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. However it’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 20 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, nevertheless, cracked. Since 2023, screwworm has resurged by 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 appeal to wave after wave of flies. And since cattle are saved in confined herds, outbreaks can ripple by dozens of animals. By the point the injury is seen, it’s often superior and the flies have unfold out. In simply the previous 12 months, such infestations have led to a $1.3 billion loss within the Mexican cattle export trade, in line with the Mexican Nationwide Agricultural Council.
Screwworm outbreaks can spiral quickly, crippling complete herds. Cows are costly to boost, feed, breed, and slaughter — that’s why the American livestock trade treats screwworms as a nightmare state of affairs, worse than mad cow or foot-and-mouth illness. In Texas alone, a screwworm outbreak may drain as a lot as $1.8 billion a 12 months from ranchers and the broader economic system, in line with 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 And it’s now dusting off outdated, confirmed methods and spending actual cash.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 Could, the USDA put $21 million to renovate an current facility in Metapa, Mexico, to provide sterile flies to manage the unfold. Earlier this 12 months, 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, growing doable locations the place screwworms can thrive. Industrial livestock farming has scaled up enormously, that means a single outbreak may communicate by herds sooner than ever. In the meantime, the Trump administration pushed out round 15,000 USDA workers and terminated a screwworm monitoring challenge. We now have newer, higher instruments to combat off these, however these are but to be accepted. And parasites don’t look ahead to paperwork.
What precisely is a New World screwworm?
The New World screwworm is a parasitic fly discovered at present 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 abnormal flies, screwworm flies love recent 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 injury. Their scientific identify, Cochliomyia hominivorax, which interprets to man-eater, and their widespread identify, screwworm, seize their horror: a spiral fly that eats its host alive.
After feeding for as much as per 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 notably 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 not often deadly with therapy. In animals, it’s a distinct story: A single untreated wound can host 1000’s of larvae that actually eat a cow from the within out, resulting in blood loss, infections, and collapse. Complete herds might be decimated.
However there’s an achilles heel: Feminine screwworms mate solely as soon as of their life — a novel 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 hundreds of thousands of {dollars}’ price 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 method to make use of the fly’s biology towards itself. If they might discover a approach to get the feminine flies to mate with sterile mates, they might cease the flies’ inhabitants in its tracks. And that’s how the sterile insect method (SIT) was developed.
The SIT is pretty easy: Rear enormous numbers of screwworms in a lab and sterilize the pupae by radiation (a discovery from the post-war atomic age when scientists realized they might 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 hundreds of thousands. They wake in heat air and do what flies do: They mate. These pairings then produce nothing. For those who do 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 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 at present’s debates round genetically modified mosquitoes, screwworms by no means stirred a lot controversy. The method 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 method 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 light-weight on livestock. It’s precisely the sort 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 hundreds of 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 in the event 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 Central America seeded recent outbreaks in new areas, and local weather shifts — greater 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 include 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 accomplish that a lot. The screwworm entrance continued to advance.
Earlier this 12 months, 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 unit 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.
It is a lot of mobilization for a freaky bug, however it underscores how in a different way the US treats livestock threats.
Hen flu has killed greater than 160 million birds within the US since 2022, in line with the Coalition to Cease Flu, and USDA not too long ago rolled out a $1 billion package deal to shore up defenses. The technique there’s nonetheless mass culling of chickens and reduction 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 hundreds of thousands {dollars} a 12 months to take care of.
A part of it’s merely worth: Cattle is a $112 billion trade, and a screwworm outbreak may value billions a 12 months in losses. Half is political: Ranchers pressed for eradication within the Fifties and have persistently backed the sterile-fly program since. The poultry trade is simply as highly effective, however it’s cut up on hen flu technique – with many producers fearing that widespread vaccination may jeopardize US export markets. That warning reveals up within the USDA’s new $1 billion hen flu package deal: half a billion for biosecurity, $400 million in monetary reduction, and simply $100 million — 10 p.c — for vaccines.
Proper now, there aren’t any FDA-approved medicine to deal with screwworms in individuals or animals, although the company says it has “a number of regulatory pathways” to fast-track evaluations and authorize medicine. There are promising genetic instruments that enhance on SIT, however they’ll want regulatory approval and public belief earlier than they fly.
And even with recent funding, the USDA could not be capable to aptly execute its bold plans, on condition that 15,000 of the company’s employees had been let go earlier this 12 months. In the long run, it’s not simply biosecurity at stake, it’s additionally a significant supply of our meals provide and a slice of our economic system.
