A flesh-eating parasite is at our borders. What happens if it spreads? 

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-Co