Flesh-eating screwworm returns to U.S. after 60 years, threatening cattle herd

A flesh-eating parasite that had been kept out of U.S. livestock for decades has been detected in Texas, threatening the nation’s cattle industry and food supply at a time when prices are already high.

Get unlimited access to ad-free articles and exclusive content.

The case of New World screwworm was confirmed in a 3-week-old calf in La Pryor, near the U.S.-Mexico border, Agriculture Secretary Brooke Rollins said late Wednesday.

The parasitic fly’s larvae feed exclusively on the living tissue of warm-blooded animals.

While the fly is capable of infecting humans and pets, such cases are rare and pose little risk to the broader public, according to experts.

The parasite does not pose a food safety threat, but a wider outbreak could still cost the livestock industry billions of dollars and put additional pressure on beef prices that are already at record highs.

The case is the first confirmed detection of New World screwworm in Texas since 1966, and is the only confirmed case identified in the country so far, said Rollins.

It follows months of warnings from U.S. and Texas agriculture officials and cattle industry leaders, as the pest steadily moved north through Mexico toward the American border.

“For months, the screwworm has advanced rapidly through Mexico in spite of the USDA’s existing gameplan,” Texas Agriculture Commissioner Sid Miller said Wednesday, adding that “instead of using every available tool, USDA moved too slowly and relied solely on a partial solution that takes years to fully implement.”

Miller has also called on President Donald Trump to take direct control of the government’s response, and “throw every available federal resource at this threat before it becomes a full-blown agricultural disaster.”

The primary weapon against screwworm is a decades-old technique that has eliminated the parasite from the U.S. in the past — releasing sterilized male flies into affected areas. Since female flies generally mate only once, those that pair with sterile males are unable to produce offspring.

In a bid to contain the spread of the parasite, USDA said it has begun releasing sterile flies in the area and is investing heavily in new sterile flies production facilities in Texas.

It has also established a roughly 12-mile quarantine zone around the site and restricted the movement of warm-blooded animals, including livestock and pets to further strengthen the response.

State veterinarians are urging ranchers and pet owners inside the quarantine zone to follow movement restrictions while eradication efforts continue.

Rollins said the USDA is confident enough in its preparations that it believes “there is no threat of mass infestation.”

“Protecting our livestock industry is a national security issue of the utmost importance, and USDA is wasting no time in taking action,” said Dudley Hoskins, a USDA under secretary. “USDA invested heavily in the tools needed to eliminate NWS ever since cases started increasing in Central America and Mexico. The United States has defeated this pest before, and we will do it again.”

Unlike contagious livestock diseases, screwworm does not spread directly from animal to animal. Instead, female flies lay eggs in open wounds or body openings.

Once the eggs hatch, the larvae burrow into living flesh and feed on tissue, potentially causing severe infections and death of livestock if left untreated.

The U.S. cattle herd is already at its lowest level in 75 years, with a major screwworm outbreak threatening to further reduce supplies and increase costs for ranchers and consumers alike.

The most recent human screwworm case in the U.S. was identified in Maryland last year after a traveler returned from El Salvador.

The person recovered, and federal health officials found no evidence that the parasite had spread to others.