Screwworm returns to US after 60 years, threatening cattle supply

The New World screwworm, a flesh-eating parasite not seen in the United States since 1966, was confirmed in a 3-week-old calf in Zavala County, Texas, the Agriculture Department said Wednesday, threatening an already shrinking US cattle herd and raising costs for meatpackers Tyson Foods Inc. and JBS SA.

"The United States has defeated this pest before, and we will do it again," Dudley Hoskins, undersecretary for marketing and regulatory programs at the USDA, said in a statement.

The larvae were found in the calf's umbilical area, and no additional cases have been detected so far, according to the USDA's Animal and Plant Health Inspection Service. The agency established a 20-kilometer quarantine zone around the detection site, restricted movement of warm-blooded animals in the area, and is accelerating the release of 4 million sterile male screwworm flies per week by air, a strategy that succeeded in eradicating the pest in the 1960s.

The detection comes as the US cattle population is already at its lowest level in decades, squeezed by years of drought-driven herd liquidation and high feed costs. Any further supply disruption from screwworm-related quarantines or culling would push cattle prices higher, compressing margins for Tyson and JBS, which have been grappling with elevated input costs. The USDA also suspended imports of live cattle, horses and bison from Mexico, where the parasite had been advancing northward, with a case confirmed in Coahuila state roughly 25 miles from the Texas border.

Quarantine and containment efforts

The USDA and Texas Animal Health Commission have activated a unified incident command and deployed ground-release chambers for sterile flies in addition to the aerial dispersal program. Officials said they are intensifying surveillance across South Texas and urged livestock owners to monitor animals for unusual wounds or larvae around body openings. The parasite, which burrows into living tissue of warm-blooded animals, can cause severe injury or death if untreated, though officials stressed the food supply remains safe as screwworms do not infest meat products.

Industry and market implications

The National Cattlemen's Beef Association said it has been working with the USDA for more than a year to prepare for a potential incursion. "We defeated this harmful pest before in the 1960s by investing heavily in sterile fly dispersal, and NCBA will continue using every tool in the toolbox to ensure Screwworm is eradicated in the U.S. once again," CEO Colin Woodall said in a statement.

For Tyson and JBS, the timing is particularly challenging. The two largest US meatpackers have been navigating a tight cattle supply environment that has driven up slaughter costs and squeezed wholesale beef margins. Any escalation of the outbreak — requiring broader quarantine zones or depopulation of affected herds — would further tighten availability and raise raw material costs. Tyson shares have fallen 12 percent this year through Wednesday's close, while JBS' New York-listed shares are down 8 percent, as investors priced in persistent margin pressure from elevated livestock prices.

The USDA said it invested heavily in sterile fly production facilities in Mexico and South Texas over the past year as the parasite moved north. "All models showed New World Screwworm entering the country in 2025; however, thanks to the hard work across the entire Trump administration and our industry, state, and local partners, we were able to buy time for this moment," Hoskins said.

This article is for informational purposes only and does not constitute investment advice.