Deadly Screwworm Parasite Found in US Threatens Cattle Herd

A deadly cattle parasite was confirmed in the U.S. for the first time in nearly a decade, posing the latest threat to a U.S. herd already at its lowest level in 75 years.

The U.S. Department of Agriculture said a suspected case from South Texas, which was reported Wednesday, tested positive for the new world screwworm. The affected animal in Zavala County is a three-week-old calf, and the larvae was detected in its umbilical area.

Agriculture Secretary Brooke Rollins said on a late Wednesday call with reporters that this is the only case that the agency is tracking right now, and that there is “no reason to believe” that this will result in an establishment of the pest in the U.S.

The agency is implementing quarantines, movement controls and surveillance in a 20-kilometer (12-mile) zone around the detection, and has also expedited the release of sterile flies to prevent the pest’s spread.

The USDA said the food supply is safe as screwworm does not infest meat or other food sources.

The confirmed case “should not be cause for panic, but rather time for the full deployment of the next phase of the whole-of-government response USDA has been planning for months,” Senate and House Agriculture Committee Chairmen John Boozman and GT Thompson said in a statement.

The detection of the parasite, which is deadly to cattle, comes at a dire time for the U.S. beef and cattle industry, with a smaller herd having already sent consumer prices to record highs. The market earlier Wednesday reacted to reports on the case, which was at the time unconfirmed. Tyson Foods Inc. dropped 4.2%, settling at a five-month low, while JBS NV closed at the lowest price since U.S. trading began about a year ago.

The U.S. has been trying to contain the spread of the pest, which can kill an animal within days, as cases across the Mexican border posed a threat. The USDA paused imports of live Mexican cattle and built new facilities to disperse sterile flies. Still, the number of cases has recently accelerated in Mexico, with the closest detection in a goat about 25 miles away from the US border, according to the US Department of Agriculture.

The screwworm’s presence in Mexico and the ensuing halt to the live cattle trade has exacerbated the supply tightness for U.S. meatpackers, who have been facing operational losses as they pay for fewer and costlier animals. That shortage has also sent consumer beef prices to all-time highs, challenging President Donald Trump’s promise to ease grocery prices.

The U.S. Meat Export Federation said it doesn’t expect any disruptions to U.S. beef exports, but will “monitor the situation carefully for any trading partner actions that could interrupt beef trade.”

The screwworm was last detected in the U.S. in 2016 among deer in the Florida Keys, before being eradicated by early 2017, according to the USDA. The most recent occurrence involving infected cattle came in a 1976 outbreak that impacted Texas’s economy by as much as $375 million in non-inflation-adjusted dollars, the agency said.

While the risk to humans is low, the U.S. confirmed one case of the parasite last year in an individual who had traveled from Central America.

The USDA in its previously prepared plan said in the event of a localized outbreak, it would initiate movement restrictions for animals in and out of an infested zone. A larger outbreak spanning multiple counties would require the use of animal drugs, which the U.S. Food and Drug Administration has already issued emergency use authorization for. Rollins said on the call with reporters that a plane is already headed to South Texas with a “lot of that treatment stockpile.”

Shares of Elanco Animal Health Inc., which makes one of those treatments, settled at the highest in nearly a month on Wednesday, and Zoetis Inc. shares also rose.

Top photo: Beef cattle at a farm near Clinton, Missouri, on Saturday, Nov. 15, 2025. Photographer: Clayton Steward/Bloomberg.