What to know about the New World screwworm fly and its reappearance in the US

The New World screwworm fly is a flesh-eating parasite. (AP: Denise Bonilla/US Department of Agriculture via AP)

The New World screwworm fly is threatening the $US113 billion United States cattle industry for the first time in more than a half century, with an infestation from its flesh-eating larvae confirmed in south Texas.

The infestation was discovered in a single 3-week-old calf in La Pryor, Texas, about 161 kilometres south-west of San Antonio and 80 kilometres from the US-Mexico border.

Federal and state officials had been working to keep the parasite from reaching Texas, home to $US17 billion worth of the nation's cattle, making it the industry's number one state.

The deadly flies were detected in Mexico late in 2024 after years of being contained at the southern end of Panama.

The fly was an annual warm-weather scourge of cattle ranchers from at least the 1930s through the 1960s, until the US eradicated the pest by breeding sterile male flies and dropping swarms of them from planes to mate with wild females.

The United States Department of Agriculture (USDA) said the most recent case was the first in Texas since 1966.

Here is what to know about the fly, the threat it poses and the response.

The New World screwworm fly in the Western Hemisphere and its Old World cousin in Africa and Asia are unusual among flies because their larvae, or maggots, eat live flesh and fluids instead of dead material.

Brooke Rollins (centre) watches efforts in Texas to combat the spread of the parasite. (AP: Eric Gay)

Females lay their eggs in open wounds and mucous membranes after mating only once in their months-long lives.

Any warm-blooded animal, including wildlife, pets and occasionally even humans, can be infested.

Livestock are vulnerable because of how they're handled, Lee Haines, an associate research professor of biological sciences at the University of Notre Dame, said in an email on Thursday.

Standard practices with cattle can break the skin, including shearing and de-horning, or even moving them in and out of corrals can cause scrapes and cuts.

Birth would also make a mother and calf vulnerable, she said.

Stephen Diebel, a Texas rancher and president of the Texas and Southwestern Cattle Raisers Association, said even wounds "as small as a tick bite" could put cattle at risk.

Death can result if an infestation is not treated, though a dozen treatments have been approved for use in a variety of species.

In decades past, ranchers had tens of millions of dollars in losses — potentially billions in today's dollars.

But agriculture officials we