At one point, hopes grew that at least some of the horses could be saved. But secondary infections set in and the pythiosis seemed to return in some cases. One pony died in October, another on Dec. 3. Two weeks later, Lyra, the first case discovered this year, was euthanized after she was no longer able to stand.

“It’s horrible,” Bowden said. “I’ve seen grown men bawl like babies when we have to put a horse down.”

It would be hard to overstate the cultural and economic role the horses have played here for centuries. They are long-feral descendants of domesticated livestock, and local legend has it that they first swam ashore as refugees from a foundering Spanish ship in the 1600s. Biologists, though, say they are more likely remnants of animals introduced by mainland settlers.

Whatever the origin story, they have long been a defining feature of island life, cared for by folks, like Bowden, who grew up with them on Chincoteague and beloved by visitors from around the world, including many entranced by the 1947 children’s classic “Misty of Chincoteague.” With about 1.3 million visitors a year, the herd is a 150-horsepower economic engine that keeps the refuge near the top of Virginia’s most popular tourist destinations.