Flesh-eating screwworm fly prompts livestock import ban at US-Mexico border

The U.S. Department of Agriculture has once again closed its southern border to livestock imports—not people—after a dangerous flesh-eating parasite was found further north in Mexico than previously reported.

The pest, known as the New World screwworm fly, lays eggs in wounds on warm-blooded animals. Its larvae feed on live flesh, posing a serious threat to cattle, horses, and other livestock. American officials fear the parasite could reach Texas, potentially reviving a problem that caused devastating agricultural losses before being eradicated in the U.S. decades ago.

"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," said U.S. Agriculture Secretary Brooke Rollins in a statement Wednesday.

How close is the parasite to the U.S. border?

The backstory:

The screwworm had been largely contained to Panama since the 1970s, when the U.S. eradicated it by releasing sterile male flies to collapse the population. The parasite reemerged in southern Mexico late last year, prompting the U.S. to suspend livestock imports in May.

After signs of progress, the USDA allowed limited border reopening at five ports—three this month, and two more by Sept. 15. But now the fly has been detected 185 miles northeast of Mexico City—roughly 370 miles from Texas, and 160 miles further north than previously known.

What we know:

The U.S. has halted imports of live cattle, horses, and bison from Mexico again.

The USDA is investing $30 million in containment efforts, including a new sterile-fly production site in southern Mexico by July 2026.

An additional facility in southern Texas is being prepared to hold and release sterile flies if needed.

What we don't know:

Officials haven’t said how long the livestock ban will remain in place.

It’s unclear whether the new detection marks the start of a broader northward spread or an isolated case.

The USDA hasn’t confirmed whether consumer beef supplies could be affected in the near term.

The other side:

Mexican President Claudia Sheinbaum criticized the U.S. response, saying officials were exaggerating the risk.

"From our point of view, they took a totally exaggerated decision to closing the border again," Sheinbaum said Thursday.

"Everything that scientifically should be done is being done."

Mexican officials say the number of infected animals is actually down nearly 19% since late June, with 392 active cases currently reported.

Meanwhile, U.S. Reps. Tony Gonzalez (TX) and Kat McCammack (FL) urged the Trump administration to fast-track approval of existing treatments for screwworm infestations in livestock, citing regulatory delays tied to product labeling.

What's next:

The USDA’s plan to breed and deploy sterile flies is central to stopping the parasite’s spread. Officials hope that with sufficient funding and international cooperation, the fly can be contained before it reaches the U.S. border.

Increased surveillance, funding, and strategic releases are expected in the coming months—particularly if more northern cases are confirmed in Mexico.