Screwworm returns to the US as a forgotten eradication triumph quietly unravels

On June 3, a flesh-eating screwworm larva turned up in a calf near La Pryor, Texas — the first real infestation on US soil since the 1980s, followed by dozens more cases across Texas and New Mexico. The New World screwworm is unusual among flies in that its maggots feed on living tissue, burrowing into any open wound and drawing yet more flies until the infection, if untreated, kills the animal. Before it was wiped out, it made livestock ranching in the Southwest a grinding ordeal: animals had to be inspected every day or two, ranchers kept hands whose sole job was checking for worms, and a single 1935 USDA survey counted over 1.2 million infections and 180,000 dead animals in Texas alone.

The US didn’t get lucky — it engineered the parasite out of existence. The ‘sterile male technique,’ developed out of USDA research that began in 1929 (including entomologist Emory Cushing’s insight that screwworm was a distinct species feeding only on living flesh, which invalidated older carrion-bait traps), floods infested regions with millions of sterilized male flies. Because females mate only once, most matings produce no offspring, and sustained releases collapse the population. Over decades this cleared screwworm from the US, Mexico, and Central America, after which a US-Panamanian body, COPEG, held a permanent ‘barrier’ at the Darién Gap by air-dropping sterile flies weekly to block reinvasion from endemic South America.

Around 2023 that barrier failed, and the pest has been marching north ever since. The deeper lesson is institutional: the program worked so well that the scale of the original catastrophe was forgotten, and the maintenance that kept it contained was allowed to lapse. Re-eradicating screwworm from North and Central America is now underway but will likely take years — a reminder that eradication is a standing commitment, not a one-time win.