In September, Mexican authorities confirmed a case of a deadly flesh-eating parasite in the state of Nuevo León, less than 70 miles (113km) from the Texas border.
The finding, in an eight-month-old cow near the busy Monterrey-to-Laredo highway, marks the northernmost detection of Cochliomyia hominivorax – the New World screwworm – in decades and the closest it’s been to the United States since its eradication there more than half a century ago.
The discovery has set off alarm bells among US ranchers and veterinarians, who still remember the devastation this flesh-eating insect once caused. The screwworm, a metallic blue-grey blowfly, looks ordinary enough. That is, until its larvae begin burrowing into the flesh of living animals.
“The adult looks a lot like blow flies we get around my lab in North Carolina,” says Prof Max Scott, an entomologist at North Carolina State University who has spent his career studying the pest. “The difference is this blowfly is an obligate parasite — the females have to lay their eggs in a living host.”
Within hours of being laid in a wound or natural opening, the eggs hatch into maggots that spiral deep into flesh. The larvae feed for several days before dropping to the ground to pupate, leaving the host disfigured or dead.
Each female fly can lay hundreds of eggs, and untreated infestations often attract other species of flies, accelerating the animal’s decline.
In livestock, screwworms can cause severe weight loss, hide damage and even death – a multibillion-dollar problem for the cattle industries of South America.
A gruesome past
The New World screwworm was once widespread across the southern United States, Mexico and the Caribbean. It wasn’t until the mid-20th century that an unlikely combination of genetics, radiation, and aerial logistics finally brought it under control.
After World War II, US Department of Agriculture (USDA) scientists developed the sterile insect technique, a pioneering genetic control method. The idea was simple but radical: breed millions of screwworm flies in captivity, sterilise them with gamma radiation, and release them into the wild.
Females mate only once, so if they mated with a sterile male, they produced no offspring, driving the population toward collapse.
In the 1950s, tests in Curaçao proved the method could work. By the early 1960s, Florida was screwworm-free. The programme expanded west and south, eventually eradicating the fly from the entire continental US, Mexico, and much of Central America by the early 2000s.
The eradication campaign wasn’t cheap – reportedly costing more than $750 million – but it saved the livestock industry billions in losses each year and allowed US cattle production to boom in the years that followed.
A small production facility in Panama, jointly run by the US and Panamanian governments, maintained a permanent barrier. Every week, they release 100 million sterile flies along the Panama–Colombia border, periodically developing new strains to ensure they are still effective.
The system worked flawlessly for two decades – until it didn’t. Around 2023, cases began appearing again in Panama. Within two years, the parasite had crossed into Costa Rica, Nicaragua, and Honduras. Now it has reached northern Mexico.
What went wrong?
No one knows exactly why the barrier failed, but Scott says there were “warning signs that there were problems along the border”.
“They were getting more cases than they normally would,” he says. “It seems now clear in hindsight that the strain of screwworm was losing effectiveness and probably should have been changed more quickly than it was.”
Farmers not complying with protocol when transporting their livestock also likely contributed to its spread and may explain how cases jumped faster than the natural populations of flies could otherwise travel.
The Panama facility is now operating beyond capacity, producing roughly 110 million sterile flies per week, according to Scott. But during the original eradication campaign through Mexico, the facility there was producing between 500 and 700 million a week. “That’s the kind of scale that was needed,” Scott says.
In short, as things stand, there simply aren’t enough sterile flies to suppress the current outbreak.
Read more:
- 10 deadly parasites
- See how insects are turned into zombies by these fungal parasites
- Are there any organisms which humans parasitise?
An old enemy at the gates
For now, there’s no sign that screwworms have breached the US border. The USDA has deployed nearly 8,000 traps across Texas, Arizona and New Mexico and says no flies have been detected. But vigilance is high.
“The risk is real,” Scott warns. “The best chance to stop it was further south, around the Isthmus of Tehuantepec. There are already reports of cases north of that, so it's going to make it very difficult to stop it with the current capacity of the plant.”
The US government has responded with what it calls a ‘five-pronged plan’. That includes building two new fly-release facilities in Texas, one at Moore Air Force Base capable of dispersing 100 million sterile flies a week, and a larger production plant planned for southern Texas that could triple that capacity.
The USDA is also investing $100 million in new control technologies, from electron-beam and x-ray sterilisation to genetically engineered flies.
The screwworm’s large-scale return would be catastrophic for US livestock producers. A single outbreak could cost billions in animal losses, quarantines, and trade restrictions. When the parasite re-emerged in Florida in 2016 – having hitched a ride on imported deer from the Caribbean – it infected 145 animals and took nearly a year to eradicate.
Though human infections are rare, they can be harrowing. The parasite’s Latin name, Cochliomyia hominivorax, literally means “man-eating fly”. Most human cases involve travellers in affected regions, who suffer infestations in wounds or nasal passages.
“It’s not a major public health issue,” Scott says, “but you do need to be aware if you’re travelling to areas where it’s present.”
A modern genetic upgrade
Scott believes the old sterile-fly approach can still work — but only if supported by new biotechnology. His lab at NC State has spent years developing genetically engineered screwworm strains that produce only males, a major efficiency boost.
The traditional sterile insect technique involves releasing both males and females, which means many males end up mating with sterile females.
As a result, Scott says, the method requires very high radiation doses to ensure those females are fully sterile. His team’s approach, by contrast, produces only males. They can then concentrate on sterilising just these males, allowing researchers to release fewer, healthier flies which are more likely to mate, but still won't produce offspring.
These male-only strains were successfully field-tested in Panama in 2018 but were never rolled out because the existing method was working. With the parasite’s northward march, that may change. Regulatory approval for genetically modified screwworms could now become a priority.
Beyond that, researchers are exploring ‘gene-drive systems’ using CRISPR technology to bias inheritance so that almost all offspring carry a sterility or male-only gene. In theory, this could reduce the number of flies required for suppression by an order of magnitude.
“I hope there’ll be funding to develop more efficient technologies,” Scott said, “because they offer the promise that instead of needing to release 100 million sterile flies, you might get by with 10 million – and that would make genetic control much more feasible.”
For now, the fight remains conventional: aerial releases, intensive trapping, and cross-border cooperation. But even with those defences, the screwworm’s ability to move north faster than expected has rattled scientists.
“I really hope the number of flies they’ve got will keep it out of the US,” Scott says. “But we just don’t know.”
For now, the border holds – just. And with each new case creeping northward, the question becomes less whether the screwworm can return to the United States, and more whether we’re ready when it does.
Read more: