The New World screwworm fly poses a serious risk to the $113 billion U.S. cattle sector for the first time in over fifty years, following confirmation of an infestation by its flesh-eating larvae in south Texas.

Officials identified the case in a single three-week-old calf in La Pryor, Texas, roughly 100 miles southwest of San Antonio and 50 miles from the U.S.-Mexico border. Federal and state authorities had worked to prevent the parasite from entering Texas, which holds $17 billion in national cattle value and ranks as the top state for the industry.

The deadly flies appeared in Mexico late in 2024 after remaining contained for years at the southern end of Panama.

The fly plagued cattle ranchers each warm season from the 1930s into the 1960s until the U.S. eliminated the pest through release of sterile males dropped from aircraft to mate with wild females. The USDA noted this marks the first Texas case since 1966.

Key details follow about the fly, its dangers and the ongoing response.

The New World screwworm fly in the Western Hemisphere and its Old World relative in Africa and Asia stand apart from other flies because their larvae consume live tissue and fluids rather than decaying matter. Females deposit eggs in open wounds and mucous membranes after a single mating during their lives that last several months.

Any warm-blooded creature, including wildlife, pets and rarely humans, remains susceptible to infestation.

Livestock face higher exposure due to routine handling, noted Lee Haines, an associate research professor of biological sciences at the University of Notre Dame, in a Thursday email. Common cattle procedures such as shearing and de-horning, or even transport through corrals, create skin breaks and scrapes. Birth also leaves both mother and calf open to attack, she added.

Stephen Diebel, a Texas rancher and president of the Texas and Southwestern Cattle Raisers Association, pointed out that even minor injuries like a tick bite can expose cattle to danger.

Untreated cases can lead to death, although a dozen approved treatments exist for various species. Ranchers in past decades suffered losses reaching tens of millions of dollars, or potentially billions when adjusted to current values.

Agriculture officials stressed that the fly does not affect food supplies, and U.S. Agriculture Secretary Brooke Rollins indicated it is unlikely to harm beef output, positive news amid already high consumer prices.

Federal, state and industry leaders have raised alarms about the fly advancing through Mexico toward the U.S. since a southern Mexico case emerged in November 2024.

Authorities viewed the pest as eradicated from Central and North America nearly two decades ago until an outbreak in Panama triggered an emergency there early in 2023 under the joint U.S.-Panama program formed in 1994. Cases later spread to Costa Rica and Nicaragua that same year.

Edward Burgess, a University of Florida entomologist who researches the fly, explained that it multiplies rapidly and travels far via hosts such as wild deer. Outside Panama, sterile-fly production and release programs have mostly ended, he said.

It proves difficult to contain because of the insect speed in movement and reproduction, Burgess noted.

By June 2 the parasite had affected more than 171,700 animals and 2,000 people across Central America and Mexico, per the U.S. Centers for Disease Control and Prevention, which also reports 10 human deaths.

Beginning in May 2025, Rollins restricted livestock entries at the border and credited the action on Thursday with postponing the fly arrival in Texas by one year.

Rollins has stated that Mexican officials have not sufficiently limited animal movement inside the country, a claim Mexican authorities dispute.

Haines highlighted climate change as a major factor in the spread of this tropical species that favors warm conditions. Rising temperatures expand its range while fewer severe cold events allow survival in areas once marginal, she said.

Texas State Veterinarian Bud Dinges established a 12-mile quarantine zone covering much of Zavala County, including La Pryor, plus a portion of adjacent Uvalde County. Animals may not exit without inspection.

Local ranchers worry about spread among wildlife, especially deer, similar to a brief 2016 outbreak in the Florida Keys. That marked the prior U.S. animal case, though the CDC confirmed one last year in a Maryland man who had visited El Salvador and recovered.

Zavala County Sheriff Eusevio Salinas said Thursday that state personnel are installing multiple road checkpoints to enforce the quarantine.

Officials indicated the checkpoints would operate for three to four days, after which the situation should be contained, Salinas added.

Diebel, whose family ranch lies about 200 miles east of the zone, reported that ranchers are administering preventive injections and treating wounds from practices such as ear tagging while monitoring animals closely for illness.

The USDA has released sterile flies across south Texas since February from a new dispersal center. Releases now occur twice weekly, totaling 4 million flies, with another 4 million placed weekly as pupae, according to Rear Admiral Michael Schmoyer of the USDA response team.

Sterile-male releases represent a proven and highly effective method. Males mate with multiple partners while females mate once, so pairing with a sterile male prevents egg hatching and drives population decline until eradication.

With production sites beyond Panama closed for years, the USDA determined sterile-fly output was insufficient. It allocated $21 million for a new breeding facility in southern Mexico set to begin operations next month.

The USDA is also investing $750 million in a Texas factory capable of producing up to 300 million sterile flies weekly, expected to open next fall.