With Mexican cattle again barred this month from entry to the United States over fears of spreading a flesh-eating parasite, ranchers and veterinarians in Mexico hundreds of miles from the border are fighting what has U.S. agricultural authorities so on edge.
In the southern state of Chiapas, which borders Guatemala, the New World screwworm fly’s rapid spread appears to have caught most ranchers off guard, despite memories of previous outbreaks in the 1980s and 1990s.
Mexico is building a plant with U.S. support in Chiapas to produce sterile flies, which have proven effective at stopping the spread, but it won’t be ready until next year. Meanwhile, the price of medicines used to treat livestock infected with the screwworm have soared in price.
That has led some to fall back on home remedies like applying gasoline or lime to open wounds to coax out the worms.
In addition to the cost of the medicine, treatment requires careful monitoring and usually involves multiple courses. Any open wound, even very small ones, are an invitation to the fly to lay its eggs.
Veterinarian Alfredo Chávez left Chiapas to study in 1989, so he says he missed seeing the effects of that outbreak, but now he’s seen cases multiply in his corner of the state over the past month.
He’s heard of dozens of cases in the area now and treated about a dozen himself. It’s not just cows either — sheep, pigs, cats and dogs are targets as well.
Armed with a pair of blue tweezers and an aerosol spray that helps draw the maggots out, Chávez moves from animal to animal. He puts maggots in plastic tubes as samples, which he provides to agricultural authorities.
But beyond providing the tubes and encouraging ranchers to report cases, he said that the government hasn’t provided much help.
“We’ve faced it alone,” he said Wednesday.
The U.S. had just gradually started to reopen the border to cattle imports this month after an earlier suspension in May, when the Trump administration said that it would close it again after an infected animal was found in the Gulf coast state of Veracruz. While prevalent in Central America, the concern is that the fly is moving north.
U.S. officials worry that if the fly reaches Texas, its maggots could cause large economic losses, something that happened decades ago.
Ranch caretaker Edi Valencia Santos said that Mexican government officials have come to his community to talk to people with livestock, but so far without resources. He has had five infected animals on the ranch.
Despite cattle in this region going to domestic consumption rather than to the U.S., the presence of the screwworm in Mexico has frozen cattle exports to the U.S. nationwide.
Valencia said that he remembers the small planes distributing sterile flies during those earlier outbreaks, so is optimistic they will eventually help, but for now the costs are piling up on ranchers.
“It’s a big, big problem in Chiapas,” he said.
Correction, July 28, 2025: A previous version of this story misidentified one of the Associated Press authors.
