A Maryland resident who traveled to El Salvador got here dwelling 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. Nevertheless it’s additionally a warning signal of a a lot larger risk creeping towards US borders. One that might rattle the spine of American agriculture.
For the previous twenty years, screwworms have been held at a distance by an invisible barrier alongside the Panama-Colombia border by a joint US-Panama program that usually 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 entice wave after wave of flies. And since cattle are stored in confined herds, outbreaks can ripple by dozens of animals. By the point the injury is seen, it’s normally 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 business, in response to the Mexican Nationwide Agricultural Council.
Screwworm outbreaks can spiral quickly, crippling whole herds. Cows are costly to boost, feed, breed, and slaughter — that’s why the American livestock business 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 12 months 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 previous, 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 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 struggle off this rising risk?
Consultants say that the context has shifted within the final 50 years. Local weather change is warming habitats, growing potential locations the place screwworms can thrive. Industrial livestock farming has scaled up enormously, that means a single outbreak might converse by herds quicker than ever. In the meantime, the Trump administration pushed out round 15,000 USDA staff and terminated a screwworm monitoring undertaking. We now have newer, higher instruments to struggle off these, however these are but to be authorised. And parasites don’t anticipate 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 just like home flies that swarm your native dumpster. However in contrast to these strange flies, screwworm flies love contemporary wounds.
Feminine screwworm flies are drawn to 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 title, Cochliomyia hominivorax, which interprets to man-eater, and their widespread title, 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 devour 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 special 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 will 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 thousands and thousands of {dollars}’ price of cattle annually. 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 means to make use of the fly’s biology in opposition to itself. If they may discover a technique 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 easy: Rear big numbers of screwworms in a lab and sterilize the pupae by 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 case you try this at a enough scale and for an extended sufficient time, the inhabitants will ultimately 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 whole. Ranching teams pushed the USDA for eradication, Texas cattlemen even wrote letters to USDA urging the company to develop SIT. And in contrast to at present’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, mentioned 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 way in which to a slender band of dense rainforests between Panama and Colombia known 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 known 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 if you happen to additionally knock down lively infestations.
This system prices about $15 million yearly and is funded largely by USDA, with Panama contributing a small share. “It was one of many biggest achievements of the USDA within the Twentieth century,” Scott mentioned.
However, in 2023, the firewall cracked.
Smuggling of cattle by 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 include than the slender Darién. COPEG has been operating 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 12 months, the USDA dedicated $21 million to convert an previous 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 might formally authorize USDA to construct and fund the brand new Texas facility.
It is a lot of mobilization for a freaky bug, but it surely underscores how otherwise 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 lately rolled out a $1 billion package deal to shore up defenses. The technique there may be nonetheless mass culling of chickens and reduction checks to producers — rather more reactive than proactive. Screwworm, in contrast, has lengthy been handled like a nationwide safety risk, defended by a prevention firewall that prices tens of thousands and thousands {dollars} a 12 months to take care of.
A part of it’s merely worth: Cattle is a $112 billion business, and a screwworm outbreak might price 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 business is simply as highly effective, but it surely’s break up on chicken flu technique – with many producers fearing that widespread vaccination might jeopardize US export markets. That warning exhibits up within the USDA’s new $1 billion chicken 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 are not any FDA-approved medication to deal with screwworms in folks or animals, although the company says it has “a number of regulatory pathways” to fast-track critiques and authorize medication. 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 might not be capable to aptly execute its formidable plans, provided that 15,000 of the company’s workers have been let go earlier this 12 months. Ultimately, it’s not simply biosecurity at stake, it’s additionally a serious supply of our meals provide and a slice of our financial system.
