The US has closed its southern border again to livestock imports, saying a flesh-eating parasite has moved further north in Mexico than previously reported.
Mexico’s president was critical Thursday, suggesting that the US is exaggerating the threat to its beef industry from the parasite, the New World screwworm fly. The female flies lay eggs in wounds on warm-blooded animals, hatching larvae that are unusual among flies for feeding on live flesh and fluids instead of dead material.
American officials worry that if the fly reaches Texas, its flesh-eating maggots could cause large economic losses, something that happened decades ago. The US largely eradicated the pest in the 1970s by breeding and releasing sterile male flies to breed with wild females, and the fly had been contained in Panama for years until it was discovered in southern Mexico late last year.
The US closed its southern border in May to imports of live cattle, horses and bison but announced June 30 that it would allow three ports of entry to reopen this month and another two by Sept. 15. However, since then, an infestation from the fly has been reported 185 miles  northeast of Mexico City, about 160 miles  further north than previously reported cases. That was about 370 miles  from the Texas border.
“The United States has promised to be vigilant,” US Department of Agriculture Secretary Brooke Rollins said in a statement Wednesday announcing the border closing. “Thanks to the aggressive monitoring by USDA staff in the US and in Mexico, we have been able to take quick and decisive action to respond to the spread of this deadly pest.”
In Mexico, President Claudia Sheinbaum said authorities there were following all established protocols to deal with the northernmost case. Mexican authorities said the country has 392 infected animals, down nearly 19 percent since June 24.
“From our point of view, they took a totally exaggerated decision to closing the border again,” Sheinbaum said. “Everything that scientifically should be done is being done.”
Three weeks ago, Rollins announced plans for combating the parasite that include spending nearly $30 million on new sites for breeding and dispersing sterile male flies. Once released in the wild, those males would mate with females, causing them to lay eggs that won’t hatch so that the fly population would die out.
The USDA hopes a new fly factory will be operating in southern Mexico by July 2026 to supplement fly breeding at an existing complex in Panama. The agency also plans to open a site in southern Texas for holding sterile flies imported from Panama, so they can be released along the border if necessary.
Also Thursday, US Reps. Tony Gonzalez, of Texas, and Kat McCammack, of Florida, urged the Trump administration to quickly approve the use of existing anti-parasite treatments for New World screwworm fly infestations in livestock. They said labeling requirements currently prevent it.