A Maryland resident who traveled to El Salvador got here house final month with an unwelcome memento: larvae of the New World screwworm burrowing of their flesh.
The affected person has since recovered from the painful parasite, and Maryland well being officers confirmed that there’s no signal of transmission to anybody else. However the case is historic: It’s the primary time in additional than half a century {that a} human within the US has been contaminated by a screwworm.
By itself, it’s a medical oddity — a one-off, travel-linked case that doesn’t pose a direct threat to Individuals. But it surely’s additionally a warning signal of a a lot larger menace creeping towards US borders. One that would rattle the spine of American agriculture.
For the previous twenty years, screwworms had been held at a distance by an invisible barrier alongside the Panama-Colombia border by a joint US-Panama program that frequently floods the area with sterile flies. That barrier has, nonetheless, cracked. Since 2023, screwworm has resurged by means of Central America and into Mexico.
As a result of the flies lay their eggs in any open wounds, infestations escalate quick: A single lower can entice wave after wave of flies. And since cattle are saved in confined herds, outbreaks can ripple by means of dozens of animals. By the point the harm is seen, it’s often superior and the flies have unfold out. In simply the previous yr, such infestations have led to a $1.3 billion loss within the Mexican cattle export trade, in response to the Mexican Nationwide Agricultural Council.
Screwworm outbreaks can spiral quickly, crippling total herds. Cows are costly to lift, feed, breed, and slaughter — that’s why the American livestock trade treats screwworms as a nightmare situation, worse than mad cow or foot-and-mouth illness. In Texas alone, a screwworm outbreak might drain as a lot as $1.8 billion a yr from ranchers and the broader financial system, in response to an estimate by the US Division of Agriculture.
The US has a historical past of eliminating these flies earlier than. And it’s now dusting off outdated, confirmed methods and spending actual cash. In Might, the USDA put $21 million to renovate an current facility in Metapa, Mexico, to provide sterile flies to manage the unfold. Earlier this yr, the USDA additionally introduced its $8.5 million plan to construct a plant in south Texas. However is a Twentieth-century toolkit sufficient to combat off this rising menace?
Consultants say that the context has shifted within the final 50 years. Local weather change is warming habitats, rising potential locations the place screwworms can thrive. Industrial livestock farming has scaled up enormously, that means a single outbreak might communicate by means of herds sooner than ever. In the meantime, the Trump administration pushed out round 15,000 USDA staff and terminated a screwworm monitoring mission. We now have newer, higher instruments to combat off these, however these are but to be authorized. And parasites don’t await paperwork.
What precisely is a New World screwworm?
The New World screwworm is a parasitic fly discovered as we speak throughout elements of South America and the Caribbean. They’ve shiny blue-gray our bodies and look much like home flies that swarm your native dumpster. However not like these strange flies, screwworm flies love contemporary wounds.
Feminine screwworm flies are interested in warm-blooded animals, and lay their eggs in open cuts or pure openings like ears or nostrils. Every feminine can lay as much as 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, inflicting excessive ache and tissue harm. Their scientific identify, Cochliomyia hominivorax, which interprets to man-eater, and their frequent identify, screwworm, seize their horror: a spiral fly that eats its host alive.
After feeding for as much as every week, the larvae wriggle again out of the wound and drop to the bottom, the place they pupate within the soil earlier than rising as grownup screwworm flies — able to repeat the cycle.
What makes screwworms significantly brutal is that they solely eat residing flesh. A single infested wound turns into a beacon for extra flies, resulting in layer upon layer of larvae actually consuming the host alive. Infestations in people are excruciating and disfiguring, however hardly ever deadly with remedy. In animals, it’s a unique story: A single untreated wound can host hundreds of larvae that actually eat a cow from the within out, resulting in blood loss, infections, and collapse. Total herds could be decimated.
However there’s an achilles heel: Feminine screwworms mate solely as soon as of their life — a singular organic quirk that has underpinned the US’s management technique for many years.
How the US beat screwworms
Screwworms as soon as terrorized the American South and the Western US, and killed thousands and thousands of {dollars}’ value of cattle every year. By the mid-Twentieth century, the fly was costing America’s ranchers as much as $100 million yearly.
However beginning within the Fifties, USDA scientists discovered a approach to make use of the fly’s biology in opposition to itself. If they may discover a approach to get the feminine flies to mate with sterile mates, they may cease the flies’ inhabitants in its tracks. And that’s how the sterile insect approach (SIT) was developed.
The SIT is pretty simple: Rear big numbers of screwworms in a lab and sterilize the pupae by means of radiation (a discovery from the post-war atomic age when scientists realized they may make flies infertile with out killing them). Then these freshly sterilized pupae are packed onto twin-engine planes, timed so the flies hatch within the air. These flies are then sprayed out over the forest and ranchlands by the thousands and thousands. They wake in heat air and do what flies do: They mate. These pairings then produce nothing. In the event you try this at a ample scale and for a protracted sufficient time, the inhabitants will finally collapse.
The primary eradication program within the American Southeast ran by means of the Fifties adopted by a bigger push throughout Southwest, costing roughly $42 million in complete. Ranching teams pushed the USDA for eradication, Texas cattlemen even wrote letters to USDA urging the company to broaden SIT. And in contrast to as we speak’s debates round genetically modified mosquitoes, screwworms by no means stirred a lot controversy. The approach was focused, pesticide-free, and spared different bugs, which is why it was an unusually “inexperienced” pest management, stated Max Scott, a professor of entomology at NC State College. By 1966, the fly was gone.
The approach then was adopted in Mexico and elements of Central America, pushing the flies all the best way to a slender band of dense rainforests between Panama and Colombia referred to as the Darién Hole. The Pan-American freeway famously stops there, the area is sparsely populated, treacherous to cross, and lightweight on livestock. It’s precisely the form of chokepoint the place a organic “firewall” can maintain.
Since 1998, a US-Panama program referred to as Fee for the Eradication and Prevention of Screwworm (COPEG) has held the road on the Darién Hole. Planes drop off thousands and thousands of sterile flies every week, and inspectors patrol the frontier city (not the deep Darién itself) to identify infestations, pluck out maggots manually, and deal with wounds with pesticides — as a result of SIT solely works when you additionally knock down energetic infestations.
This system prices about $15 million yearly and is funded principally by USDA, with Panama contributing a small share. “It was one of many biggest achievements of the USDA within the Twentieth century,” Scott stated.
However, in 2023, the firewall cracked.
Smuggling of cattle by means of Central America seeded contemporary outbreaks in new areas, and local weather shifts — larger temperatures and humidity — aided their unfold. By spring 2025, Mexico was reporting detections as far north as Oaxaca and Veracruz, a stretch of land far wider and tough to comprise than the slender Darién. COPEG has been working flat out, turning out round 100 million larvae every week. However even at most capability, the plant can solely achieve this a lot. The screwworm entrance continued to advance.
Earlier this yr, the USDA dedicated $21 million to convert an outdated fruit-fly plant in Metapa, Mexico, to churn out 60 million to 100 million sterile screwworms weekly to be launched in southern Mexico, the place the brand new entrance is. It’s additionally constructing a manufacturing facility in Edinburg, Texas, with 3 times that capability. Congress is attempting to lock this in with the STOP Screwworms Act, a bipartisan invoice that may formally authorize USDA to construct and fund the brand new Texas facility.
This can be a lot of mobilization for a freaky bug, nevertheless it underscores how in another way the US treats livestock threats.
Hen flu has killed greater than 160 million birds within the US since 2022, in response to the Coalition to Cease Flu, and USDA just lately rolled out a $1 billion package deal to shore up defenses. The technique there’s nonetheless mass culling of chickens and aid checks to producers — rather more reactive than proactive. Screwworm, against this, has lengthy been handled like a nationwide safety menace, defended by a prevention firewall that prices tens of thousands and thousands {dollars} a yr to keep up.
A part of it’s merely worth: Cattle is a $112 billion trade, and a screwworm outbreak might price billions a yr in losses. Half is political: Ranchers pressed for eradication within the Fifties and have constantly backed the sterile-fly program since. The poultry trade is simply as highly effective, nevertheless it’s break up on fowl flu technique – with many producers fearing that widespread vaccination might jeopardize US export markets. That warning reveals up within the USDA’s new $1 billion fowl flu package deal: half a billion for biosecurity, $400 million in monetary aid, and simply $100 million — 10 % — for vaccines.
Proper now, there are not any FDA-approved medicine to deal with screwworms in folks or animals, although the company says it has “a number of regulatory pathways” to fast-track opinions and authorize medicine. There are promising genetic instruments that enhance on SIT, however they are going to want regulatory approval and public belief earlier than they fly.
And even with contemporary funding, the USDA could not be capable to aptly execute its bold plans, provided that 15,000 of the company’s workers had been let go earlier this yr. Ultimately, it’s not simply biosecurity at stake, it’s additionally a significant supply of our meals provide and a slice of our financial system.
