When the United States Department of Agriculture reported last week that had detected a case of New World screwworm in a calf in Texas, ecologist Jeremy Radachowsky was not surprised.
Radachowsky, director of the Wildlife Conservation Society for Mesoamerica and the Western Caribbean, had long warned about the resurgence of the botfly: a species with a life cycle which looks like the plot of “Alien”.
The worms incubate exclusively in wounds or orifices of warm-blooded animals such as cows, dogs, horses and humans. Furthermore, theDespite being caused by ectoparasites, screwworm and botfly have different characteristics, starting with the fly.
Botflies are caused by blowfly larvae human dermatosis. They penetrate the skin and take a few days to develop.
The screwworm is caused by the fly Cochliomyia hominivorax which is attracted to cuts in the skin, wounds. The fly lays its eggs in small wounds or wounds, such as the navel of newborn calves.
The larvae hatch and start feeding on the animal’s living tissue, causing inflammation, pain and a bad smell that attracts other flies to lay eggs.
The parasite had previously been eradicated in North and Central America through a program multimillion-dollar, decades-long fly sterilization effort led by the United States.
But Radachowsky and other researchers have been warning for years that illegal cattle smuggling accelerated the return of the botfly to its ceded territory in Central America. It has since spread north, reaching Mexico, Texas and, this week, the New Mexico.
Cattle trafficking is a long-standing problem in Central America, where organized crime groups smuggle animals, some of which carry the blow fly, across borders without adequate health examinations, according to a 2022 report do think tank InSight Crime.
The report notes that cattle trafficking is profitable in itself, but the phenomenon also allows criminal groups to launder money through smuggled cattle and control territories by clearing jungles to make room for huge cattle ranches.
The influx of cattle and their traffickers into Central American forests has had serious consequences, Radachowsky said, including reduced tree cover, increased violence and the spread of new diseases.
“Every cow that is transported illegally has the potential to carry botfly and other diseases,” Radachowsky said. “Something that is also really scary is the possibility of transmission of bird flu through livestock and tuberculosis.”
The USDA (United States Department of Agriculture) and the Mexican Ministry of Agriculture announced new efforts to breed and release sterilized flies to hinder the spread of botfly.
The last time the botfly arrived in Texas, in the 1970s, the outbreak caused hundreds of millions of dollars in livestock losses.
But Radachowsky warns that unless the botfly is tackled 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 considerable action in things that only they can do, in order to put an end to this illicit activity,” he said.
Until then, the botfly threatens to cause to the beef industry in the southwestern United States.
Blame game
Texas Agriculture Commissioner Sid Miller has been a critic of the US response to botfly, demanding that the USDA (US Department of Agriculture) Start using the Swamp Fly Adult Suppression System (SWASS), a type of pesticide and bait, in addition to releasing sterile flies.
“I have been pushing the USDA to bring SWASS back into use for over a year,” Miller said in a statement Monday. He added that he has already provided information about the technique to Secretary of Agriculture Brooke Rollins “on three separate occasions because we know this tool works.”
Last week, Miller even made a personal appeal so that US President Donald Trump ordered the USDA (US Department of Agriculture) to implement the pest control tool.
O USDA he answered Miller’s allegations. The department’s botfly control task force posted on social media that the SWASS method uses carcinogenic chemicals and “would also attract and kill the sterile flies we are deploying.”
At a press conference, USDA Undersecretary Scott Hutchins said the technique is problematic for the environment and is “no longer viable.”
There is a lot of blame being thrown around. Rollins criticized the Mexican government for failing to crack down on “drug trafficking and illegal immigration, allowing this plague to spread rapidly throughout southern Mexico.”
Mexican President Claudia Sheinbaum’s office declined to comment when contacted by CNN.
Although experts have suggested that a recent migratory wave through the Darién, south of Panama, may have included animals carrying the blowfly, this is not a disease that is transmissible from person to person.
In July 2025, the USDA closed southern borders to cattle from Mexico to prevent transmission. Rollins defended the controversial closure by saying the measure has stopped blowflies from crossing the border before.
“We do not agree with this measure,” Sheinbaum said when the closure was announced. “The Mexican government has been working on all fronts since the first moment we were alerted about the botfly.”
Shortly after the US discovered its first cases of botfly, Mexico closed its frontier for American cattle.
At the request of cattle ranchers, Mexico carried out numerous operations and raids on the southern border to stem the flow of illegal cattle. But the botfly continued north.
Sheinbaum acknowledged to reporters last year that “it is sometimes difficult to control the passage of cattle coming into our country from Central America.”
Meanwhile, Mexican farmers have struggled with botfly. In September 2025, a farmer in Chiapas, near the border with Guatemala, lamented the difficulty of protecting his calves from the plague.
“They get the worms two or three days after birth, and that complicates things because we have to come and continue treating them,” said Fidel Gutiérrez. He told the CNN at the time he had lost a cow to botfly the previous summer, costing his small farm more than $1,000.
Not just cows
The blow fly was once the pest of farmers throughout the southern and southwestern United States.
It received its scientific name, Cochliomyia hominivorax, which is Latin for “man-eater,” when Charles Coquerel, a French naval surgeon, found a specimen on Devil’s Island in French Guiana, where the flies often deposited hundreds of eggs in the noses of unsuspecting prisoners.
“Unfortunately, science finds itself practically powerless to stop these terrible devastations,” lamented Coquerel in his original report.
A century later, Coquerel’s complaint found an answer. American entomologists Edward F. Knipling and Raymond Bushland discovered that bombarding New World blowfly pupae with gamma rays rendered the males sterile.
The two theorized that flooding nature with the irradiated, powerless flies could wipe out the species entirely.
After some tests in Florida, an experiment on the Caribbean island of Curaçao in 1954 managed to eradicate the botfly in seven weeks. Successive releases of sterile flies by the United States Department of Agriculture (USDA) over the next decade managed to initially eradicate the botfly in the USA in 1966.
Mexico and other Latin American countries joined the fight against botfly shortly thereafter, with Mexico eliminating it in 1991. In 2006, botfly was banned in Panama.
However, the fly began to reappear in 2023, likely resurfacing in Panama among animals during a migratory wave heading north.
“When the botfly broke through the blockade of Darién,” Radachowsky recalled, referring to a 65-mile stretch of roadless jungle between Colombia and Panama, “it traveled relatively slowly through Panama and then reached Costa Rica.”
Then, in 2024, Radachowsky noticed something scary: the blow fly, which can travel from if conditions were favorable, it was moving at a much greater speed.
“Once it got to Nicaragua, it started to spread very, very quickly throughout the rest of Central America,” he said. “It was moving more than a thousand kilometers in two months.”
Radachowsky and other ecologists analyzed a map of the places where the botfly had appeared and realized that the species was spreading in the meat of illegally smuggled cattle: the cases of transmission coincided with the path of known trafficking routes.
It’s not just cattle that bring the fly to the north. On Monday, the United States Department of Agriculture (USDA) reported that a dog from southern New Mexico is the botfly in the state.
Andrés Lira, a Mexican ecologist who has studied the botfly for years, says that dogs are the main vectors of spread.
“If you look at the current numbers, cattle in general come first,” said Lira. “In second place are canines. It’s highly prevalent in dogs today.”
Lira noted that the presence of botfly in dogs is exacerbated by limited animal control services in Mexico and other parts of Latin America.
“These pets that we don’t take good care of are probably spreading this a lot more than we realize,” Lira said.
As for solutions, Lira is skeptical about the possibility of completely eradicating the botfly in South America, even with a massive sterilization program. After all, she is native to this hemisphere. South American farmers have already learned how to deal with the effects of botfly on their livestock.
“We are talking about a huge territory,” said Lira. “The fly is native. My impression is that we will have to learn to live with it.”
Lira, who is currently in Germany on an exchange program, said she has already received calls from European food regulators to draw up an action plan in case the fly crosses the Atlantic.
“They see what’s happening in the Americas,” Lira said, “and they’re really worried.”
