The last four of the Chincoteague ponies afflicted by an infection sometimes called “swamp cancer” have been euthanized, authorities said.

The ponies were part of a celebrated herd of wild horses that live on Assateague Island, on Virginia’s Atlantic Coast. At least seven had been infected.

Although the ponies’ care was the best available, the decision was made to euthanize the last four that had the disease, a spokeswoman for the Chincoteague Volunteer Fire Company said Friday on Facebook. The company helps care for the horses.

The ponies, Shadow, Lightning, Calceti’n and Elusive Star, “just couldn’t fight this off,” Denise Bowden, a public relations officer, wrote in the post. The herd has been traced back hundreds of years. Each year, the ponies swim across a channel to Chincoteague Island, where some are sold.