By Mackenzie Tatananni

The first case of New World screwworm, a flesh-eating parasite that typically feeds on livestock, has been detected in the U.S after six decades. It might sound like something out of science fiction, but it has very real implications for a beef industry already under pressure.

New World screwworm was found in a calf in Texas, about 50 miles from the U.S.-Mexico border, officials said Wednesday. The parasitic fly lays its eggs on warm-blooded animals, where the hatched maggots burrow into and feed on the surrounding flesh.

The species is endemic to South America and parts of the Caribbean. While it was officially eradicated from the U.S. in 1966, it is no new threat to the continent, as an outbreak has been raging in Mexico after spreading north of Panama in 2023. As of early May, Mexico had 1,717 active cases in animals, according to the country's National Service of Agro-Alimentary Health, Safety, and Quality.

The U.S. halted all livestock imports from Mexico during an earlier outbreak in May 2025, which was extended last July. The ban remains in place today.

The latest flare-up comes as the U.S. beef cattle herd is already strained. Earlier this year, the U.S. Department of Agriculture said the total inventory of cattle and calves had reached a 75-year low, continuing a slide that began in 2019. As of the start of January, the nation's cattle inventory totaled 86.2 million, down from 86.5 million in the previous year.

The shortage is driving up livestock prices, with the tight supply projected to last at least two more years. This has placed a burden on Tyson Foods and JBS, two of the largest beef producers in the country. Tyson acknowledged last month that soaring livestock prices have continued to eat into its profits.

The halt "has placed incremental pressure on beef supplies over the last year," Raymond James analyst Brian Vaccaro wrote, adding that the accelerating spread of New World screwworm "limits visibility as to when cattle imports may resume."

Shares of Tyson and JBS are both in the red for the year, down 2.6% and 16%, respectively. The benchmark S&P 500 has gained 10% over the same period.

While painful for beef producers, the outbreak creates an opportunity for animal health companies like Elanco Animal Health. On Thursday, Elanco issued a news release describing the emergence of the parasite as a threat "that has not been seen north of the Florida Keys since the fly was eradicated from the United States more than 50 years ago."

The company has six products on the market targeting screwworm larvae, including an aerosol wound treatment sanctioned by the Environmental Protection Agency. In pets, Elanco's family of Credelio tablets for dogs and cats have been granted either conditional approval or emergency use authorization by the Food and Drug Administration.

Peers Zoetis and Merck are also positioned to benefit. Zoetis manufactures an injection called Dectomax-CA1, which is conditionally approved by the FDA for use in cattle. Merck Animal Health, meanwhile, offers a pour-on solution targeting screwworm infections.

Write to Mackenzie Tatananni at mackenzie.tatananni@barrons.com

This content was created by Barron's, which is operated by Dow Jones & Co. Barron's is published independently from Dow Jones Newswires and The Wall Street Journal.

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June 04, 2026 10:23 ET (14:23 GMT)

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