A flesh-eating parasite is at our borders. What happens if it spreads? 

A Maryland resident who traveled to El Salvador came home last month with an unwelcome souvenir: larvae of the New World screwworm burrowing in their flesh.

The patient has since recovered from the painful parasite, and Maryland health officials confirmed that there’s no sign of transmission to anyone else. But the case is historic: It’s the first time in more than half a century that a human in the US has been infected by a screwworm.

On its own, it’s a medical oddity — a one-off, travel-linked case that doesn’t pose a direct risk to Americans. But it’s also a warning sign of a much bigger threat creeping toward US borders. One that could rattle the backbone of American agriculture.

For the past two decades, screwworms were held at a distance by an invisible barrier along the Panama-Colombia border by a joint US-Panama program that regularly floods the region with sterile flies. That barrier has, however, cracked. Since 2023, screwworm has resurged through Central America and into Mexico.

Because the flies lay their eggs in any open wounds, infestations escalate fast: A single cut can attract wave after wave of flies. And since cattle are kept in confined herds, outbreaks can ripple through dozens of animals. By the time the damage is visible, it’s usually advanced and the flies have spread out. In just the past year, such infestations have led to a $1.3 billion loss in the Mexican cattle export industry, according to the Mexican National Agricultural Council.

Screwworm outbreaks can spiral rapidly, crippling entire herds. Cows are expensive to raise, feed, breed, and slaughter — that’s why the American livestock industry treats screwworms as a nightmare scenario, worse than mad cow or foot-and-mouth disease. In Texas alone, a screwworm outbreak could drain as much as $1.8 billion a year from ranchers and the wider economy, according to an estimate by the US Department of Agriculture.

The US has a history of eliminating these flies before. And it’s now dusting off old, proven strategies and spending real money. In May, the USDA put $21 million to renovate an existing facility in Metapa, Mexico, to produce sterile flies to control the spread. Earlier this year, the USDA also announced its $8.5 million plan to build a plant in south Texas. But is a 20th-century toolkit enough to fight off this emerging threat?

Experts say that the context has shifted in the last 50 years. Climate change is warming habitats, increasing possible places where screwworms can thrive. Industrial livestock farming has scaled up enormously, meaning a single outbreak could speak through herds faster than ever. Meanwhile, the Trump administration pushed out around 15,000 USDA employees and terminated a screwworm monitoring project. We have newer, better tools to fight off these, but those are yet to be approved. And parasites don’t wait for paperwork.

What exactly is a New World screwworm?

The New World screwworm is a parasitic fly found today across parts of South America and the Caribbean. They have shiny blue-gray bodies and look similar to house flies that swarm your local dumpster. But unlike those ordinary flies, screwworm flies love fresh wounds.

Female screwworm flies are attracted to warm-blooded animals, and lay their eggs in open cuts or natural openings like ears or nostrils. Each female can lay up to 200 eggs at a time, which hatch some 12 to 24 hours later. Upon hatching, the larvae twist into flesh like corkscrews tearing deeper as they feed, causing extreme pain and tissue damage. Their scientific name, Cochliomyia hominivorax, which translates to man-eater, and their common name, screwworm, capture their horror: a spiral fly that eats its host alive.

After feeding for up to a week, the larvae wriggle back out of the wound and drop to the ground, where they pupate in the soil before emerging as adult screwworm flies — ready to repeat the cycle.

What makes screwworms particularly brutal is they only consume living flesh. A single infested wound becomes a beacon for more flies, leading to layer upon layer of larvae literally eating the host alive. Infestations in humans are excruciating and disfiguring, but rarely fatal with treatment. In animals, it’s a different story: A single untreated wound can host thousands of larvae that literally eat a cow from the inside out, leading to blood loss, infections, and collapse. Entire herds can be decimated.

But there’s an achilles heel: Female screwworms mate only once in their life — a unique biological quirk that has underpinned the US’s control strategy for decades.

How the US beat screwworms

Screwworms once terrorized the American South and the Western US, and killed millions of dollars’ worth of cattle each year. By the mid-20th century, the fly was costing America’s ranchers up to $100 million annually.

But starting in the 1950s, USDA scientists found a way to use the fly’s biology against itself. If they could find a way to get the female flies to mate with sterile mates, they could stop the flies’ population in its tracks. And that’s how the sterile insect technique (SIT) was developed.

The SIT is fairly straightforward: Rear huge numbers of screwworms in a lab and sterilize the pupae through radiation (a discovery from the post-war atomic age when scientists realized they could make flies infertile without killing them). Then these freshly sterilized pupae are packed onto twin-engine planes, timed so the flies hatch in the air. These flies are then sprayed out over the forest and ranchlands by the millions. They wake in warm air and do what flies do: They mate. Those pairings then produce nothing. If you do that at a sufficient scale and for a long enough time, the population will eventually collapse.

The first eradication program in the American Southeast ran through the 1950s followed by a larger push across Southwest, costing roughly $42 million in total. Ranching groups pushed the USDA for eradication, Texas cattlemen even wrote letters to USDA urging the agency to expand SIT. And unlike today’s debates around genetically modified mosquitoes, screwworms never stirred much controversy. The technique was targeted, pesticide-free, and spared other insects, which is why it was an unusually “green” pest control, said Max Scott, a professor of entomology at NC State University. By 1966, the fly was gone.

The technique then was adopted in Mexico and parts of Central America, pushing the flies all the way to a narrow band of dense rainforests between Panama and Colombia called the Darién Gap. The Pan-American highway famously stops there, the region is sparsely populated, treacherous to cross, and light on livestock. It’s exactly the kind of chokepoint where a biological “firewall” can hold.

Since 1998, a US-Panama program called Commission for the Eradication and Prevention of Screwworm (COPEG) has held the line at the Darién Gap. Planes drop off millions of sterile flies each week, and inspectors patrol the frontier town (not the deep Darién itself) to spot infestations, pluck out maggots manually, and treat wounds with insecticides — because SIT only works if you also knock down active infestations.

The program costs about $15 million annually and is funded mostly by USDA, with Panama contributing a small share. “It was one of the greatest achievements of the USDA in the 20th century,” Scott said.

But, in 2023, the firewall cracked.

Smuggling of cattle through Central America seeded fresh outbreaks in new regions, and climate shifts — higher temperatures and humidity — aided their spread. By spring 2025, Mexico was reporting detections as far north as Oaxaca and Veracruz, a stretch of land far wider and difficult to contain than the narrow Darién. COPEG has been running flat out, turning out around 100 million larvae each week. But even at maximum capacity, the plant can only do so much. The screwworm front continued to advance.

Earlier this year, the USDA committed $21 million to convert an old fruit-fly plant in Metapa, Mexico, to churn out 60 million to 100 million sterile screwworms weekly to be released in southern Mexico, where the new front is. It’s also building a factory in Edinburg, Texas, with three times that capacity. Congress is trying to lock this in with the STOP Screwworms Act, a bipartisan bill that would formally authorize USDA to build and fund the new Texas facility.

This is a lot of mobilization for a freaky bug, but it underscores how differently the US treats livestock threats.

Bird flu has killed more than 160 million birds in the US since 2022, according to the Coalition to Stop Flu, and USDA recently rolled out a $1 billion package to shore up defenses. The strategy there is still mass culling of chickens and relief checks to producers — much more reactive than proactive. Screwworm, by contrast, has long been treated like a national security threat, defended by a prevention firewall that costs tens of millions dollars a year to maintain.

Part of it is simply value: Cattle is a $112 billion industry, and a screwworm outbreak could cost billions a year in losses. Part is political: Ranchers pressed for eradication in the 1950s and have consistently backed the sterile-fly program since. The poultry industry is just as powerful, but it’s split on bird flu strategy – with many producers fearing that widespread vaccination could jeopardize US export markets. That caution shows up in the USDA’s new $1 billion bird flu package: half a billion for biosecurity, $400 million in financial relief, and just $100 million — 10 percent — for vaccines.

Right now, there are no FDA-approved drugs to treat screwworms in people or animals, though the agency says it has “multiple regulatory pathways” to fast-track reviews and authorize drugs. There are promising genetic tools that improve on SIT, but they will need regulatory approval and public trust before they fly.

And even with fresh funding, the USDA may not be able to aptly execute its ambitious plans, given that 15,000 of the agency’s staff were let go earlier this year. In the end, it’s not just biosecurity at stake, it’s also a major source of our food supply and a slice of our economy.