The United States once defeated a ravenous, parasitic fly with mass releases of sterile insects. A genetically engineered strain could strengthen that defense, while CRISPR gene drives remain a more contentious possibility.
The flesh-eating New World screwworm (NWS)—a parasitic fly whose maggots feed on the living tissues of warm-blooded animals—has returned to the United States, creating a challenging agricultural biosecurity emergency across the ranchlands, border corridors, and wildlife habitat of Texas and New Mexico.
The U.S. Department of Agriculture (USDA) has confirmed 45 cases across 13 counties: 44 in Texas hill country and one in New Mexico, affecting cattle, sheep, goats, and dogs. The Centers for Disease Control (CDC) currently describes the risk to the U.S. public as very low. One travel-associated human case was confirmed in August 2025 in a person who returned from El Salvador.
"This is not just a Texas problem; it's a national security threat to our food supply," said Texas Agriculture Commissioner Sid Miller. "The only proven way to eliminate this pest is to destroy the fly itself through an aggressive program."
[View the daily New World Screwworm Tracker here]
Officials have set up zones of quarantine covering parts of 22 Texas counties, hoping that such measures will block further disease spread. Warm-blooded animals within designated infested zones cannot be moved outside those areas without prior state authorization. To bolster defenses, the Food and Drug Administration (FDA) issued an Emergency Use Authorization (EUA) for "CLiK Extra," a preventive wound spray authorized for specified livestock and certain captive or captured wildlife. It is not authorized for pets and will not treat an active infestation. The authorization also includes slaughter-withdrawal and milk restrictions.
The economic ripple effects are already visible: Canada has suspended imports of cattle, bison, horses, pigs, sheep, goats, cervids, and camelids from Texas. Wildlife experts warn that millions of wild white-tailed deer could be especially difficult to protect or treat. USDA estimates that an outbreak on the scale of Texas's 1976 resurgence could cost the state's producers about $733 million a year and the wider Texas economy roughly $1.8 billion.
While state and federal officials use surveillance, movement controls, wound treatments and mass releases of sterile flies, genetics is beginning to expand the toolkit. The nearest-term advance is NovoFly, a genetically engineered male-only strain intended to make the existing sterile-insect program more efficient. Farther ahead lies a more consequential possibility: a self-propagating CRISPR gene drive designed to suppress or even eliminate the screwworm. The two approaches carry very different benefits, risks, and regulatory challenges.
The pest and its return
On June 3, the USDA made an ominous discovery. They confirmed a case of NWS in a three-week-old calf in South Texas. It was a shock—the first identified case of screwworm in the continental U.S. since 1976 (a subsequent outbreak in 2016–17 killed endangered deer in the Florida Keys before sterile-fly releases eliminated it). The 1976 resurgence infested an estimated 1.49 million cattle and 332,600 sheep and goats in Texas; producers spent $132.1 million that year, before adjustment for inflation, to manage it. The current regional resurgence began in Panama and Costa Rica in 2023, reached Mexico in November 2024, and continued northward.
What is the fearsome screwworm, how does it spread, and why now? The "worm" is the larval stage of Cochliomyia hominivorax, a type of blowfly with three stripes on its body, named for its reputation as a "man-eater." The female fly can lay 200 to 300 eggs in a wound or body opening, such as the eyes, ears, nose, or mouth of livestock, pets, and even, on rare occasions, humans and wildlife. The eggs hatch into maggots equipped with mouth hooks that tear living tissue. After about a week of feeding, the larvae drop off the animal and burrow into the ground to pupate. The life cycle concludes when the adult screwworm fly emerges and flies off to reproduce.
Unlike common wound-associated maggots that consume dead tissue, the larvae of the screwworm feed only on living tissue. This makes them especially gruesome and difficult to confront, since farmers and pet owners in afflicted areas must constantly monitor their animals for behavioral changes, open wounds with foul-smelling discharge, or unusual swelling. Every maggot must be removed from the flesh, sometimes surgically, and the wound cleaned and treated under veterinary guidance. Treatments may include antiparasitic medications such as ivermectin—yes, it has a legitimate use even though it has no benefit in treating COVID in humans—alongside preventive applications like the newly authorized CLiK Extra wound spray.
Response strategies
Past eradication campaigns pioneered a highly successful method known as the sterile insect technique, or SIT. Screwworm pupae are exposed to gamma radiation, producing sterile adult flies. By releasing enormous numbers of them, the program floods an infested area with sterile males that mate with wild females, resulting in a diminished number of viable fertilized eggs and, therefore, fewer flies. Because female screwworm flies generally mate only once, their eggs do not hatch, and the wild population drops over successive generations. The approach worked. SIT eliminated screwworm from the southeastern United States in 1959 and the Southwest in 1966. The campaign then moved south: Mexico was declared free of the pest in 1991, and eradication reached Panama in 2006. But in 2023, an outbreak moved north from Central America, overwhelming the defense system.
Animal movement appears to have accelerated the regional spread. The Wildlife Conservation Society reported that screwworm hot spots tracked known cattle-smuggling routes through Central America and Mexico. That evidence argues the need for stronger animal traceability and enforcement, although it does not establish that cartels caused the specific U.S. infestations.
The problem is complicated. Although the smuggling of unscreened cattle across the border is the proximate cause of the spread of screwworms, a contributing factor is cutbacks in federal agencies such as the USDA's Animal and Plant Health Inspection Service (APHIS), which lost 25% of its staff, including more than 300 veterinary services employees, due to cutbacks by Elon Musk's Department of Government Efficiency (DOGE). DOGE also gutted the U.S. Agency for International Development (USAID), which had housed a plan to monitor, contain, and respond to the spread of screwworm across the U.S.-Mexico border.
What's being done now to stop the screwworm infestation? The USDA's publicly accessible Dashboard tracks and monitors confirmed animal cases and wild-fly detections, while state agencies administer infested zones, inspections, and animal-movement rules.
Currently, screwworm infestations are not considered a food-safety threat, so meat and dairy products will continue to be sold as usual. But this could change.
To discourage further spread, the agency is expanding both production and dispersal of sterile flies. The Pacora plant currently produces about 100 million sterile flies a week. A renovated facility nearby in Mexico is ramping toward another 100 million. Moore Air Base in Edinburg, Texas, has an operating dispersal center capable of releasing up to 100 million flies a week, and a separate production plant under construction there is scheduled to begin producing 100 million a week in November 2027 and eventually to reach 300 million. Together, USDA says, the expanded network could approach 500 million sterile flies a week—the scale used during the original eradication campaign.
The most immediate advance is the genetically modified NovoFly, developed by USDA's Agricultural Research Service and North Carolina State University. Its conditional female-lethal system kills females early inside the production facility, yielding male-only batches. Those males are still irradiated before release, so they are sterile and cannot pass the engineered trait into the wild.
The efficiency gain could be substantial. Conventional facilities rear both sexes, and released sterile females can mate with sterile males, diverting those males from wild females. A male-only strain could effectively double the useful male output from the same production capacity while reducing rearing costs. EPA granted NovoFly a limited emergency exemption on June 8, 2026. Production must still be scaled up, and additional jurisdictions may need to approve its use.
Could CRISPR gene drives go further?
The use of sterile flies to curb an insect infestation suggests a more radical possibility— a "gene drive" that could spread a suppressive trait without the continuous mass release of irradiated insects. It's been dubbed an "extinction drive."
Gregory Kaebnick, a senior bioethics scholar at The Hastings Center, has examined this exact question—exploring whether gene drives could be deployed to deliberately drive the parasite toward extinction. This CRISPR-mediated technique acts like a genetic fast-forward button. It overrides the usual 50-50 odds of natural inheritance, allowing engineered traits—in this case perhaps female sterility or lethality—to pass through nearly 100% of offspring and potentially crash a pest population from the inside out. Unlike NovoFly, such a drive is designed to reproduce and spread in the wild.
The CRISPR-based drive contains a Cas9 enzyme and guide RNA that cut the corresponding unmodified DNA sequence inherited from the other parent. The cell's own repair machinery may then copy the drive sequence into the cut site. The critical result: Far more than half of the offspring can inherit the drive.
Let's break down how the drive could work. One approach would target a gene essential for fertility or female viability—for example, disrupting a gene needed for female flies to develop properly. As the drive spreads, fewer fertile females are produced, and the population could collapse.
Laboratory experiments have already crashed caged populations of malaria-carrying Anopheles gambiae mosquitoes using a drive targeting female development. Screwworm is biologically different, however: the fly is the pest, and its larvae directly cause myiasis. Genomic research has identified possible screwworm targets for future genetic control, but a screwworm gene drive remains theoretical.
Population-replacement drives offer another strategy for mosquitoes: They can spread traits that prevent the insect from carrying a pathogen. That model does not translate neatly to screwworm, however, because C. hominivorax is itself the harmful organism. Suppression is the relevant goal.
There are several reasons—practical, ecological, and regulatory—that this radical approach has not yet been tested, let alone implemented in screwworm. Previous gene-drive proposals have spurred intense opposition from activist environmentalists and civil society groups, who raise the potential for a 'Butterfly Effect.' They warn about possible transboundary spread, ecological effects, resistance, and the difficulty of recalling a self-propagating organism.
A 2016 call by Friends of the Earth International leading a coalition of 160 NGOs warned of "serious and potentially irreversible threats to biodiversity," warning that "once released, these engineered organisms cannot be recalled." Even today, many NGOs lobby to suspend gene drive implementation indefinitely. No gene-drive insect has yet been released into the open environment for disease control.
Many researchers have opposed blanket bans, arguing that contained work is necessary to learn whether gene drives can be made safe and effective. Imperial College London evolutionary geneticist Professor Austin Burt, a pioneer in gene drive research, has called attempts to halt studies "somewhat ridiculous." "If the point is that this could be something that could be of great benefit, you would want to be exploring it safely rather than shutting down research." Molecular parasitologist Andrea Crisanti has cautioned that blanket bans would be "a disaster for developing the technology."
The National Academies has called for phased testing, ecological risk assessment, public engagement, and clarified oversight, because some effects could be irreversible. Bioethicist Gregory Kaebnick and ecologist James Collins have argued that deliberately eradicating screwworm may be ethically defensible, while emphasizing that extinction decisions should be rare, species-specific, and publicly deliberated.
For most researchers and health experts, the possibility of limiting disease and devastation outweighs the speculative ecological risks of suppressing or eliminating a destructive pest species. The present outbreak supplies a powerful reason to pursue that research carefully—and to build the scientific and regulatory capacity before the next barrier fails.
Kathleen L. Hefferon is an instructor in microbiology at Cornell University. Find Kathleen on X @KHefferon.
Henry I. Miller, a physician and molecular biologist, is the Glenn Swogger Distinguished Fellow at the Science Literacy Project. He was the founding director of the FDA's Office of Biotechnology. Find Henry on X @henryimiller