The sun is high, the temperature is higher and a Texas cattle rancher along the U.S.-Mexico border is checking his cows. He owns hundreds, so checking them all can take time, and with a new threat, it takes longer. That new threat is screwworm flies, and the Department of Agriculture is warning ranchers to keep their eyes open because an invasion may be coming.
The new world screwworm might sound like a cartoon, but what it does is not funny. The pest can infect livestock and even domesticated animals like cats and dogs, and, if left untreated, can kill them. It can affect humans if a female fly lays eggs on an open wound, according to the Centers for Disease Control and Prevention.
“The New World screw worm is actually a parasitic fly larva … that can burrow into the flesh of a living animal and can cause serious complications, which can lead to the death of that animal if it is untreated,” USDA Secretary Brooke L. Rollins told the press during a call on Tuesday.
While the parasite hasn’t hit the U.S., Mexico is dealing with nearly 2,000 active cases and has recorded more than 26,000 since late 2024. And those active cases are getting uncomfortably close, with the nearest detected in a goat in Coahuila, Mexico, just 25 miles from the U.S. border.
The US has beaten this before, so why is it so worried?
Sixty years ago, while the U.S. was busy in Vietnam, entomologists were celebrating a victory over screwworms. That’s because the U.S. had just successfully eradicated the pests from the country.
The scientists used an incredible new form of pest control, which they called the sterile fly technique. This is when researchers use gamma or X-ray radiation to sterilize the male screwworm flies. This process works because the radiation makes them sterile but doesn’t reduce the flies’ drive to compete for a mate.
Further programs following the eradication in the ‘60s have pushed the worms south, past Mexico. But the bugs have kept creeping back north.
Rollins blamed drug cartels for the screwworms’ northward migration.
A massive response
The U.S. is mounting a large response to the potential for a new invasion, spending nearly $800 million on eradication, including $750 million for a new facility in South Texas that will produce sterile flies.
“That facility will be capable of producing up to 300 million additional sterile flies per week, which is what we really need to begin pushing this back to South America,” Rollins said.
The USDA is also spending $20 million on a joint action plan with Mexico’s agricultural authority, which funds surveillance, animal movement controls and thousands of traps across the border region. Another $21 million is going to convert a facility in Metapa, Mexico, into a sterile-fly dispersal site.
The officials on the press call on Tuesday said a confirmed case in the U.S. would lead to quarantines and restrictions on animal movements. Other responses would include trapping the flies and releasing more sterile flies to collapse any reproducing population.
“This remains at the very top of my priority list,” Rollins said. “We will not rest until new world screwworm has been eliminated once and for all.”
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