The New World screwworm fly is threatening the $US113 billion United States cattle industry for the first time in more than a half century, with an infestation from its flesh-eating larvae confirmed in south Texas.
The infestation was discovered in a single 3-week-old calf in La Pryor, Texas, about 161 kilometres south-west of San Antonio and 80 kilometres from the US-Mexico border.
Federal and state officials had been working to keep the parasite from reaching Texas, home to $US17 billion worth of the nation’s cattle, making it the industry’s number one state.
The deadly flies were detected in Mexico late in 2024 after years of being contained at the southern end of Panama.
The fly was an annual warm-weather scourge of cattle ranchers from at least the 1930s through the 1960s, until the US eradicated the pest by breeding sterile male flies and dropping swarms of them from planes to mate with wild females.
The United States Department of Agriculture (USDA) said the most recent case was the first in Texas since 1966.
Here is what to know about the fly, the threat it poses and the response.
Being unusual makes the flies a threat
The New World screwworm fly in the Western Hemisphere and its Old World cousin in Africa and Asia are unusual among flies because their larvae, or maggots, eat live flesh and fluids instead of dead material.
Brooke Rollins (centre) watches efforts in Texas to combat the spread of the parasite. (AP: Eric Gay)
Females lay their eggs in open wounds and mucous membranes after mating only once in their months-long lives.
Any warm-blooded animal, including wildlife, pets and occasionally even humans, can be infested.
Livestock are vulnerable because of how they’re handled, Lee Haines, an associate research professor of biological sciences at the University of Notre Dame, said in an email on Thursday.
Standard practices with cattle can break the skin, including shearing and de-horning, or even moving them in and out of corrals can cause scrapes and cuts.
Birth would also make a mother and calf vulnerable, she said.
Stephen Diebel, a Texas rancher and president of the Texas and Southwestern Cattle Raisers Association, said even wounds “as small as a tick bite” could put cattle at risk.
Death can result if an infestation is not treated, though a dozen treatments have been approved for use in a variety of species.
In decades past, ranchers had tens of millions of dollars in losses — potentially billions in today’s dollars.
But agriculture officials were quick to note that the fly did not infest food, and US Agriculture Secretary Brooke Rollins said it’s unlikely to damage beef production — welcome news given that consumers were already facing record prices.
Officials sounded alarms for nearly 2 years
Federal and state officials and cattle industry leaders have been sounding public alarms about the fly’s movement through Mexico and towards the US since a case was confirmed in southern Mexico in November 2024.
Officials had considered the pest eradicated from Central and North America nearly two decades before an outbreak in Panama prompted a state of emergency there early in 2023, according to the joint US-Panama program established in 1994 to stop the parasite.
Cases jumped to Costa Rica and Nicaragua later that year.
Edward Burgess, a University of Florida entomologist who studies the fly, said it reproduced quickly and was carried across wide areas by its hosts, namely wild animals such as deer.
Outside of Panama, he said, programs that produced and released sterile flies had largely shut down.
“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 sickened more than 171,700 animals and 2,000 people across Central America and Mexico, according to the US Centers for Disease Control and Prevention.
There have been 10 human deaths, the CDC said.
Starting in May 2025, Ms Rollins closed border entries to livestock and on Thursday she credited that move with delaying the fly’s arrival in Texas by a year.
Ms Rollins has argued that the Mexican government has not done enough to control animals moving within the country, a suggestion Mexican authorities have rejected.
A sterilised screwworm fly during release as part of the Mexican government’s fight to stop the spread. (AP: Fernando Llano)
But Ms Haines said climate change was a key element in the spread of a tropical species that thrives in warm weather.
Warmer temperatures are expanding the fly’s habitat and cold snaps that killed them off each year in marginal habitats are becoming less frequent and less severe, she said.
Officials quarantine a swath of Texas
Texas State Veterinarian Bud Dinges imposed a 20-kilometre quarantine zone covering much of Zavala County, home to La Pryor, and a small part of neighbouring Uvalde County.
Animals cannot leave that zone without being inspected.
Local ranchers are concerned that the fly will spread among wildlife, particularly deer, as a small, short-lived outbreak did in the Florida Keys in 2016.
That was the last time a US case was confirmed among animals, though the CDC confirmed a case last year in a Maryland man who had travelled to El Salvador and recovered.
Zavalas County Sheriff Eusevio Salinas said on Thursday that state officials were setting up several road checkpoints in the county to enforce the quarantine.
“They said they were going to do that for three to four days, and hopefully after that it’s already under control,” Mr Salinas said.
In Texas, shots and fly drops
Mr Diebel, whose family ranch is about 322 kilometres east of the quarantine zone, said ranchers were proactively giving injections that prevented screwworm infestation.
They’re also taking extra care to treat wounds from ear tagging and other practices and keeping a close eye for signs of illness.
The USDA has been dropping sterile flies in south Texas since February, when it opened a centre for dispersing them in south Texas.
A test container of dyed fly pupae are displayed at a Screwworm Sterile Fly Production Facility in Texas. (AP Photo: Eric Gay)
It is now dropping them twice a week, for a total of 4 million flies, and it’s also putting 4 million more a week in the ground as pupae, flies in the stage between larvae and adult, said Rear Admiral Michael Schmoyer, a member of the USDA’s response team.
Releasing sterile flies is both time-tested and highly effective.
While males are “promiscuous,” in the scientific sense, females are not, and if their one mating hook-up is with a sterile male, no eggs from that female will hatch.
Once sterile males are prevalent enough, the fly’s population declines and then dies out.
But with sites outside Panama shut down for years, the USDA didn’t think sterile flies were being bred fast enough.
It invested $US21 million in a new fly-breeding facility in southern Mexico that is expected to start operations next month.
The USDA is also spending $US750 million to build a fly factory in southern Texas that can produce up to 300 million sterile flies a week.
It is expected to begin operating by the end of next year.
AP
