An infestation of the flesh-eating screwworm parasite may have been detected in South Texas, and a sample is at a federal government laboratory for testing, the U.S. Department of Agriculture said on Wednesday.

The case was “presumptive positive,” the USDA said on a call, according to a source who took part in the briefing. The result means a preliminary test was positive but needs to be officially confirmed.

If confirmed, the infestation would pose a new and serious threat to U.S. cattle ranchers who have been preparing for screwworm for more than a year, and would represent a setback for U.S. efforts that cost millions of dollars to keep the pest out.

The fly can infest people and pets, but the risk to humans is low, experts said. The fly poses no food safety issues.

“We have already activated personnel on the ground and are working with local partners,” USDA said on X.

Widespread market chatter about the suspected case hung over cattle futures, which traders said have long been sensitive to threats of New World screwworm.

The parasite has been moving north through Mexico for more than a year. Market players believe a screwworm infestation in the U.S. could reduce demand for beef from consumers nervous about the flesh-eating pest, but would be bullish in the long term by reducing the U.S. cattle supply.

Samples from two calves on a cattle ranch in La Pryor, Texas, were taken on Tuesday and sent for testing, Texas Representative Don McLaughlin told Reuters, adding he had seen photos and videos of the animals.

A photo seen by Reuters, which McLaughlin said was one of those he also saw, shows multiple larvae resembling the screwworm inside a bloody circular wound on an animal.

McLaughlin said one of the suspected infestations was detected in an umbilical cord wound of a calf, though it was unclear if that was the case seen in the photo. Reuters could not immediately verify the photo.

“At this point, it’s unconfirmed that it’s the New World screwworm,” McLaughlin, a Republican, said. “It looks like it, but it’s unconfirmed.”

Outbreak Could Shrink US Cattle Supply

The Texas Animal Health Commission told Reuters on Wednesday there has been no confirmed case of New World screwworm in Texas.

Screwworms are parasitic flies whose females lay eggs in wounds of any warm-blooded animal. Once the eggs hatch, hundreds of screwworm larvae use their sharp mouths to burrow through living flesh, eventually killing their host if left untreated. An outbreak of screwworm in the U.S. could cause $1.8 billion in damage to Texas’ economy alone and would likely raise U.S. beef prices by shrinking the U.S. cattle supply, experts have said.

Earlier in the week, the USDA said that McLaughlin, who had said a case was confirmed one mile south of the Texas border, had inaccurate information. The USDA confirmed a case 25 miles south of the Texas border in Mexico’s Coahuila State on Tuesday.

The U.S. has kept its border with Mexico closed to cattle imports for more than a year to prevent the destructive parasite from reaching U.S. border states. The United States has spent millions of dollars to slow its advance through Mexico, investing in sterile fly production facilities, expanding trapping efforts and increasing livestock surveillance.

(Reporting by Cassandra Garrison in Mexico City and Heather Schlitz and Tom Polansek in Chicago. Editing by Emily Schmall, Deepa Babington, Rod Nickel and Cynthia Osterman)