A flesh-eating parasite is making headlines again. The New World screwworm—an insect that was wiped out of the U.S. nearly 60 years ago—is causing trouble just south of the border. And now, the U.S. Department of Agriculture (USDA) is stepping in fast to stop it from coming back.
The New World screwworm isn’t your average housefly. Its larvae—aka maggots—don’t just feed on dead things. They burrow into living flesh, attacking livestock (and sometimes people) through open wounds. The infection can be devastating if not caught in time. This pest caused millions in damages to the U.S. livestock industry until it was finally wiped out in the U.S. in 1966, thanks to a clever technique where sterilized male flies were released to prevent the population from reproducing.
Even after clearing screwworms out of the U.S., scientists knew they’d keep creeping north from South America unless there was a permanent solution. So the USDA, together with regional partners, moved their fly-fighting operations further south, eventually landing in Panama—the narrowest point of Central America. That’s where they built what became known as the Great American Worm Wall. Every week, millions of sterilized screwworm flies are grown and chilled (so they stay calm). They’re then loaded into planes that fly precise routes over the jungles, sprinkling sterile flies across the landscape like biological confetti. These air drops create an invisible but powerful line of defense—one that stops screwworms from spreading north and keeps the rest of the Americas protected. It’s worked for more than 30 years.
However, in late 2024, Mexico reported new screwworm cases in its southern states. By May 2025, the parasite had made its way closer to the U.S., prompting the USDA to take a major step. On May 11, 2025, the U.S. paused all imports of live cattle, horses, and bison from Mexico.
Screwworms are a real economic and health threat. Before eradication, they cost U.S. farmers up to $100 million a year in losses. What makes screwworms so worrying is how quickly they can spread—especially in livestock. If even one infection goes unnoticed, it can lead to an outbreak that could affect entire herds and put our food supply at risk. It’s pretty rare, but humans can also get infected, especially if there’s an untreated wound in places where screwworms are active. Thankfully, it’s treatable, but it’s painful—and something we definitely don’t want to deal with.
The screwworm is a chilling reminder of how quickly a once-eradicated pest can make a comeback. With cases emerging further north, the situation is unfolding in real time—and the response will need to match the urgency. Whether the current efforts are enough to keep the parasite from crossing into the U.S. remains to be seen. This is one bug we don’t want back in anyone’s backyard.
