Screwworms in Texas: What an infestation means for livestock, beef supply and the economy

The return of the New World screwworm fly in Texas has ranchers and government agencies scrambling to contain what could threaten a $113 billion cattle industry in the U.S. 

As state and federal officials work to eradicate the latest cattle infestation – the first one in more than 50 years – here’s what to know about the flesh-eating parasite and why livestock are so vulnerable. 

What is a screwworm? 

The backstory:

The New World screwworm fly is dangerous because unlike other flies, their larvae, or maggots, eat live flesh and fluids instead of dead material. Females lay their eggs in open wounds and mucous membranes after mating only once in their monthslong lives.

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

The fly was a huge problem for cattle ranchers from the 1930s through the 1960s, until the U.S. eradicated it by breeding sterile male flies and dropping them from planes to mate with wild females. 

Livestock are especially vulnerable to the deadly flies because of how they’re raised, Lee Haines, an associate research professor of biological sciences at the University of Notre Dame, told The Associated Press. Practices like shearing and dehorning can break the skin, and even moving them in and out of corrals can cause scrapes. Birthing a calf also makes a mother and calf more susceptible. 

The last time a U.S. case was confirmed among animals was during a short-lived outbreak in the Florida Keys in 2016. In that outbreak, the flies spread through deer. 

The CDC also confirmed a case last year in a Maryland man who had traveled to El Salvador and recovered.

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

By the numbers:

As of June 3, screwworms have sickened more than 171,700 animals and 2,070 people across Central America and Mexico, according to the U.S. Centers for Disease Control and Prevention. There have been 10 human deaths, the CDC says.

Screwworms found in Texas

Local perspective:

In Texas, a screwworm infestation was discovered last week in a single 3-week-old calf in La Pryor, Texas, about 100 miles southwest of San Antonio and 50 miles from the U.S.-Mexico border. It was the first one found in Texas since 1966. 

A second confirmed case was found days later in a one-month-old calf in Zavala County, Texas, about 5.6 miles from the first case.

Texas is now enforcing a 12-mile quarantine zone covering much of Zavala County, home to La Pryor, and a small part of neighboring Uvalde County. Animals cannot leave that zone without being inspected.

Texas officials say part of the eradication plan involves the major sterile fly dispersal and production facility in Edinburg. Most of the efforts will be in Zavala and Uvalde counties.

What they're saying:

Officials said the focus now is containment and education, urging ranchers and pet owners to inspect animals daily for even small wounds, FOX 7 in Austin reports. 

RELATED: Flesh-eating screwworm in Texas: Gov. Abbott signs statewide disaster declaration

"Any larvae on a living animal is a reportable issue and what that does is lets us mobilize more quickly so that we can get to the animal, provide treatment to it, eliminate those fly larvae from becoming adults and perpetuating the problem," Texas A&M Department of Entomology Dr. Phillip Kaufman said.

"It's terrible. It's like right out of a horror movie," Texas Agriculture Commissioner Sid Miller said.

Screwworm border closure delayed their return

The federal government stopped allowing Mexican livestock imports a year ago in an effort to prevent the screwworm from returning to the U.S., according to Reuters. 

Agriculture Secretary Brooke Rollins said the livestock border closure helped to delay the deadly fly’s return by a year, but it didn’t come without economic consequences. 

Beef industry has taken a hit

Dig deeper:

The border closure has had a negative economic impact on the U.S. beef industry, particularly in Texas, as many ranchers raised cattle imported from Mexico. Mexico, meanwhile, has capitalized on the U.S. industry’s shrinking by expanding its beef exports, Reuters reports. 

Texas is home to $17 billion worth of the nation’s cattle. It’s the top state in the nation for cattle production. 

But officials do point out that screwworms don’t infest food. Rollins said it’s unlikely to damage beef production, though Texas Gov. Greg Abbott said it could cause production issues.