A Maryland resident who traveled to El Salvador came home last month with an unwelcome souvenir: larvae of the New World screwworm burrowing in their flesh.

The patient has since recovered from the painful parasite, and Maryland health officials confirmed that there’s no sign of transmission to anyone else. But the case is historic: It’s the first time in more than half a century that a human in the US has been infected by a screwworm.

On its own, it’s a medical oddity — a one-off, travel-linked case that doesn’t pose a direct risk to Americans. But it’s also a warning sign of a much bigger threat creeping toward US borders. One that could rattle the backbone of American agriculture.

For the past two decades, screwworms were held at a distance by an invisible barrier along the Panama-Colombia border by a joint US-Panama program that regularly floods the region with sterile flies. That barrier has, however, cracked. Since 2023, screwworm has resurged through Central America and into Mexico.

Because the flies lay their eggs in any open wounds, infestations escalate fast: A single cut can attract wave after wave of flies. And since cattle are kept in confined herds, outbreaks can ripple through dozens of animals. By the time the damage is visible, it’s usually advanced and the flies have spread out. In just the past year, such infestations have led to a $1.3 billion loss in the Mexican cattle export industry, according to the Mexican National Agricultural Council.

Screwworm outbreaks can spiral rapidly, crippling entire herds. Cows are expensive to raise, feed, breed, and slaughter — that’s why the American livestock industry treats screwworms as a nightmare scenario, worse than mad cow or foot-and-mouth disease. In Texas alone, a screwworm outbreak could drain as much as $1.8 billion a year from ranchers and the wider economy, according to an estimate by the US Department of Agriculture.

The US has a history of eliminating these flies before. And it’s now dusting off old, proven strategies and spending real money. In May, the USDA put $21 million to renovate an existing facility in Metapa, Mexico, to produce sterile flies to control the spread. Earlier this year, the USDA also announced its $8.5 million plan to build a plant in south Texas. But is a 20th-century toolkit enough to fight off this emerging threat?

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