The Parasitic Screwworm Has Crossed Into Texas, and the Tools That Beat It Once May Not Hold the Line
Federal inspectors confirmed New World screwworm in a 3-week-old calf near La Pryor, Texas, on June 3, 2026, the first detection in the state since 1966.
Agriculture officials moved quickly to contain concerns. The USDA has said it remains confident in its preparations and believes there is “no threat of mass infestation.” But a 12-mile quarantine now restricts the movement of any warm-blooded animal, household pets included, around the infested ranch.
For this one calf, the confidence is reasonable. For the path that brought the fly to it, less so. The screwworm has spent the better part of two years pushing north out of Central America, and it reached Texas at the moment the American cattle herd can least afford a new enemy.
A parasite unlike the rest
Most flies are a nuisance. The screwworm (Cochliomyia hominivorax) is something worse. The female lays her eggs in the open wound or mucous membrane of a living, warm-blooded animal, and when the larvae hatch they burrow inward and feed on living tissue. The wound widens as they go.
The infestation can kill its host, and the fly does not confine itself to cattle. It infests wild mammals like bison, pets and, occasionally, people. The pain from this infestation is severe enough that human patients are given morphine before doctors can remove the maggots, Kevin Esvelt, a biologist at the MIT Media Lab, said.
A single female lays as many as 300 eggs on one wound, and they hatch in under a day to begin feeding on living tissue, Texas A&M entomologists note. A missed infestation is seldom just one animal.
The United States knows the cost of screwworm infestation because it paid it for decades. According to the University of Texas at Austin, screwworm drained an estimated $200 million a year from the country’s meat and dairy industries in the 1950s, over $2 billion in today’s money. The fly was driven out of the U.S. in the 1970s and pushed back to a containment line in Panama, where it held until late 2024.
The La Pryor case appears to be among the very few confirmed U.S. detections since eradication, following the contained Florida Keys outbreak in 2016 and a single traveler’s infection in Maryland in 2025.
The worst possible timing
The calf sits at the exposed edge of a $15 billion industry. Texas runs more cattle than any other state, and the Texas Tribune described the screwworm as a parasite “poised to harm” that business directly.
Keeping the fly out is worth an estimated $796 million a year to U.S. livestock producers alone, by an FAO assessment of past eradication programs, but that benefit stays invisible until the day it fails.
The wider market has little slack to give. The U.S. cattle herd has fallen to about 86.2 million head, its lowest level since 1951, and the breeding cow herd that determines future supply sits at a low not seen since the early 1960s. Years of drought and high input costs drove ranchers to thin their herds, and analysts do not expect supply to recover before 2028. Beef prices have set records as a result, with ground beef climbing above $6.70 a pound and average retail beef reaching roughly $9.64 a pound this spring, up about 13% year over year.
After screwworms spread through Mexico, officials restricted the southern border import of Mexican cattle in 2025. That move reduced a supply that in normal years adds as many as 20,000 feeder cattle a week to U.S. feedlots, close to 5% of placements. A domestic outbreak on top of that would pull animals out of a herd at its lowest levels in several decades and push retail prices past the records they have just set.
And the domestic price spike is only half the exposure risk. U.S. beef exports run around $10 billion a year, according to USDA trade data, and foreign buyers tend to shut their borders at the first quarantine notice, which would strand product that has nowhere else to go.
The 20th-century playbook
The campaign that beat the screwworm the first time is widely regarded as a major achievement in applied biology. Scientists irradiate male flies to sterilize them, then release them by the million. Females mate once in their short lives, and a female that pairs with a sterile male lays eggs that never hatch. Sustain the releases across a wide enough area for long enough, and the population starves itself of offspring and collapses.
The USDA is running that playbook again, at scale. It has converted a fruit-fly plant in southern Mexico into a screwworm breeding facility for $21 million, opened a sterile-fly dispersal center in southern Texas, and broken ground on a $750 million production factory at Moore Air Base in Edinburg meant to reach 100 million sterile flies a week by late 2027. The long-term target is 300 million flies a week.
After the first containment, the U.S. closed its sterile-fly factories and leaned for decades on a single plant in Panama, which releases only about 20 million flies a week over the Darién Gap. The country won the first battle in the screwworm war, then let the machine that won it go idle.
Sterile insect releases work best at a defined frontier or against a small, isolated incursion. They are slow and extraordinarily expensive against a large, established population spread across a continent. There are simply too many screwworms in South America to control through irradiated releases, Esvelt told Freethink, which is why researchers there began hunting for something stronger. A moat of sterile flies can hold a line. It can’t easily drain the reservoir pressing against it.
There is also the time factor. By the USDA’s own schedule, the Edinburg factory does not reach 100 million flies a week until late 2027, which leaves the near-term response leaning on imported sterile flies and the long-standing facility in Panama. But this timeline could struggle against a fly whose numbers can rebound quickly in warm weather, and the calf in La Pryor turned up at the start of summer.
Biotech to the rescue — the gene drive approach
A gene drive is a tool that aims to make a genetic trait spread faster through reproduction than would occur under typical inheritance patterns. It alters how genetic traits are passed on across generations in specific engineered systems. Normally, a parent passes a given gene to about half its offspring. A drive engineered with CRISPR technology can, in some designs, increase inheritance rates toward nearly 100%, so an introduced trait spreads through a population far faster than ordinary inheritance would allow.
For screwworm, a design under study in Uruguay couples a drive with a gene that leaves only female offspring sterile. Released males carry the drive and keep mating. They pass it down through generations while the number of fertile females falls until the population can no longer sustain itself. It is the logic of the sterile-fly program made self-propagating, but it doesn’t require a factory turning out hundreds of millions of insects every week.
Some have raised concerns that, once released, a gene drive is no longer just a laboratory technology and may be difficult to control. But researchers argue they have now built “bio-contained” drives that persist for only a set number of generations, as well as separate constructs meant to switch an existing drive off, the kind of built-in brakes that turn a one-way release into a reversible one.
Colossal Biosciences, a Dallas-based biotechnology company focused on de-extinction technology, has put reversibility at the center of its pitch. They emphasize that their gene drives are designed to be both safer and reversible. Chief executive Ben Lamm has said the company’s approach is intended to be “safer than what has ever been dispersed in the wild,” adding that “we have the ability to roll it back.” The company’s broader approach to invasive species relies on engineered animals that produce only male offspring and live out their natural lives, an alternative to poison and mass culling.
Lamm frames the screwworm as one front in a far larger war. Invasive species impose a significant economic burden worldwide, yet much of the genetic-engineering talent that could help address the problem remains focused on human medicine rather than ecological or agricultural applications. He likened conventional control methods, traps, pesticides and culling, to “archaic ways of treating cancer” set against powerful tools that already exist.
Colossal has described its work on biocontainment technologies for invasive species as relevant to broader public-sector discussions. Whether that advisory role translates into deployment authority is now a live question because the screwworm is no longer a forecast.
“The controlled release of self-limiting biocontainment gene drives can help address challenges at a scale not achievable with other available techniques,” Lamm said. “The issue should be treated seriously as a threat to agriculture, and that safer, reversible tools should be considered as part of long-term response strategies.”
The policy question
The United States has no established rapid pathway for authorizing the release of a gene-drive insect, and jurisdiction is split among agencies that have never approved one. Building that pathway now, while the only confirmed case is a single quarantined calf, is a different exercise than improvising it during a multi-state outbreak.
Cost is perhaps the weakest objection. Past screwworm-eradication programs have returned on the order of $5 to $10 for every dollar invested, by FAO estimates, which makes a new, potentially science-backed tool valuable for helping to reduce risks associated with outbreaks that history has shown can cost billions.
The sterile-fly program bought the country 50 years of relief, and it may yet hold this incursion to one ranch in Zavala County. But the argument from Lamm and from the researchers in Uruguay is narrower and more uncomfortable: the method that won the last screwworm battle was built for a smaller, more contained enemy than the one now standing on the riverbank. Whether to add a new instrument to the arsenal is a decision the country can make deliberately today, or under duress later.
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