By Kirk Maltais

A case of flesh-eating New World screwworm parasites in a Texas calf marks the first time since the 1960s that the flies have been discovered in a livestock animal in the U.S. In response, the Department of Agriculture is implementing plans to contain the spread of the parasite, while market-watchers attempt to understand their possible effects on beef prices and supply.

What's a New World screwworm?

The New World screwworm fly is a parasitic type of blowfly common in Central and South America. It reproduces by laying its eggs inside of an open wound or space in a warm-blooded animal -- most often livestock like cattle, although they can infect birds, dogs, wild animals, and even humans. However, they are not commonly found in people. The larvae burrow into the tissue of the affected organism, beginning what's called myiasis, or the consumption of healthy flesh.

The Department of Agriculture said that New World screwworm is not a food safety issue.

The latest outbreak of New World screwworm infections began in Panama in 2023, despite efforts dating back to the 1970s to keep populations under control. They have slowly traveled north, pushing through Central America into Mexico in late 2024. Cases gradually moved through Mexico until being found in the U.S. this week.

What does it mean for beef?

Cattle accounts for around 86% of all New World screwworm cases reported in the current outbreak throughout Central America. Nearly 57,000 bovines have been affected since January 2024, according to Copeg, the group founded in 1994 by the partnership between the U.S. and Panama.

The spread of NWS has been slow-moving. Most livestock traders and producers were expecting a case to be ultimately confirmed in the U.S. months ago.

"The 'discovery' is like the scene in 'Austin Powers' with the steamroller; we have seen this coming for months," said Karl Setzer, founder of Consus Ag Consulting in Grand Ledge, Mich.

CBOT live cattle futures were up 1.6% to roughly $2.42 a pound Thursday. Setzer says that many traders are opting to buy the news after selling on the rumor.

What's the trade impact?

Livestock trade has been suspended by the USDA between ports at the southern border since July 2025. This suspension remains active.

The shutdown has prevented U.S. meatpackers from getting feeder cattle from Mexico, depriving them from additional cattle supplies at a time when beef prices are record-high. The arrival of screwworm may also have an effect on beef exports leaving the U.S., with more rigorous inspection of exported meat and potential fluctuation in demand for that beef.

The extended shutdown may keep prices for beef elevated, as the U.S. cattle herd is already the smallest since the 1950s.

"If the infestation continues to spread within Texas, the resulting restrictions and increased risk could further delay herd rebuilding efforts, limiting production potential and increasing reliance on imports," said Steiner Consulting Group analysts in a note.

What are the symptoms?

Warm-blooded creatures infected with the parasitic screwworm larvae show visible signs once the eggs are hatched and visible maggots begin feeding on the living animal's flesh. The area becomes tender and painful, and emits a foul odor as the infection advances, according to the USDA.

How to fight the infections?

Combating spreading screwworm infections has involved releasing sterilized flies into the wild, which then mate with the regular flies and render them unable to lay more eggs. This was the primary method used to eliminate screwworm when it last impacted U.S. livestock in the 1960s.

A key issue is the early detection of infections in cattle and other livestock animals.

"The burden falls hardest on farmers who must monitor animals scattered across vast open rangeland, often going unobserved for days at a time," said Lee Haines, an associate professor of biological sciences with the University of Notre Dame.

-----------

Write to Kirk Maltais at kirk.maltais@wsj.com


(MORE TO FOLLOW) Dow Jones Newswires

06-04-26 1609ET