The U.S. has closed its southern border again to livestock imports, saying a flesh-eating parasite has moved farther north in Mexico than previously reported.

Mexico’s president was critical Thursday, suggesting that the U.S. 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 U.S. 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 U.S. 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 farther north than previously reported cases. That was about 370 miles from the Texas border.

“The United States has promised to be vigilant,” U.S. 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 U.S. 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% 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.

Also Thursday, U.S. 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.

U.S. technology

The U.S. government is preparing to breed billions of sterile male flies and dump them out of airplanes over Mexico and southern Texas to fight the pest.

“It’s an exceptionally good technology,” said Edwin Burgess, an assistant professor at the University of Florida who studies parasites in animals, particularly livestock. “It’s an all-time great in terms of translating science to solve some kind of large problem.”

The U.S. Department of Agriculture plans to ramp up the breeding and distribution of adult male flies — sterilizing them with radiation before releasing them.

It is more effective and environmentally friendly than spraying the pest into oblivion.

The USDA plans to open a fly distribution center in southern Texas by the end of the year so that it can import and distribute flies from Panama if necessary.

“A thousand-pound bovine can be dead from this in two weeks,” said Michael Bailey, president elect of the American Veterinary Medicine Association.

Veterinarians have effective treatments for infested animals, but an infestation can still be unpleasant — and cripple an animal with pain.

Don Hineman, a retired western Kansas rancher, recalled infected cattle as a youngster on his family’s farm.

“It smelled nasty,” he said. “Like rotting meat.”

The New World screwworm fly is a tropical species, unable to survive Midwestern or Great Plains winters, so it was a seasonal scourge. Still, the U.S. and Mexico bred and released more than 94 billion sterile flies from 1962 through 1975 to eradicate the pest, according to the USDA.

The numbers need to be large enough that females in the wild can’t help but hook up with sterile males for mating.

One biological trait gives fly fighters a crucial wing up: Females mate only once in their weekslong adult lives.

Female flies can lay their eggs in wounds on any warm-blooded animal, and that includes humans.

Decades ago, the U.S. had fly factories in Florida and Texas, but they closed as the pest was eradicated.

The Panama fly factory can breed up to 117 million a week, but the USDA wants the capacity to breed at least 400 million a week. It plans to spend $8.5 million on the Texas site and $21 million to convert a facility in southern Mexico for breeding sterile fruit flies into one for screwworm flies.

In one sense, raising a large colony of flies is relatively easy, said Cassandra Olds, an assistant professor of entomology at Kansas State University.

But, she added, “You’ve got to give the female the cues that she needs to lay her eggs, and then the larvae have to have enough nutrients.”

Fly factories once fed larvae horse meat and honey and then moved to a mix of dried eggs and either honey or molasses, according to past USDA research. Later, the Panama factory used a mix that included egg powder and red blood cells and plasma from cattle.

In the wild, larvae ready for the equivalent of a butterfly’s cocoon stage drop off their hosts and onto the ground, burrow just below the surface and grow to adulthood inside a protective casing making them resemble a dark brown Tic Tac mint. In the Panama factory, workers drop them into trays of sawdust.

Security is an issue. Sonja Swiger, an entomologist with Texas A&M University’s Extension Service, said a breeding facility must prevent any fertile adults kept for breeding stock from escaping.

Dropping flies from the air can be dangerous. Last month, a plane freeing sterile flies crashed near Mexico’s border with Guatemala, killing three people.

In test runs in the 1950s, according to the USDA, scientists put the flies in paper cups and then dropped the cups out of planes using special chutes. Later, they loaded them into boxes with a machine known as a “Whiz Packer.”

The method is still much the same: Light planes with crates of flies drop those crates.

Burgess called the development of sterile fly breeding and distribution in the 1950s and 1960s one of the USDA’s “crowning achievements.”

Some agriculture officials argue now that new factories shouldn’t be shuttered after another successful fight.

“Something we think we have complete control over — and we have declared a triumph and victory over — can always rear its ugly head again,” Burgess said.