Published 4:49 pm Monday, May 12, 2025
The USDA again closed the Mexican border to live cattle, bison and horse imports as the New World screwworm, a flesh-eating maggot, spreads farther north toward the U.S.
Agriculture Secretary Brooke Rollins announced the border closure May 12 in response to new cases of screwworm near Oaxaca and Veracruz, about 700 miles from the U.S.
The USDA had closed the border between Nov. 22 to Jan. 31 after screwworm was detected in the southern Mexican state of Chiapas. The border was reopened Feb. 1 after the countries agreed on a plan to contain the parasite.
Rollins complained in a letter in April to her counterpart in Mexico that Mexican authorities were not cooperating fully with the U.S. to execute the plan, which includes dropping millions of sterile male flies from the air.
Mexican President Claudia Scheinbaum said at a press conference her government disagreed with the closure.
“The Mexican government had been working on all fronts since the moment the alert on the screwworm was issued,” she said. “We hope that this measure, which we consider unfair, will be lifted very soon.”
Cattlemen’s groups praised the renewed closure as a necessary precaution to protect U.S. livestock. About a million head of live cattle cross into the U.S. from Mexico annually, though crossings are way down so far this year.
“I think it exemplifies the vulnerability the U.S. has as a result of not maintaining self-sufficiency in beef production,” R-CALF CEO Bill Bullard said. “This should be an awakening and a signal that the U.S. needs to rebuild its cattle herd.”
As maggots, New World screwworms (Cochliomyia hominivorax) corkscrews with sharp teeth into wounds or openings of animals. The parasite is endemic in the Caribbean and most of South America.
The screwworm became a problem in the Southwest and Southeast U.S. in the 1930s. In the 1950s, scientists discovered how to breed males sterilized with gamma radiation.
The sterile males are normal in all other ways, but frustrate efforts to reproduce. By 1966, the screwworm was eradicated in the U.S., though there were short-lived outbreaks in Texas and Florida in the 1970s. By 1986, the U.S. and Mexico had driven the parasite out of Mexico.
To contain the parasite, the USDA breeds and sterilizes up to a million male flies a week at a laboratory in Panama and releases them on the isthmus to create a biochemical barrier.
The screwworm broke through the barrier over the past two years, moving north through Costa Rica, Nicaragua, Honduras and Guatemala before being found in a cow in November in Chiapas.
Rollins threatened to re-close the border in an April 11 letter to Mexico’s agriculture secretary, Julio Berdegue. Mexico was charging tariffs on the U.S. aircraft, equipment and sterilized flies brought into Mexico to wage war against the screwworm, Rollins complained.
Mexican authorities also were limiting aerial spraying to six days a week. The closure was averted by a new April 30 agreement between the counties.
A 6-day-old calf in Veracruz and 5-year-old horse in Oaxaca, however, were later found infected, moving the parasite closer to the U.S. and prompting Rollins to close the border indefinitely.
“Secretary Berdegue and I have worked closely on the (screwworm) response. However, it is my duty to take all steps within my control to protect the livestock industry in the United States from this devastating pest,” Rollins said in a statement.
