A flesh-eating parasitic fly that burrows into living animals and eats them from the inside has now been detected in 34 animals across the United States, and the scientists trying to stop it want you to know they have roughly one-fifth of the sterile flies they'd actually need to push it back. The New World screwworm is here, it's moving fast, and the strategy we're betting on worked sixty years ago under completely different conditions. Sleep tight.
What the Screwworm Actually Does (It's as Bad as It Sounds)
Let's be clear about what we're talking about. The New World screwworm is a parasitic fly that lays its eggs in open wounds on living animals. The larvae hatch and literally eat the host alive, burrowing deeper as they feed. Untreated infestations kill. The United States eradicated this thing in 1966 and then spent the better part of six decades congratulating itself about it.
The fly has now been detected in 34 animals in the US, most of them in Texas, with one case in New Mexico, according to The Guardian. So far it's livestock and pets. No confirmed wildlife detections on the US side yet. But here's the thing about that "yet" -- conservationists with camera traps in Central American forests have been watching jaguars, pumas, tapirs, deer, peccaries, and even porcupines come up covered in screwworm wounds deep in the interior of those forests, far from any cattle operation. The fly doesn't need cattle anymore. It's gone wild, literally.
The Sterile Fly Math Is Not Encouraging
The primary weapon against screwworm is a genuinely clever piece of science: breed massive numbers of flies, irradiate them so they're sterile, then release the males to mate with wild females. The eggs don't fertilize. The population crashes. It worked in the 1960s. Officials are currently dropping 100 million sterile flies across the southwest and Mexico, which The Guardian reports may slow the northward spread but will absolutely not eradicate it from the region.
How many do they actually need? About 500 million, according to Phillip Kaufman, professor and department head of entomology at Texas A&M University. "What we lack are sufficient flies in order to start pushing the population back south," Kaufman told The Guardian. So we're at 20 percent capacity on the one tool we know works, racing to open new breeding facilities -- one launched in Mexico in late June, another planned for Texas in late 2027. Late 2027. The fly is moving now.
New techniques are being explored: raising only sterile males instead of both sexes, developing better bait traps. But as Kaufman put it plainly, "We can't stop doing things we know work in order to try things that don't have any data to support." Science takes time. The screwworm does not.
Illegal Cattle Trafficking Is Driving This and Nobody Is Talking About It
Here is the part of this story that should make you furious. Jeremy Radachowsky, director of the Mesoamerica and Caribbean program at the Wildlife Conservation Society, has been watching the screwworm spread through camera footage and he knows exactly how it's moving. "It basically shot northward through Central America" after breaking through the Darien Gap in 2022, he told The Guardian, covering thousands of kilometers in four or five months -- "at the speed of a truck, and exactly along those illegal cattle-trafficking routes that we had already documented."
When you see screwworm jump 50 or 100 miles, Kaufman explained, that's not an adult fly crossing the distance on its own. They don't do that. "It was people," transporting livestock and pets across borders without health and safety checks. The illegal cattle trade is the delivery system. The screwworm is the package.
Radachowsky told The Guardian that eradication efforts are almost entirely focused on the fly itself -- sterile releases, traps, eradication programs -- while the root cause gets almost no attention. "If you're only using sterile fly technique and you have other factors that are moving beyond your efforts to blanket those areas with sterile flies, you're never going to have the capacity to clear huge areas," he said. Plug the hole, not just the flood.
1966 Called and the Situation Is Completely Different Now
There's an assumption embedded in US agricultural policy right now that goes something like this: the sterile fly technique worked before, so it will work again. Radachowsky, who has been watching this unfold from the ground, thinks that assumption is dangerously naive.
"The problem is in the 60s, when we first eradicated screwworm, we didn't have this high density of cattle like we do now," he told The Guardian. The human population and cattle population across Central America are completely different. Illicit cattle movement across borders barely existed back then. The conditions that allowed the eradication to succeed have fundamentally changed, and the playbook hasn't caught up.
Making things worse: because the screwworm has been classified as a foreign animal disease pest in the US, scientists haven't been legally allowed to study it in American research facilities for fifty years. "We were not permitted to have the fly in research facilities in the US for the last 50 years, because it's such a dangerous organism," Kaufman told The Guardian. So basic questions -- like which odors attract the fly, which would let scientists build effective bait traps -- are only now being investigated through fresh USDA grants. We banned ourselves from studying the thing we were afraid of, and now we're surprised we don't know much about it.
The Screwworm Is the Least Scary Thing Here
As bad as the screwworm is, the conservationists watching this unfold are more worried about what comes next. The same illegal cattle corridors that moved the screwworm north are also open doors for tuberculosis, brucellosis, hoof and mouth disease, and bird flu, The Guardian reports.
"It frightens me that we're not learning the main lesson here," Radachowsky said. The lesson is that porous borders for livestock, combined with an illegal trade that moves animals hundreds of miles without health checks, is a disease superhighway. The screwworm just happened to be the first passenger that showed up visibly enough to panic everyone. The infrastructure for the next one is intact and humming along.
The Dingo Take
Let's put this together. A parasitic fly that eats animals alive is moving north through illegal cattle-trafficking routes, we have one-fifth of the sterile insects needed to fight it, our second breeding facility won't be ready until 2027, and the scientists who know what to do point out that we've spent fifty years legally prohibited from studying this thing in our own country. This is a policy failure dressed up as a natural disaster.
The political will to crack down on illegal cattle trafficking has never materialized because it's expensive, complicated, and crosses borders that multiple governments would rather not acknowledge are porous. So instead we'll spend years and hundreds of millions of dollars on a sterile fly program that experts are already saying can't work at current scale, while the underground livestock trade keeps running the express lane northward. Brilliant strategy. Very normal country behavior.
Radachowsky's warning about other diseases using the same routes deserves to be the headline on every agricultural policy meeting in Washington right now. It won't be. We'll wait until hoof and mouth disease shows up in Kansas or bird flu hitchhikes north on an unmarked truck, and then we'll act surprised and throw money at it and call it unprecedented. It is not unprecedented. People are watching it happen in real time, on camera, in the jungle, and saying so out loud. Whether anyone with actual power is listening is a different question entirely.
