A flesh-eating parasitic fly called New World screwworm is back in the U.S. for the first time since eradication in the 1960s. The U.S. Department of Agriculture reported the detection Wednesday in La Pryor, Texas – about 50 miles from the border – in a 3-week-old calf.
“USDA and Texas officials have taken and are taking immediate action to contain and eradicate in south Texas,” said agency Secretary Brooke Rollins.
The presence of New World screwworm could spell trouble for the ranch economy. Screwworms lay eggs in the flesh of live animals, which can cause infections, disease and death if left untreated.
In a press call Thursday afternoon, the USDA told reporters that the Animal and Plant Health Inspection Service has been deployed to the area, and a 20-kilometer quarantine and movement control zone has been implemented around La Pryor.
Rollins said the USDA is prepared to handle any potential outbreaks with the help of federal, state and local partners.
“If we all work together and follow animal treatment and the movement restriction guidance, there is no reason to believe that this incursion will result in any sort of establishment of the pest,” Rollins said.
Texas state veterinarian and Texas Animal Health Commission Executive Director Bud Dinges said the calf is doing “much better” after treatment, and no animals had moved in or out of the quarantine zone.
“This is a highly treatable condition,” Dinges said. “If you do get a case, we may have tools in our toolbox to prevent a devastating case.
Officials report no risk to food safety at this time. While the screwworm can also infect humans, the risk to public health is considered very low. The USDA is asking livestock and pet owners in the area to monitor their animals – including nearby wildlife – and work with a veterinarian to report any potential cases.
“Quick notification leads to quick detection and quick response to stop this pest from spreading,” Dinges said.
Texas ranching organizations are hard at work coordinating a response. Texas & Southwestern Cattle Raisers Association President Stephen Diebel said collaborations with the USDA have positioned the state to “respond quickly and effectively.”
“We have identification, treatment and reporting protocols in place. We are exceptionally grateful for the leadership and response of our partners on this issue,” Diebel said in a press release. “Texas ranchers and landowners may be on the front lines of this challenge, but they are not facing it alone.”
Fighting the flies
The USDA estimates the absence of screwworms saves the U.S. economy nearly $3 billion annually, based on late-1990s figures.
The pests were mostly contained near the border of Panama and Colombia for about 30 years, but the barrier was breached in 2022. In the past four years, they’ve made their way north through Mexico.
“As it began to move up a couple of years ago, all the models predicted that no matter what happened or what we did, that the New World screwworm would be on our side of the border last summer and early fall,” Rollins said. “We obviously didn't accept that as an absolute.”
A facility in Panama has been producing sterile flies for decades. The sterilized males are then dropped from airplanes. When sterile male screwworms mate with females in the wild, no eggs are produced.
USDA partners have said more sterile flies must be produced to fight potential outbreaks.
With the help of U.S. investment, a facility to produce sterile screwworms will be completed within the month in Mexico. A $750 million Texas facility is under construction that will produce 300 million sterile flies per week at full capacity, but it will not be finished until next year.
“We went on ‘Trump speed’ to get that permitted, thanks to the Department of War and our Army Corps of Engineers. What would have taken a couple of years to even get a permit just took a couple of months,” Rollins said. “And again, we broke ground on that several months ago, and are moving out quickly.”
Before eradication, the pest devastated the ranching industry.
Kansas Farmers Union Executive Director Nick Levendofsky said his dad dealt with screwworms on the family ranch as a child.
“He mentioned some of the tactics on how they dealt with it. It was pretty rudimentary – they did use things like kerosene and gasoline and all that, and they poured into the wounds. It did the job – it killed the screwworm – but then you're also dealing with everything else that goes on when you're using chemicals like that to deal with it.”
Before a medicine called “Smear 62” was invented to kill screwworms on livestock, physical extraction or home remedies – usually benzol and pine tar oil – were the only way to treat the infections.
Levendofsky said controlling the screwworm population during the summer is key to preventing spread further northward.
“In Kansas, it gets cold enough in the wintertime that it does kill the screwworm, but these summer months are going to be pretty crucial for tracking it and ensuring that we can stay ahead of it,” Levendofsky said.
The USDA reported a case 25 miles south of the Texas-Mexico border on Monday, telling reporters that the agency was prepared if the fly did cross over.
“We are preparing to implement the very moment the fly makes it into our country,” Rollins said in the Monday press call. “USDA and our state partners will work hand in hand to address the incident, and we will communicate every step of the process with the public.”
Two days later, that scenario became a reality. A 29-page response playbook outlines plans to monitor any outbreaks and reduce spread to other animals.
Producers respecting animal movement guidelines will help curb any spread outside of the quarantine zone, said Rollins.
“This fly typically moves great distances because humans move animals. These flies do not fly to new areas on their own,” Rollins said. “Without disrupting too much commerce and too much of our livestock producers' ability to make a living, it will entail all people respecting the common good of all Texas producers and follow our well-thought-out animal movement restrictions.”
Sterile flies will be dispersed in the area to reduce the population.
In May 2025, Rollins suspended imports of Mexican cattle to curb potential infections in the country. It has remained closed since then, but Rollins said partnering with Mexico will be key to mitigating outbreaks.
“We've conducted extensive audits and verification missions throughout Mexico to assess their surveillance, movement controls, prevention, and response activities,” Rollins said. “There obviously was a lot to be desired in the Mexican response, but we made some progress, and we will continue.”
A top meat industry group in Mexico told Reuters in May that the country is processing more beef domestically – rather than selling live cattle across the border – to offset the impact of the import ban. The group reported an $1.8 billion loss to Mexico’s livestock sector.
Rollins acknowledged closing the border had economic impacts in the U.S. too, but screwworm could cause more damage.
“We will obviously keep the ports closed until further notice,” Rollins said.
This story was produced in partnership with Harvest Public Media, a collaboration of public media newsrooms in the Midwest and Great Plains. It reports on food systems, agriculture and rural issues.
