A dangerous parasite that hasn’t been seen in the United States for more than fifty years has returned, creating serious concerns for America’s $113 billion cattle industry. Officials have verified that the New World screwworm fly has infected livestock in southern Texas.
Authorities discovered the outbreak in a 3-week-old calf located in La Pryor, Texas, positioned roughly 100 miles southwest of San Antonio and 50 miles from the Mexican border. Both federal and state agencies had been working to prevent this parasite from entering Texas, which houses $17 billion in cattle value and leads the nation in this agricultural sector.
Officials detected these dangerous insects in Mexico during late 2024 following years of successful containment in Panama.
From the 1930s through the 1960s, this pest regularly plagued cattle operations during warm seasons, until American authorities eliminated it through a unique approach involving breeding infertile male flies and releasing massive quantities from aircraft to mate with wild females. According to the USDA, this recent discovery marks the first Texas case since 1966.
The New World screwworm fly found in the Western Hemisphere, along with its Old World relative in Africa and Asia, stands apart from other flies because its larvae consume living tissue and bodily fluids rather than decomposing matter. After mating just once during their months-long lifespan, females deposit eggs in open injuries and mucous membranes.
All warm-blooded creatures, including wild animals, domestic pets, and sometimes humans, face potential infestation risks.
Cattle face particular vulnerability due to standard handling procedures, explained Lee Haines, an associate research professor of biological sciences at the University of Notre Dame, in Thursday email correspondence. Common cattle management activities that break skin include shearing and de-horning, while moving animals through corrals can create scrapes and cuts. The birthing process also leaves both mothers and calves at risk, she noted.
Stephen Diebel, a Texas rancher and president of the Texas & Southwestern Cattle Raisers Association, emphasized that even injuries “as small as a tick bite,” can endanger cattle.
“These flies can lay eggs in very, very small places,” he stated.
Researchers and cattle organizations report that infected wounds develop terrible odors and cause severe suffering or death when infestations go untreated. During previous decades, ranchers experienced tens of millions in financial losses — equivalent to potentially billions in current currency.
However, agricultural authorities emphasized that this fly does not contaminate food products.
Federal and state officials along with cattle industry representatives have been issuing public warnings about the fly’s movement through Mexico toward the United States since confirming a case in southern Mexico during November 2024.
This spread has severely damaged Mexico’s beef sector, especially after U.S. Agriculture Secretary Brooke Rollins shut down border entry points to livestock imports in July 2025 to keep the fly from entering Texas.
Mexico has documented thousands of infestations, and Rollins has contended that the Mexican government hasn’t taken sufficient action to regulate animal movement within their borders, a claim Mexican officials have disputed. Rollins has also criticized former Democratic President Joe Biden’s administration, claiming that inadequate border security contributed to the flies’ migration.
However, Haines identified climate change as a crucial factor in the expansion of this tropical species that flourishes in warm conditions and previously vanished during cold periods in the United States.
“The cold snaps that once suppressed stray populations in marginal northern regions are becoming rarer and less severe, thus removing a natural biological check on the flies’ migration north,” she explained. “Warmer temperatures are also expanding the geographical band of suitable habitat northward.”
Texas State Veterinarian Bud Dinges established a 12-mile quarantine zone encompassing most of Zavala County, which includes La Pryor, and a small section of adjacent Uvalde County. Animals cannot exit this area without inspection. Dinges has encouraged residents to examine their animals — including pets — and to “stay put.”
Rollins explained that the fly cannot travel hundreds or even dozens of miles independently. “The only way this spreads is through animal movement,” she stated.
Area ranchers worry the fly will spread among wild animals, especially deer. The previous U.S. outbreak primarily affected deer in the Florida Keys during 2016, although officials confirmed one case last year in a Maryland man who had visited El Salvador and recovered. During the 2016 Florida situation, authorities eliminated the fly within six months by releasing sterile males to mate with females.
In Texas, Haines forecasted, “Their numbers will continue to expand in wildlife populations.”
Rollins reported that the USDA has been releasing millions of sterile male flies in south Texas since February to prevent the insects’ expansion. Officials plan to maintain this strategy.
The USDA established a center in south Texas during February to distribute flies bred in Panama, and invested $21 million in a new fly-breeding facility in southern Mexico expected to begin operations next month.
Diebel, whose family operation sits approximately 200 miles east of the quarantine area, said ranchers are preventively administering injections that block screwworm infestation. They’re also taking additional precautions to treat injuries from ear tagging and other procedures while closely monitoring for illness signs.
“Surveillance is one of the biggest things — just constantly monitoring those cattle,” Diebel explained.
He said he wouldn’t be shocked to see additional isolated cases confirmed, but added, “I’m very confident we can keep this at bay.”
Government and industry representatives express confidence in containing the fly within the United States because the most effective eradication method is both proven and highly successful: releasing sterile males into natural populations. While males are “promiscuous,” scientifically speaking, females are not, and if their single mating encounter involves a sterile male, none of that female’s eggs will develop.
When sterile males become sufficiently abundant — with millions released weekly — the fly population decreases and eventually disappears.
The United States closed its own fly production facilities after eliminating the pest decades ago, leaving only an international breeding operation in Panama in the Western Hemisphere until the new Mexican facility opens. Nevertheless, the USDA is also investing $750 million to construct a fly factory in southern Texas capable of producing up to 300 million sterile flies weekly.
“The sterile insect is not only the most effective tool we have, but it is also considered one of the most environmental friendly insect pest control methods ever developed,” Rollins stated.
