We beat screwworm once. Here’s how we’re trying to do it again

A Texas rancher in the 1940s notices the small wound on his calf first thing in the morning. It’s nothing unusual—perhaps the calf snagged on barbed wire. But when the rancher returns at noon to check on the animal, the wound has grown and is raw and weeping. Closer inspection reveals hundreds of little, white larvae writhing and burrowing into the flesh: screwworm maggots. If left untreated, the calf will be dead within days.This isn’t an isolated horror story: the flesh-eating parasite devastated livestock operations from Florida to Texas for much of the 20th century, when it killed thousands of animals every year and cost more than $1 billion annually in today’s dollars. Back then screwworm was simply a fact of life, a gruesome tax levied by nature on anyone who raised animals.Then came one of the most remarkable triumphs in pest control. Through a brilliant combination of science and engineering, screwworm was eliminated in the U.S. in 1966. The victory seemingly closed a chapter in agricultural history. But then, on June 3, 2026, screwworm reared its ugly head once more: larvae were found in a three-week-old calf in Zavala County, Texas, followed within days by confirmed cases in three other counties, as well as in a dog in New Mexico that had traveled across state lines. The U.S. Department of Agriculture confirmed the first wildlife detection on October 21: an elk killed on a game ranch in Pecos County, Texas. There have been more than 60 confirmed cases in domestic animals since then and an additional case in another Texas elk. With hunting season now underway, more infections could emerge, particularly in wildlife, as hunters encounter affected animals.On supporting science journalismIf you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.A New World Screwworm.USDA Media by Lance CheungThe return of New World screwworm to U.S. soil for the first time in decades follows years of warnings from veterinarians and entomologists who have watched the parasite creep steadily northward through Central America and Mexico, propelled by a rise in temperatures, shifts in wildlife movements and gaps in surveillance. Its reappearance now poses an urgent threat to ranchers, wildlife and public health.It’s not 1966 anymore. The U.S. can’t simply rerun the old playbook. Instead officials are trying to pair proven methods with modern technology, such as artificial intelligence, drones and thermal cameras, to keep the once eliminated parasite from regaining a foothold.A Fly That Lives Up to Its NameThe New World screwworm’s scientific name, Cochliomyia hominivorax, comes in part from a rough translation of “man-eater” in Latin. Unlike most other blowflies, which eat carrion, it feeds on the living tissue or flesh of warm-blooded animals—including people—to complete its life cycle.The cycle begins when a female fly lays 200 to 300 eggs at a time in an open wound or orifice such as the eyes, nose or mouth. Within 24 hours, the eggs hatch into larvae and immediately start burrowing headfirst into the flesh using hooklike mouthparts. As they feed, they literally spiral or screw their way in, inspiring their common name. When sated, the larvae fall to the ground, lodge themselves into soil and emerge as adults to continue the vicious cycle.The results are devastating. Bacteria can infect the wounds where maggots are present, leading to sepsis. Without proper wound care and quarantine, infected animals die quickly, often within a week.In the past, ranchers and veterinarians could only slow the spread of screwworm but never stop it. The reproductive rate of the fly was too high, and the geography it covered was too vast.But in the 1930s USDA scientists Edward Knipling and Raymond Bushland came up with a protocol called the sterile insect technique (SIT). It’s founded on a basic biological process: female blowflies only mate once. If the male is sterile, she lays unfertilized eggs that never hatch, effectively removing them from the population. Release enough sterile males to outnumber wild males, and you can drive the population into collapse without a single drop of pesticide.In response to a confirmed New World Screwworm case in Tom Green County, federal ground teams deployed sterile flies using specially designed ground based release chambers across the affected area. The pupae are shipped from Panama in coolers to keep them dormant, are then measured into precise amounts and coated with a non-toxic green, fluorescent dye to distinguish them from wild pupae.USDA Media by Lance CheungAfter some trial and error, the technique succeeded in 1954, when it was used on the island of Curaçao. As screwworms tore through the local goat population, researchers packed sterile males into paper boxes and dropped them from aircraft. The boxes burst open on impact, scattering the flies across the landscape. They quickly swamped the wild population, and in about eight months, screwworm was eliminated from the island.Florida was cleared by 1959. Texas proved more challenging, given its vast ranching operations, but it, too, was eventually declared screwworm-free in 1966, and the effort pushed south through Mexico and into Central America.South America’s size, terrain and political complexity made it too difficult to tackle, so scientists established a suppression zone at Panama’s Darién Gap, the narrow neck of land where Central and South America meet. There sterile flies were continuously released to prevent northern migration. Screwworm was functionally eliminated everywhere north of that line by 2002. Not anymore.How screwworm came backFor decades, the system worked. But by 2023, a perfect storm of factors had converged to create the conditions for the screwworm’s return.Climate change has expanded the areas that have suitably warm, humid conditions for more of the year for screwworm to thrive, meaning less seasonal die-off and more generations of screwworms per year.Political instability in Panama, a surge in drug‑trafficking activity and heavy human migration through the Darién Gap made it increasingly hard to maintain the Panama Barrier. At the same time, U.S. federal budget pressures periodically forced cutbacks in surveillance. Early in 2025 the Trump administration’s U.S. DOGE Service sharply reduced funding for the USDA’s Animal and Plant Health Inspection Service and ended a USAID program that had monitored and contained New World screwworm in Central America. Border policies have also oscillated: in the past two years, Mexico-U.S. livestock import rules have relaxed and tightened repeatedly, creating windows in which infected animals could slip through. And even when border quarantines are in place, illegal livestock and wildlife crossings continue.An external camera captures the start of a flight's sterile New World Screwworm fly dispersal in predetermined zones over Texas along the U.S./Mexico border on July 2, 2026.USDA Media by Lance CheungAdding to the strain, wild screwworm may be evolving to recognize and reject mass-reared sterile males. This behavior, known as assortative mating, would undermine the sterile insect technique the eradication program relies on.A warning of the current infestation came in a 2016 outbreak in the Key deer population of Florida—these were the first U.S. cases in decades. Though that outbreak was contained by strictly monitoring the deer and releasing sterile flies, it exposed critical vulnerabilities: the difficulty of detecting early cases, the danger to wildlife populations with no natural resistance and the risks associated with island and isolated ecosystems.Those same problems were still in play as the screwworm made its real return, moving steadily northward through Central America. In November 2024 screwworm was detected in a cow at an inspection checkpoint in the state of Chiapas in Mexico—the first detection that far north in the country in decades. By late 2025, isolated cases were being found in northern Mexico, within about 100 miles of the Texas border. In January 2026 the USDA shifted sterile fly dispersal efforts into Texas as a precaution. By February, a new sterile fly dispersal facility had opened at Moore Air Base in Texas, the first domestic installation the U.S. has ever built. None of it came fast enough.Where Do We Go from Here?For the first several months of the new infestation, screwworm cases were confirmed only in domestic animals, located in Texas and New Mexico, and early efforts to restrict trade and impose quarantine zones seemed to be working. But after hunting season began in September, the USDA confirmed the first U.S. wildlife detection in the Texas elk. With a dog in Arizona now among the reported cases and almost four dozen new domestic detections so far in November alone, veterinarians, ranchers and wildlife managers are bracing for more cases, particularly if the parasite gains a foothold in deer, elk or feral hogs that share ranges with livestock.So what is being done to prevent screwworm from gaining ground this time around?The core response to the outbreak remains the approach that worked 60 years ago: sterile fly release. The new sterile fly facility at Moore Air Base has been releasing 100 million flies weekly since February 2026. A separate facility there is projected to begin operations in 2027, scaling up to producing 300 million flies per week. More flies are also being sent out across Mexico.But the question on many researchers’ minds is whether these sterile flies are sufficient.On June 16, the USDA announced funding for 40 projects totaling around $105 million to help battle screwworm. Much of that investment is going toward next-generation control tools, but only a couple of these are usable now or will be in the very near future. The most advanced is NovoFly, a genetically engineered male-only strain. Conventional programs must rear and sterilize both sexes because they can’t easily be separated, even though only males are needed, so a male-only strain dramatically increases efficiency. The Environmental Protection Agency granted an emergency exemption for NovoFly use in the U.S. in June; Mexico has yet to approve it, pending public consultation. As of November, NovoFly was not yet being used operationally, and its deployment timeline remained uncertain.Researchers are also testing a technology called eBeam sterilization. It uses high-energy electrons to sterilize flies instead of the current method, which involves the radioactive isotope cobalt 60. Early lab results are promising, and pilot evaluations are underway, though it would take years to adapt production facilities to use this technique. Yet another approach uses a synthetic scent, called swormlure-5, that mimics an open wound, luring adult flies into traps. Paired with insecticide, swormlure-5 has cut local fly populations by up to 90 percent, and it is now used in border traps for surveillance.More speculative approaches, such as CRISPR-based gene editing to create sterile males, are likely many more years away from being implemented.A U.S. Department of Agriculture Wildlife Services employee operates a drone equipped with a thermal imaging camera during wildlife surveillance at Texas Parks and Wildlife Department's Chaparral Wildlife Management Area in South Texas.Maegan Lanham / Texas Parks & WildlifeThe current response also draws on surveillance technologies that were unavailable during the last major eradication campaign. USDA teams are using drones equipped with thermal sensors and high-resolution cameras to survey livestock and wildlife across vast ranches, looking for abnormal heat signatures, visible wounds and unusual behavior that could signal infestation. The USDA carried out more than 2,000 drone surveillance flights and observations of nearly 40,000 animals in Texas between June 4 and October 22. In addition, AI tools are being developed to analyze the images, flag suspicious animals for closer inspection and help prioritize field investigations, while predictive models use weather, habitat and outbreak data to forecast where the pest may spread. Trail cameras, thousands of insect traps and enhanced monitoring of animal movements add further layers of surveillance.If this pest is not contained, the economic stakes are enormous. A sustained outbreak could mean livestock deaths, costly inspections and treatment, movement restrictions and more disruption to cross-border trade. Monitoring wild animals, which are harder to inspect and treat than livestock, is another challenge, and veterinary capacity is also a concern. Many U.S. veterinarians have never encountered screwworm, making rancher education and rapid diagnosis essential.The coming months will show whether the current response is working. Progress would look like sustained declines in new infestations and wild-fly detections, followed by long stretches without cases. Historically, eradication programs have used six months without detection as a technical benchmark and 12 months as a basis for declaring an area officially free of screwworm, though it depends on the local circumstances. The question we have now is whether the response is suppressing the outbreak or merely chasing its spread. The tools exist; the challenge is deploying them at sufficient scale for long enough to make a lasting change.