Animal health companies are repurposing old drugs to kill the New World screwworm, a flesh-eating parasite affecting cows, sheep, and other mammals that was detected in the US last week.
The New World screwworm is a type of fly that lays eggs in wounds on animal skin. When the larvae hatch, they eat the animal’s flesh. An infestation can seriously injure or even kill cows or other mammals. Prevalent in South America, the screwworm can also feed on wild animals, domestic pets, and sometimes humans.
As the pest inched closer to the US last year, the Food and Drug Administration began issuing emergency authorizations for drugs to kill it, the first-ever authorizations for animal health products. But experts caution that drugs are only a temporary solution to help farmers minimize the damage until the government can eradicate the parasite entirely.
“This particular infestation is so serious that treating it . . . is not really a great solution,” says Jay Stallons, founder of the animal health advisory firm Tiresias Consulting. “It’s more effective and efficient to eradicate it.”
Last week, the US Department of Agriculture (USDA) announced that it had found the parasite on a cow in Texas. The agency has since reported several other cases, in Texas and New Mexico.
“We have a molecule. And there’s early data that shows that the molecule could work.”
The screwworm used to antagonize farmers in the US but was eliminated in the 1960s by sterilizing males with gamma radiation and releasing them into the wild. Female flies mate only once, so the influx of sterile males caused populations to plummet.
By 2006, officials in North and Central America had used the sterile insect technique to limit the parasite’s northernmost distribution to the isthmus of Panama. Subsequent periodic releases of sterile males on the strip of land separating North and South America established a barrier preventing the flies from returning.
But in 2022 the flies started spreading north, possibly on smuggled cattle. As the flies advanced, government officials braced for the screwworm’s inevitable return to the US.
In August 2025, the FDA announced that it would accept applications for emergency approvals for screwworm drugs. Animal health firms combed through their portfolios to find useful molecules.
A few weeks later, the animal health company Zoetis used evidence from several small studies in South America to secure the first US authorization for a screwworm drug. The product contained the antiparasitic doramectin, which Zoetis was already selling to control worms, lice, and mites. In February, Boehringer Ingelheim used data from Brazil and Argentina to get emergency authorization for ivermectin.
These drugs were a logical place to start, as both are avermectins—a family of old, widely used antiparasitic molecules, Stallons says. “Avermectins have been around forever, so there’s information out there about basically any parasite you can imagine.”
The animal health firm Elanco took a similar tack to identify a screwworm medication for dogs and cats, says Casey Locklear, the company’s medical lead for parasiticides. Elanco found South American studies showing that lotilaner, an isoxazoline Elanco sells as a flea and tick medicine, could expel screwworms from infested dogs.
“We can’t study it here in the United States right now, but we have a knowledge base,” Locklear says. “We have a molecule. And there’s early data that shows that the molecule could work.”
In total, the FDA has approved more than 10 products for screwworm treatment. They include a powder with the insecticides coumaphos and propoxur, tablets containing the neonicotinoid nitenpyram, and a topical solution with the isoxazoline fluralaner.
The US is trying to encourage further innovation to manage screwworm. In January, the USDA announced $100 million in funding for enhancing sterile fly production, improving screwworm traps, and researching treatments that could prevent or control the pest. The agency is also building a facility in Texas to breed sterile males and increase distribution in the US.
Matt Lewis, founder of the agricultural advisory firm Ophelia Partners, says drugs are a small part of the solution. “You’re only throwing a Band-Aid on the problem,” he says.
Fixing the issue will involve ramping up the sterile fly program again and pushing the flies south. Then, Lewis says, governments should improve surveillance at border crossings to make sure the flies don’t come back.
Stallons adds that farmers shouldn’t expect companies to develop new drugs to fight the screwworm. He predicts that the sterile insect technique will successfully eliminate the flies from the US before a company can develop, test, and market a new drug.
In South America, where the flies are endemic, cattle ranchers must keep costs low to stay competitive. They can afford old molecules like avermectins, which have the added benefit of warding off other parasites. But drug developers would likely have a hard time selling expensive new molecules in the region, Stallons says.
“A lot of parasiticides suffer the same fate as antibiotics in humans,” he says. “It’s very hard to build a business case, because the market is just not very favorable.”
