Summary
- A once-eradicated parasite that incubates in the wounds of warm-blooded animals has returned to the US.
- Experts link the resurgence of New World screwworm to illegal cattle trafficking by organized crime groups in Central America.
- The outbreak threatens billions in damage to the beef industry as officials debate control methods.
AI-generated summary was reviewed by a CNN editor.
When the US Department of Agriculture reported last week that it detected a case of New World screwworm in a Texas calf, ecologist Jeremy Radachowsky was not surprised.
Radachowsky, the Mesoamerica and Western Caribbean director for the Wildlife Conservation Society, had long warned of the resurgence of the screwworm fly: a species with a life cycle that sounds like the plot of “Alien.”
Screwworms incubate exclusively in the wounds or orifices of warm-blooded animals such as cows, dogs, horses and human beings. The parasite had previously been eradicated in North and Central America through a multimillion-dollar, decades-long program of fly sterilization led by the United States.
But Radachowsky and other researchers have warned for years that illegal cattle smuggling has quickened the return of screwworm to its ceded territory in Central America. It has since spread northward to Mexico, Texas and, as of this week, New Mexico.
Cattle trafficking is a long-standing issue in Central America, where organized crime groups smuggle livestock, some of which carry screwworm, across borders without legitimate health screenings, according to a 2022 report from the think tank InSight Crime.
The report notes that cattle trafficking is lucrative on its own, but the phenomenon also allows criminal groups to launder money through smuggled cattle and control territory via jungle deforestation to make room for massive cattle ranches.
The influx of cattle and their traffickers into the forests of Central America has had serious consequences, Radachowsky said, including receding tree cover, growing violence and the spread of new diseases.
“Every cow that is being moved illegally has the potential to carry a screwworm and other diseases,” Radachowsky said. “Something that’s really frightening as well is that you have avian flu transmitted by cattle and tuberculosis.”
The USDA and the Mexican Agriculture Department have announced new efforts in breeding and releasing sterilized flies to hamper the spread of screwworm. The last time the screwworm wriggled its way into Texas, in the 1970s, the outbreak caused hundreds of millions of dollars in cattle losses.
But Radachowsky warns that unless screwworm is stopped at the source, the problem will remain.
“What we really need are the governments of the United States, Mexico and (Central American countries) to come together and take considerable action in things that only they can do, in order to shut down this illicit activity,” he said.
Until then, the screwworm threatens to cost billions of dollars in damage to the beef industry in the southwestern United States.
Texas Agriculture Commissioner Sid Miller has been a critic of the US response to screwworm, demanding that the USDA begin using the Screwworm Adult Suppression System (SWASS), a type of pesticide and bait, in addition to sterile fly releases.
“For over a year, I’ve been pushing USDA to bring SWASS back into the fight,” Miller said in a statement Monday. He added that he has given Agriculture Secretary Brooke Rollins information about the technique “three separate times because we know this tool works.”
Miller last week even made a personal plea that US President Donald Trump direct the USDA to deploy the pest management tool.
The USDA has pushed back on Miller’s claims, with the Department’s screwworm task force writing on social media that SWASS uses carcinogenic chemicals and “would also attract and kill the sterile flies we are deploying.” At a Monday press conference, USDA Undersecretary Scott Hutchins said that the technique is environmentally problematic and “no longer really viable to utilize anymore.”
There is plenty of blame to go around. Rollins has criticized the Mexican government for not cracking down on “cartel trafficking and immigration, allowing the pest to spread quickly across southern Mexico.”
Mexican President Claudia Sheinbaum’s office declined to comment when reached by CNN.
While experts have suggested that a recent wave of migration through the Darien Gap south of Panama may have included animals carrying screwworm, it is not a disease that can be transmitted from person to person.
The USDA closed southern border ports to livestock from Mexico in July 2025 to prevent transmission. Rollins has credited the controversial closure with keeping the screwworm from crossing the border earlier.
“We do not agree with this measure,” Sheinbaum said when the closure was announced. “The Mexican government has been working on all fronts from the very first moment we were alerted to the screwworm.”
Soon after the US discovered its first cases of screwworm, Mexico closed its border to American livestock.
At the urging of ranchers, Mexico has carried out numerous stings and raids at the southern border to stanch the flow of illegal cattle. But the screwworm continued its march north.
Sheinbaum acknowledged to reporters last year that “sometimes it is difficult to control the passage of cattle coming from Central America into our country.”
Mexican farmers have struggled with screwworm in the meantime. In September 2025, one farmer in Chiapas, near the Guatemala border, bemoaned the difficulty of keeping his calves safe from the pest.
“They get the worms within two or three days of birth, and that complicates things because we have to come and keep treating them,” Fidel Gutíerrez said. He told CNN at the time that he had lost one cow to screwworm the summer before, at a cost of over $1,000 for his small farm.
Screwworm was once the bane of ranchers throughout the southern and southwestern United States. It earned its scientific name, Cochliomyia hominivorax, Latin for “man-eater,” when Charles Coquerel, a French naval surgeon, encountered a specimen from Devils Island in French Guiana, where the flies often laid hundreds of eggs in unsuspecting prisoners’ noses.
“Science unfortunately finds itself nearly powerless to halt these terrible ravages,” Coquerel lamented in his original report.
A century later, Coquerel’s complaint met an answer. American entomologists Edward F. Knipling and Raymond Bushland found that bombarding New World screwworm pupae with gamma rays would render the males sterile. The two theorized that flooding the wild with the irradiated, impotent flies could extinguish the species entirely.
After a few trial runs in Florida, an experiment on the Caribbean island of Curaçao in 1954 managed to banish the screwworm in seven weeks. Successive releases of sterile flies by the USDA across the United States over the next decade managed to initially eradicate the screwworm in the US in 1966. Mexico and other countries in Latin America joined the war on the screwworm soon afterward, with Mexico eliminating it in 1991. By 2006, the screwworm was banished from Panama.
Yet the fly began to make a comeback in 2023, likely reemerging in Panama among animals during a migrant surge northward.
“When screwworm broke through the Darien Gap,” Radachowsky recalled, referring to a 66-mile stretch of roadless jungle between Colombia and Panama, “it sort of traveled fairly slowly through Panama and then made its way into Costa Rica.”
Then, in 2024, Radachowsky noticed something frightening: The screwworm, which can travel six to 12 miles if conditions are favorable, was moving at a much faster rate.
“When it got to Nicaragua, it started to move really, really rapidly throughout the rest of Central America,” he said. “It was moving above maybe a thousand kilometers (roughly 621 miles) in two months.”
Radachowsky and other ecologists looked at a map of where screwworm had appeared and realized that the species was hitching a ride in the flesh of illegally trafficked cattle: the transmission instances matched the path of previously known trafficking routes.
It’s not just cattle that bring the fly north. On Monday, the USDA said that a dog from southern New Mexico is the state’s first confirmed case of screwworm. Andrés Lira, a Mexican ecologist who has studied the screwworm for years, says that dogs are a primary driver of the spread.
“If you look at the current numbers, first it’s cattle and livestock,” said Lira. “Second is canids. It’s highly prevalent in dogs today.”
Lira noted that the screwworm’s presence among dogs is compounded by limited animal control services in Mexico and other parts of Latin America.
“These companion animals that we don’t take good care of are probably spreading this much more than we could understand,” Lira said.
As for solutions, Lira is skeptical that the screwworm could be eradicated in South America entirely, even with a massive sterilization program. It is, after all, native to this hemisphere. South American farmers have learned to account for the screwworm’s effects on their livestock.
“We’re talking about a huge territory,” Lira said. “The fly is native. My impression is that we’ll have to learn to live with this.”
Lira, currently in Germany on a fellowship, said he has already fielded calls from European food regulators to come up with a battle plan in case the fly crosses the Atlantic.
“They see what’s happening in the Americas,” Lira said, “and they are really worried.”
CNN’s Jen Christensen, Valeria Leon and Rocio Muñoz Ledo contributed to this report.
Correction: An earlier version of this article included a photo of a man incorrectly identified as Texas Agriculture Commissioner Sid Miller
