
The CDC reports 10 human fatalities.
In May 2025, Rollins closed livestock border crossings and credited this measure with delaying the fly’s arrival in Texas by a year.
Haines pointed to climate change as a significant factor in the spread of this tropical species, which thrives in warm conditions.
Warmer temperatures are extending the fly’s habitat, and cold snaps that used to kill them in marginal areas are becoming less frequent and severe, she explained.
Texas State Veterinarian Bud Dinges has implemented a 20-kilometer quarantine zone covering much of Zavala County, including La Pryor, and parts of neighboring Uvalde County.
Local ranchers are worried that the fly might spread among wildlife, notably deer, as a brief outbreak did in the Florida Keys in 2016.
That was the last confirmed case among animals in the US, although the CDC verified a human case last year in a Maryland man who had traveled to El Salvador and recovered.
The USDA has been releasing sterile flies in South Texas since February, when it opened a center for their dispersal.
Currently, they are released twice a week, totaling 4 million, along with another 4 million a week placed in the ground as pupae, in the stage between larvae and adult.
With sites outside Panama closed for years, the USDA invested $21 million in a new fly-breeding facility in southern Mexico, expected to begin operations next month.
The USDA is also spending $750 million to construct a fly factory in southern Texas, capable of producing up to 300 million sterile flies weekly. The facility is set to commence operations next fall.
Related: ‘Devastating’ Flesh-Eating Parasite Is Spreading Toward The US, CDC Warns
Releasing sterile flies is a proven and effective method.
While male flies are “promiscuous” in the scientific sense, females are not. If a female mates with a sterile male, her eggs will not hatch.
As sterile males become more prevalent, the fly population diminishes and eventually dies out.
