The U.S. Department of Agriculture recently suspended imports of cattle from Mexico, after new reports showed that a destructive pest called the New World Screwworm had moved north to the Mexican states of Oaxaca and Veracruz.
The New World Screwworm – which is actually a fly – was a huge problem for U.S. livestock and wildlife until it was eradicated in the 1960s. Their screw-shaped larvae burrow into mammals’ wounds and other sensitive spots of mammals, like an umbilical cord – causing disease, infection and death.
“This is a critter that we don't want back in the U.S.,” said David Anderson, a livestock professor and extension economist with Texas A&M University. “If you have livestock, you're going to be out there all the time checking your animals for any wound. … I think it would be a pretty devastating thing if we were to get it back.”
U.S. Agriculture Secretary Brooke Rollins said in a statement that the decision to suspend imports of live animals from Mexico was to protect “our animals and safety of our nation’s food supply.”
“This is not about politics or punishment of Mexico, rather it is about food and animal safety,” Rollins said.
A return of the screwworm to the U.S. would not only hurt the ranchers raising cattle and other large animals, but it would also impact the farmers growing feed and hay, experts said.
“This affects the grain farmer in Kansas or Nebraska just as much because with a reduction in cattle numbers, that’s a reduction in demand for grain,” said Wayne Cockrell, a rancher from College Station, Texas, who chairs the cattle health committee for the Texas and Southwestern Cattle Raiser’s Association.
The USDA will evaluate the cattle import suspension on a month-to-month basis, based on eradication efforts and the screwworm’s movement.
A decades-long battle
At their height, screwworms troubled ranchers from South America all the way to the Dakotas. But they were only a seasonal problem in more northern climates, since they can’t survive freezing temperatures.
Researchers in the U.S. first eradicated the insect by strategically dropping millions of sterilized screwworms from airplanes. Female screwworms mate only once, so flooding an infested area with sterile males tamps down the population with time.
Over the decades, wave after wave of sterile screwworms pushed the population down to Panama’s border with Colombia. The barrier is maintained by a joint venture between Panama and the United States called the United States Commission for the Eradication and Prevention of Screwworm, or COPEG, which produces sterile insects in a Panama facility and coordinates the air drops and on-the-ground inspections.
The program has saved an untold number of animals’ lives, and the USDA estimates that the absence of screwworms is worth $2.8 billion per year to the U.S. economy.
But in 2022, the flies broke through COPEG’s barrier and started to spread north. No one knows exactly why this happened, but the illegal movement of people and cattle likely contributed, as well as regulators being stretched thin by the pandemic.
The fly moved north through Central America, and was detected in Mexico in November 2024.
Jenny Lester Moffitt was in charge of the screwworm program at the time as USDA undersecretary for marketing and regulatory programs. Moffitt was a Biden administration appointee and left the agency in January.
“You know, a fly knows no boundaries,” Moffitt said. “And so we needed to make sure that we took a comprehensive regional approach, ensuring that we’re rearing enough flies, but then also to…adapt where we’re dispersing flies based on the movement.”
When the USDA got word that screwworms had reached Mexico last year, the department temporarily shut down cattle imports, which lasted until February. During that time, the agency installed inspection equipment in southern Mexico, and started dropping sterile insects in the country.
These efforts have failed to stop the flies from continuing northward, however.
The fight ahead
To keep the screwworm from reaching the U.S., Moffitt and others believe that more sterile flies will be necessary.
Cockrell, with the Texas and Southwestern Cattle Raiser’s Association, visited the facility in Panama that makes the sterile flies last month with a group of other Texans. He was impressed by its size, its staff and its continuous operation. The facility can produce 100 million sterile flies per week, but he said more are needed.
“The 100 million flies out of that Panamanian facility are not enough flies to push it back down to Colombia probably in my lifetime,” Cockrell said. “It’s going to take an increased amount of flies,”
If the plant in Panama shuts down for any length of time, ranchers’ best tool to fight screwworms goes away. That’s why Cockrell is one of many in the cattle industry pushing to get a sterilized insect plant built in the U.S. as soon as possible.
A sterile insect plant can’t just go anywhere. It uses nuclear radiation to sterilize screwworms, which would complicate any regulatory review process. It also needs lots of water, a few hundred staffers and a wide berth from any neighbors who might be offended by its odor (which is “the smell of death” according to Cockrell).
Cockrell and other members of the Texas and Southwestern Cattle Raiser’s Association recently met with Agriculture Secretary Rollins to discuss the issue. Association president Carl Ray Polk Jr. stressed the need for a new screwworm plant.
“This is not going away,” Polk said. “You’re never going to eradicate New World screwworm. You’re going to push it back. You’re going to put a band-aid on it. But Texas, the United States of America, needs a facility, and needs a facility quick. You’re talking about 24 to 36 months.”
Polk said that Rollins was receptive to the idea of a new sterile fly facility. He has identified sites in south Texas that might be suitable.
U.S. Rep. Tony Gonzales, a Republican whose district includes much of Texas’ border with Mexico, also wrote a bipartisan letter to Rollins in March urging the USDA to look into building a sterilized fly facility in the southwest.
Grant Drewell, a professor and veterinarian at Iowa State University, said that at the moment it’s safe for cattle businesses to purchase animals from the southern U.S., but those animals should be inspected carefully.
“We're going to want to probably do some things to treat those cattle, inspect them when they arrive here, to try to treat them to make sure we've killed all the potential parasites,” Drewell said. “Because even though it's going to be a short term problem here in Iowa, it would still be a big problem. So we want to try to minimize that.”
This story was produced in partnership with Harvest Public Media, a collaboration of public media newsrooms in the Midwest and Great Plains. It reports on food systems, agriculture and rural issues.
