Flesh-Eating Cattle Parasite Spreads Beyond Texas as New Screwworm Cases Are Found

Two more cases of the New World screwworm have been confirmed, including one outside the main cluster in Texas, demonstrating the difficulty of stopping a pest that could potentially devastate the nation’s cattle industry, the U.S. Department of Agriculture (USDA) announced Monday.

The screwworm is actually a fly larva that eats living flesh instead of dead material. Females lay their eggs in open wounds of warm-blooded animals like cattle, but wildlife, pets, and occasionally even humans can be infested. A government program to breed sterile male flies and drop swarms of them from planes to mate with wild females had kept screwworm contained at the southern end of Panama for decades.

So far, there are four confirmed cases: three calves in Texas and a dog from neighboring Lea County, New Mexico. The dog, which the USDA initially reported as a Texas case, lives in New Mexico and was reclassified as the first in that state. The animal’s travel history is being investigated.

The first two screwworm cases were discovered last week in calves a few miles apart in south Texas. The third was announced Monday in La Salle County, southwest of San Antonio.

Scientists expect a handful of new cases could pop up in the coming days and weeks, but it doesn’t mean screwworm is spreading rapidly, said Edwin Burgess, PhD, a University of Florida entomologist who studies the fly.

“When that first case is seen, everyone is being vigilant and their eyes are on it more intensely,” Burgess said. “And when you are looking for something, you are more likely to see it.”

The USDA and the U.S. cattle industry have been racing to prevent an infestation since screwworm was detected in Mexico late in 2024. The annual warm-weather scourge of cattle ranchers had been eliminated in the U.S. in the 1960s.

So far, its reappearance hasn’t greatly affected beef prices, which are already near record levels because there are fewer cows in the U.S. Although the parasite attacks live cattle, it does not infest meat or fruit. There are also a dozen government-approved medications to treat livestock.

Canada temporarily stopped importing cattle, horses, or other livestock from Texas on Friday. The parasites prefer humid areas where temperatures are at least 77°F (25°C), making them more of a summer problem up north.

USDA Secretary Brooke Rollins will hold a news conference about the infestation Monday afternoon following a briefing at the U.S. Livestock Insects Research Laboratory in Kerrville, Texas.

Burgess said the long-term solution — breeding sterile male flies — is months away. Since wild female flies mate just once, if that encounter is with a sterile male, outbreaks can eventually be halted as the flies die out.

The USDA is working to both increase sterile fly production in plants outside the U.S. and build a massive fly factory in Texas.

However, Texas Agriculture Commissioner Sid Miller said the federal response will take too long and risks crippling the cattle industry. Instead, he says a poison bait could eliminate the screwworm problem in a few months, even if USDA and other experts say the bait hasn’t been proven to work and could poison other flies, animals, and even humans.

“What the hell is a good fly?” Miller said in an interview Monday.