Ecologist Jeremy Radakowski wasn’t surprised when the U.S. Department of Agriculture reported last week that it had detected cases of New World screwworm in calves in Texas.
Radachowski, Mesoamerica and Western Caribbean regional director for the Wildlife Conservation Society, has been warning for years about the resurgence of the screwworm fly, a species with a life cycle reminiscent of an “Alien” story.
Screw maggots hatch only in wounds or openings in warm-blooded animals such as cows, dogs, horses, and humans. The parasite had previously been eradicated in North and Central America through a decades-long, multimillion-dollar fly sterilization program led by the United States.
But Radachovsky and other researchers have long warned that illegal cattle smuggling is accelerating the return of screwworms to the ceded territories of Central America. Since then, it has spread north to Mexico, Texas, and, as of this week, New Mexico.
Cattle trafficking has been a long-standing problem in Central America, with organized crime groups smuggling screwworm-carrying animals across borders without proper health checks, according to a 2022 report from think tank Insight Crime.
The report notes that while cattle trafficking is lucrative in its own right, the phenomenon also allows criminal groups to launder money through cattle smuggling and control territory through jungle clearing to create large-scale ranches.
Radachovsky said the influx of cattle and their traffickers into Central American forests is having serious impacts, including retreating tree cover, increased violence and the spread of new diseases.
“Any cattle that are moved illegally can carry screwworm and other diseases,” Radachowski said. “What’s really scary as well is being infected with avian influenza, which is transmitted through cattle and tuberculosis.”
The USDA and Mexico’s Department of Agriculture have announced a new effort to breed and release sterile flies to prevent the spread of screwworms. The last time screwworms entered Texas was in the 1970s, and outbreaks caused hundreds of millions of dollars in losses to cattle.
But Radachowski warns that unless we stop the warts at the source, the problem will remain.
“What we really need is for the U.S., Mexican, and (Central American) governments to come together and do what only they can do to stop this illegal activity,” he said.
In the meantime, screwworms threaten to cause billions of dollars in damage to the beef industry in the southwestern United States.
Texas Agriculture Commissioner Sid Miller has criticized the U.S. response to screwworms and has called on the USDA to begin using the Screwworm Adult Suppression System (SWASS), a type of insecticide and bait, in addition to releasing sterile flies.
“For more than a year, I have been lobbying the Department of Agriculture to bring SWASS back into the fight,” Miller said in a statement Monday. He added that he provided Agriculture Secretary Brooke Rollins with information about the technique “in three separate installments.” This is because this method is known to be effective.
Last week, Miller personally petitioned US President Donald Trump to direct the Department of Agriculture to implement pest management tools.
The USDA pushed back on Miller’s claims, with the agency’s screwmaggot control team writing on social media that SWASS uses carcinogenic chemicals that “could also attract and kill the sterile flies we deploy.” U.S. Department of Agriculture Undersecretary Scott Hutchins said at a press conference Monday that the technology has environmental problems and is “no longer practical for practical use.”
There are many responsibilities. Rollins criticized the Mexican government for allowing the pest to spread rapidly into southern Mexico by failing to crack down on “cartel trafficking and immigration.”
Mexican President Claudia Sheinbaum’s office declined to comment when contacted by CNN.
Experts have suggested that recent waves of migration through southern Panama’s Darien Valley may have included animals carrying screwworm, but screwworm is not a disease that can be passed from person to person.
The USDA closed the southern border port to receive livestock from Mexico in July 2025 to prevent infection. Rollins credits the controversial closure with preventing the screwworms from crossing the border sooner.
“I do not agree with this measure,” Scheinbaum said when the closure was announced. “The Mexican government has been working on all fronts from the moment we first learned of the existence of screwworms.”
Shortly after the United States discovered its first case of screwworm, Mexico closed its borders to U.S. livestock.
At the request of ranchers, Mexico conducted a number of crackdowns and raids on its southern border to stem the influx of illegal cattle. But the screwworm continued its march north.
“It can be difficult to control the movement of cattle into our country from Central America,” Scheinbaum acknowledged to reporters last year.
Meanwhile, Mexican farmers have been battling screwworms. In September 2025, a farmer in Chiapas, near the Guatemalan border, lamented the difficulty of protecting his calves from pests.
“It gets complicated because they get infected with parasites within a few days of birth and we have to come and continue the treatment,” Fidel Gutierrez said. He told CNN at the time that he lost a cow to screwworms the summer before, costing his small farm more than $1,000.
Screwworm flies were once the bane of ranchers throughout the southern and southwestern United States. The scientific name Cochliomyia hominivorax, meaning “man-eater” in Latin, was given to French naval surgeon Charles Coquerel when he encountered a specimen he discovered on Devil’s Island in French Guiana, where the flies often deposited hundreds of eggs in the noses of unsuspecting prisoners.
“Unfortunately, science has proven largely powerless to stop these horrific destructions,” Cockerel lamented in his initial report.
A century later, Coquerel’s complaint was answered. American entomologists Edward F. Knipling and Raymond Bushland discovered that exposing New World screwworm pupae to gamma rays rendered the males infertile. The pair theorized that if the irradiated and incapacitated flies flooded the wild, they could wipe out the species entirely.
After several trials in Florida, a 1954 experiment on the Caribbean island of Curaçao successfully eradicated screwworms in seven weeks. Over the next 10 years, the Department of Agriculture released sterile flies throughout the United States, and in 1966 they succeeded in eradicating the screwworm fly in the United States. Mexico and other countries in Latin America soon joined the war against the screwworm, and Mexico eradicated the disease in 1991. By 2006, the screwworm maggot was banished from Panama.
But the fly began making a comeback in 2023, likely reappearing among animals in Panama during a surge of migrants north.
“When the warblers broke through the Darien Valley, they moved pretty slowly through Panama and into Costa Rica,” Radachowski recalled of the 106 miles of roadless jungle between Colombia and Panama.
Then in 2024, Radachowski made a terrible realization. Screw maggots, which can travel 6 to 12 miles under favorable conditions, are traveling at much faster speeds.
“Once it got to Nicaragua, it started moving really quickly throughout the rest of Central America,” he says. “We probably traveled more than 1,000 kilometers in two months.”
Radachovsky and other ecologists looked at maps of where the spinach maggots appeared and realized that they were making their way into the meat of illegally traded cattle. The transmission cases matched the routes of previously known trafficking routes.
It’s not just cows that bring flies north. On Monday, the Department of Agriculture announced that a dog in southern New Mexico was the first confirmed case of screwworm in the state. Andres Lira, a Mexican ecologist who has studied screwworms for years, says dogs are the main driver of the spread.
“If you look at the current numbers, first of all it’s cattle and livestock,” Lira said. “The second one is canines. It’s very prevalent among dogs today.”
Lira noted that the presence of warts among dogs is exacerbated by limited animal control services in Mexico and other parts of Latin America.
“Companion animals that we don’t take good care of are probably spreading the virus more than we understand,” Lila said.
As for solutions, Lira is skeptical that screwworms can be completely eradicated in South America, even with large-scale sterilization programs. After all, it is unique to this hemisphere. Farmers in South America can now explain how screwworms affect their livestock.
“We are talking about a vast territory,” Lila said. “This fly is a native species. My impression is that we have to learn to coexist with it.”
Lila, who is currently in Germany on a fellowship, said she has already responded to requests from European food regulators to come up with a plan for what to do if the flies cross the Atlantic.
“They’re watching what’s happening in the Americas and they’re really concerned,” Lila said.
CNN’s Jen Christensen, Valeria Leon and Rocio Muñoz Ledo contributed to this report.
