WASHINGTON — Judy McCullough, a rancher in Wyoming, still remembers the blood sprayed on the barn walls, the smell of burning tar and the fear of finding a maggot nestled in the broken hide of a cow.
More than 70 years ago, when McCullough was a child on her grandfather’s cattle ranch, the New World screwworm was all but certain to incite dread. Feeding on the flesh of the live cattle, it laid eggs on open wounds, killing the animals if it went untreated.
Ranchers resorted to a number of preventive measures: spraying noxious pesticides, dehorning and castrating calves in colder months that screwworm larvae could not survive, and branding using a tar mixture to minimize open flesh.
“Nothing’s nastier than this maggot,” McCullough, 79, said, recalling the aftermath of a gory dehorning. She pointed to the care that people “took of wounds, even on themselves,” given that the fly could be equally deadly to humans.
Since the 1970s, the screwworm has largely stayed out of the United States, kept at bay by an eradication campaign that has prevented the large-scale loss of livestock and wildlife and saved the cattle industry $2.3 billion a year, according to one government estimate. But after breaching a biological barrier in Panama in 2022, the flesh-eating parasite is at risk of returning, spurring the United States and Mexico to invest in increased biological countermeasures, surveillance, detection and scientific research.
The efforts reflect a scramble to address a potentially lethal threat whose spread could have wide-ranging ramifications. The cattle industry already faces high feed prices, drought and consolidation. And because the screwworm’s appetite is not limited to beef, its return could infect other farmed animals, wildlife like deer and rabbits, pets, and even people.
Former officials with experience in screwworm eradication warned that the Trump administration’s workforce reductions and cuts to agricultural research could complicate its attempt to stamp out the parasite. More than 1,300 workers in the Agriculture Department’s Animal and Plant Health Inspection Service, or APHIS, took the administration’s buyout offer this year, including 300 employees in veterinary services.
Tanya Espinosa, a spokesperson for APHIS, said the agency had exempted crucial positions like veterinarians from a hiring freeze, and that the screwworm team had grown in size. But she did not respond directly to a question about whether any of the job cuts involved employees who worked on screwworm eradication.
Dr. Kevin Shea, a former APHIS administrator, said that simply replacing workers may not be an adequate solution. “It’s not just the number. It’s the experience,” he said. “Screwworm’s a pretty specialized thing, right? Not too many people know about it or ever worked in it, and if you lose people who have that expertise, that’s a big problem.”
Rearing Millions of Sterile Flies
Fly factories will be crucial to the international effort to eradicate the screwworm.
At one facility in Panama, scientists are breeding and sterilizing 100 million flies a week. Another factory in southern Mexico, set to open by 2026, is expected to produce 100 million more.
Government scientists in the 1950s pioneered the technology used to eradicate the screwworm from the United States, blasting the flies with radiation to sterilize them. The larvae are raised in biosecure facilities on a protein-heavy diet (a mixture that can include dried bovine blood, eggs, honey, molasses and dried milk).
Once adults, the sterile flies are released in areas with known populations of screwworm. Females mate only once, meaning their union with sterilized males produces no offspring and allows the population to die out over time.
The United States used this method to push the screwworm back to Panama, where a permanent barrier at the Darién Gap, on the border with Colombia, was established in 2006 and maintained through weekly drops of millions of sterilized flies. Experts are not entirely sure how the screwworm breached that barrier in 2022, but some have pointed to increased migration of cattle and humans, changing weather conditions and the adaptability of the screwworm.
Since 2022, the screwworm has been detected throughout Central America and southern Mexico. Containing the spread has proved extremely challenging: APHIS estimates that it would take “several years of intensive efforts” to eradicate the current outbreak.
Ramping up sterile fly production is key.
In 2023 and 2024, the Agriculture Department dedicated more than $270 million to increase weekly production in Panama from 20 million to 100 million; establish additional animal checkpoints; and increase awareness in Central America. This year, the agency announced it would funnel nearly $30 million more to reopen a sterile fly production facility in southern Mexico and a dispersal facility in Texas. When completed at the end of the year, the Mexico factory will produce 60 million to 100 million flies every week.
That is still short of the estimated 500 million flies needed to push the screwworm back to Panama and reestablish the barrier at the Darién Gap. In August, a coalition of more than 170 groups representing cattle ranchers, dairymen, hog and sheep farmers, horse breeders, and wildlife managers wrote to the Agriculture Department urging the construction of a domestic fly production facility.
In the meantime, the Agriculture Department said it was exploring “all options.”
Espinosa said public outreach, strong surveillance and effective controls on the movement of animals were also important pillars to the agency’s eradication program. As part of its surveillance, the agency is using more than 9,000 existing fruit fly traps and placing hundreds of additional screwworm traps along Texas’ border with Mexico.
Gene Editing, Synthetic Bait and Other Tools
Because it will take months, if not years, for sterile fly production to ramp up to the needed level, officials on both sides of the border are looking to other solutions.
In Mexico, where authorities recently confirmed the country’s first case of screwworm in humans, researchers are looking at gene editing and researching possible evolutionary changes among flies in different regions. In Texas, a top agricultural official pointed to synthetic bait.
“We don’t have to be dependent on these sterile male flies,” said Sid Miller, the commissioner of agriculture in Texas. “Everybody’s depending on this 60-year-old technology that took 36 years to work. We’re trying to use some modern technology — that’s quicker, faster, better — to improve screwworm eradication.”
More than 2,700 cases of screwworm have been reported in cattle in southern Mexico since last November. Even though the case closest to the United States, now inactive, was 370 miles away from the border, “the situation is critical,” said Miguel Galarde, a researcher at Mexico’s National Institute of Forestry, Agriculture and Livestock Research.
Although the Mexican government will also invest in the fly factory, sterilizing flies is only one part of the solution, said Galarde. “The solution is to treat and treat and treat all wounds,” he said. “With or without future larvae.”
Galarde is part of a team focused on researching the screwworm fly. The group hopes to study the evolution of screwworm flies and possible gene-editing solutions; different types of bait or lures scientists could use in the wild; the flies’ resistance to larvicides; and the effects of those pesticides on the environment.
In countries across South America, where the screwworm is endemic, genetic tools are a hopeful solution. Uruguay’s National Institute of Agricultural Research and Institut Pasteur of Montevideo are using genome editing to create a population of male screwworms with a gene that affects fertility. When released into the wild, these males mate with females and pass on the infertility gene. Over time, the gene will cause more generations of females to become sterile, leading to a sharp population decline, said Alejo Menchaca, the director of animal health at the research institute.
But most of these techniques are still undergoing experimentation.
Detailed studies need to be done on the ecological effect these methods will have, said Moisés Vargas-Terán, an international consultant who led efforts to eradicate an outbreak of screwworm in Libya in the 1990s.
“The only viable technique right now is the sterile insect technique,” he said.
A spokesperson for the Agriculture Department said that genetically engineered organisms could enhance the department’s abilities during this outbreak.
Miller said Texas planned to deploy a synthetic bait made of insecticide and “swormlure,” a concoction of different chemicals that mimics the smell of open wounds, in every county along the Rio Grande. The bait is estimated to be 90% effective in killing the screwworm, he said. Texas is also exploring a screwworm vaccine, as well as approving the use of the deworming medication ivermectin to treat infected animals, Miller added.
For McCullough, the eradication effort in the 1960s was one she hoped the government could repeat.
“Have you read the Bible? Revelation talks about a fly that does what these guys can do. So I’m just saying I hope we can keep it out,” McCullough said. “I hope we can. Better say your prayers, man.”