A Flesh-Eating Parasite the US Spent 50 Years Eradicating Has Returned

The New World screwworm, a flesh-eating fly larvae eradicated from the US by 1966 through one of history's most ambitious biological control programs, has breached the Panama barrier and reappeared in Texas and New Mexico for the first time since the 1980s.

For the first time since the 1980s, the New World screwworm — a flesh-eating parasite whose larvae burrow into the living tissue of animals — has been found in the United States. On June 3 of this year, screwworm was detected in a three-week-old calf near the town of Del Rio, Texas. Dozens more cases have since been confirmed across Texas and New Mexico.

The screwworm (Cochliomyia hominivorax) is a species of blowfly whose maggots are uniquely predatory: unlike most fly larvae that feed on dead tissue, screwworms eat their way into living flesh, worsening the wound and attracting more flies in a grisly cycle. Before eradication, US ranchers couldn't leave their animals unattended for more than a day without risking finding them eaten alive.

The US eliminated screwworm through a breathtakingly clever program: the "sterile male technique." Using gamma radiation to sterilize millions of male flies, the USDA dropped them from planes over infested areas. Females mated with sterile males and laid eggs that never hatched. Generation by generation, the wild population collapsed. First tested on the Florida island of Sanibel, then the Caribbean island of Curaçao, the program was scaled up to eliminate screwworm from the entire US by 1966.

The barrier was then pushed south — through Mexico, through Central America, and finally to the Darien Gap on the Colombia-Panama border. At just 60 miles wide, this narrow strip of dense rainforest was the perfect choke point. For over two decades, a joint US-Panamanian facility dropped millions of sterile flies a week across the gap. It worked so well that the US collectively forgot screwworm was ever a problem.

Sometime around 2023, the barrier failed. COVID-era disruptions grounded livestock inspectors and killed millions of sterile flies during power outages at the Panama facility. Illegal cattle trafficking — sometimes tied to narcotics cartels — moved infected animals north through Central America. The Darien Gap itself degraded: dense rainforest was cleared for cattle grazing, making it easier for screwworm to spread.

By 2024, screwworm had reached every Central American country and Mexico — 18,000 cases in Panama, 8,600 in Costa Rica, 3,300 in Nicaragua. In 2025, cases in Mexico surged. And now, in mid-2026, it has crossed into the United States.

The USDA is responding aggressively. A new screwworm facility capable of producing 300 million sterile flies a week is being built at Moore Air Force Base in Texas. Existing fruit fly insectaries in Mexico are being converted for screwworm production. But experts warn that re-eradication will likely take "close to a decade of sustained work."

The screwworm program stands as a cautionary tale of success breeding complacency. When a problem is solved so thoroughly that we dismantle the very infrastructure that solved it, the old enemy only needs one breach to begin its long march back.

Sources: Construction Physics, Wikipedia