Texas, USA — When the US Department of Agriculture reported last week that it had detected a case of New World screwworm in a calf in Texas, ecologist Jeremy Radachowsky was not surprised.
Radachowsky, director for Mesoamerica and the Western Caribbean at the Wildlife Conservation Society, had been warning for some time about the resurgence of the screwworm fly — a species whose life cycle recalls the plot of the movie “Alien.”
The screwworm incubates exclusively in the wounds or orifices of warm-blooded animals such as cows, dogs, horses, and humans. The parasite had previously been eradicated in North and Central America thanks to a multi-million-dollar fly sterilization program led by the United States that lasted decades.
But Radachowsky and other researchers have been warning for years that illegal cattle smuggling has accelerated the screwworm’s return to its former territory in Central America. Since then, it has spread northward into Mexico, Texas, and, as of this week, New Mexico.
Cattle trafficking is a long-standing problem in Central America, where organized crime groups traffic animals — some carrying screwworm disease — across borders without proper health controls, according to a 2022 report by the think tank InSight Crime.
The report notes that cattle trafficking is lucrative in itself, but it also allows criminal groups to launder money through cattle smuggling and control territory by clearing jungle for huge cattle ranches.
The influx of cattle and their traffickers into Central American forests has had serious consequences, Radachowsky said, including reduced forest cover, increased violence, and the spread of new diseases.
“Every cow that is moved illegally has the potential to carry screwworms and other diseases,” Radachowsky said. “What’s really alarming is that there is also the possibility that cattle could transmit bird flu and tuberculosis.”
The USDA and Mexico’s Ministry of Agriculture have announced new initiatives to breed and release sterile flies to curb the spread of the screwworm. The last time the pest reached Texas, in the 1970s, the outbreak caused hundreds of millions of dollars in livestock losses.
But Radachowsky warns that unless the screwworm plague is stopped at its source, the problem will persist.
“What we really need is for the governments of the United States, Mexico, and Central American countries to come together and take significant actions — which only they can carry out — to end this illicit activity,” he said.
Until then, the screwworm threatens to cause billions of dollars in damage to the meat industry in the southwestern United States.
Blame Game
Texas Agriculture Commissioner Sid Miller has criticized the US response to the screwworm plague and demanded that the USDA begin using the Screwworm Adult Suppression System (SWASS), a type of pesticide and bait, in addition to releasing sterile flies.
“For over a year, I have been pressuring the USDA to use SWASS again in the fight,” Miller said in a statement Monday. He added that he had provided information on the technique to Agriculture Secretary Brooke Rollins “on three separate occasions because we know this tool works.”
Last week, Miller even made a personal call to President Donald Trump to order the USDA to implement the pest control tool.
The USDA has disputed Miller’s claims. The Department’s screwworm task force posted on social media that SWASS uses carcinogenic chemicals and “would also attract and kill the sterile flies we are using.” In a press conference Monday, USDA Under Secretary Scott Hutchins said the technique is problematic from an environmental standpoint and “is no longer viable.”
There are many culprits. Rollins has criticized the Mexican government for not cracking down on drug trafficking and illegal immigration, allowing the problem to spread rapidly through southern Mexico.
Mexican President Claudia Sheinbaum’s office declined to comment when contacted by CNN.
While experts have suggested that the recent wave of migration through the Darien Gap in southern Panama may have included animals carrying the screwworm, the disease is not transmitted from person to person.
The USDA closed southern border ports to cattle from Mexico in July 2025 to prevent transmission.
“We disagree with this measure,” Sheinbaum said when the closure was announced. “The Mexican government has been working on all fronts from the moment we were first alerted to the presence of the screwworm.”
Shortly after the US discovered its first screwworm cases, Mexico closed its border to US cattle.
Mexico has conducted numerous raids and operations on its southern border to stem the flow of illegal cattle. But the plague continued its northward advance.
Last year, Sheinbaum acknowledged to journalists that “sometimes it is difficult to control the passage of cattle from Central America into our country.”
Meanwhile, Mexican farmers have struggled with the screwworm. In September 2025, a farmer in Chiapas, near the border with Guatemala, lamented the difficulty of protecting his calves from the pest.
“The worms appear two or three days after birth, and that complicates things because we have to come and treat them continuously,” said Fidel Gutierrez. He told CNN that the previous summer he had lost a cow to the screwworm, costing his small farm more than $1,000.
Not Just Cows
The screwworm was once the nightmare of livestock farmers across the southern and southwestern United States. It earned its scientific name, Cochliomyia hominivorax, which in Latin means “man-eater,” when French naval surgeon Charles Coquerel found a specimen from Devil’s Island in French Guiana, where the flies used to lay hundreds of eggs in the noses of unsuspecting prisoners.
“Unfortunately, science is practically powerless to stop these terrible ravages,” Coquerel lamented in his original report.
A century later, Coquerel’s complaint found an answer. US entomologists Edward F. Knipling and Raymond Bushland discovered that bombarding New World screwworm pupae with gamma rays sterilized the males. They theorized that flooding nature with these irradiated, impotent flies could completely extinguish the species.
After several pilot tests in Florida, an experiment on the Caribbean island of Curacao in 1954 eradicated the screwworm in seven weeks. Subsequent releases of sterile flies by the USDA over the next decade initially eradicated the screwworm in the US in 1966. Mexico and other Latin American countries joined the fight soon after, and Mexico eliminated it in 1991. By 2006, the screwworm had been eradicated from Panama.
However, the fly began to reappear in 2023, likely in Panama, among animals during a northward migration wave.
“When the screwworm crossed the Darien Gap,” Radachowsky recalled, referring to a 106-kilometer stretch of roadless jungle between Colombia and Panama, “it traveled quite slowly through Panama and then made its way into Costa Rica.”
Then, in 2024, Radachowsky noticed something alarming: the screwworm, which can travel between 9 and 20 kilometers (6 to 12 miles) under favorable conditions, was moving at a much faster pace.
“When it reached Nicaragua, it began to move very, very quickly through the rest of Central America,” he said. “It covered perhaps more than 1,000 kilometers (621 miles) in two months.”
Radachowsky and other ecologists examined a map of where the screwworm larvae had appeared and realized the species was traveling attached to the flesh of illegally trafficked cattle: the transmission cases coincided with previously known trafficking routes.
Cattle were not the only ones carrying the fly north. This Monday, the USDA reported that a dog in southern New Mexico is the first confirmed screwworm case in the state. Andres Lira, a Mexican ecologist who has studied the screwworm for years, says dogs are a major factor in its spread.
“If we look at the current numbers, first are cows and cattle,” Lira said. “Second are canids. Today it is very common in dogs.”
Lira noted that the presence of screwworm among dogs is exacerbated by the scarcity of 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 can imagine,” Lira said.
As for solutions, Lira is skeptical about the possibility of completely eradicating the screwworm in South America, even with a massive sterilization program. After all, it is native to that hemisphere. South American ranchers have learned to control the effects of the screwworm on their livestock.
“We are talking about a huge territory,” Lira said. “The fly is native. I have the impression that we will have to learn to live with it.”
Lira, who is currently in Germany on a fellowship, said he has already received calls from European food regulators to draw up an action plan in case the fly crosses the Atlantic.
