CHINCOTEAGUE, Va. — A deadly infection took a toll on the Eastern Shore's wild pony herd in Chincoteague, and the animals' caretakers are hoping they've seen the last of the disease.

Known as "swamp cancer," the illness led to the deaths of seven ponies between October and December.

The disease is not actually cancer but rather an infection known as pythiosis, which is caused by an organism similar to a fungus that can be found in stagnant water. Horses become infected when the organism enters the bloodstream through open wounds. 

The disease then leaves ponies with lesions on their bodies and can cost thousands of dollars to treat. 

"This thing is just a nightmare," said Dr. Charles Cameron, a veterinarian at Eastern Shore Animal Hospital. He added, "Unless we catch it early, the chances of success as far as treating it is not good."

"The problem is these are wild animals," said Denise Bowden of the Chincoteague Volunteer Fire Company, which manages the herd. By living out in the wild, the ponies are not seen every day.

Dr. Cameron said a vaccine is in development but is not yet FDA approved. He's hopeful it will be made available this spring, and it will help stave off swamp cancer in the future.

There have been no new reported cases of swamp cancer in 2019.

"Right now we feel like we're getting a pretty good hold on what we can do to prevent it from happening," said Bowden.

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The Associated Press contributed to this report.