Horse Health

What is swamp cancer and how is it killing the Chincoteague wild horses?

by Mary Jo DiLonardo as published on

A rare fungus is threatening the lives of the Chincoteague ponies.

Caretakers are concerned that the fungus also could affect the wild horses in Corolla, North Carolina. (Photo: lembi/Shutterstock)

The wild ponies of Chincoteague are legendary. Made famous in Marguerite Henry’s 1947 book, “Misty of Chincoteague,” these colorful, shaggy horses actually live on nearby Assateague Island. There are two herds, separated by a fence, with one living in Virginia and one in Maryland. These sturdy, resilient horses have learned to adapt over the years by eating marsh grasses and finding fresh water in ponds. Every summer, the ponies are rounded up and made to swim the channel between the islands so some of the foals can be auctioned off to raise money for the local fire department and to help control the size of the herd.

But a recent outbreak has caused distress among the Virginia herd. Seven of the Chincoteague ponies have had to be put down due to an uncommon fungus-like infection called pythios. Also referred to as “swamp cancer,” the disease is caused by Pythiosis insidiosum, an organism that can be found in standing or stagnant water.

Despite various medications, surgeries and round-the-clock care, Chincoteague ponies Essie, Rain Dancer, Lyra, Shadow, Lightning, Calceti’n and Elusive Star were unable to battle the disease.

According to the Chincoteague Volunteer Fire Department, the rest of the herd now appear to be healthy. In early January, the group posted on its Facebook page, “All the ponies that suffered from the ‘swamp cancer’ have crossed the rainbow bridge. We currently have NO ponies that are dealing with this fungus. The rest of the herd is healthy and doing very, very well.”

How the disease is spread and treated

Pythiosis typically survives when winter temperatures aren’t cold enough to kill the fungus. Although the fungus can infect dogs, cats and even people, it’s most common in horses, according to Today’s Veterinary Nurse. In the U.S., the disease is most often found along the Gulf Coast, but cases have been found in other areas across the country, including Illinois, Missouri, Oklahoma and Wisconsin.

The fungus grows on dying or dead grass near the water. Horses often are infected when they stand in water with open cuts or sores. They can also drink, eat or inhale the dangerous spores. It cannot be spread from animal to animal or from animal to person. They have to come in direct contact with the fungus. Once the fungus is in a horse’s body, it compromises the animal’s immune system and starts triggering granular, seed-like lesions, often on the lower legs and abdomen.

The disease sometimes can be treated with a therapeutic (not preventative) “vaccine,” immunotherapy and surgery. Treatment success rates are high if the disease is caught early, Georgia equine veterinarian Kenneth Marcella writes in DVM360.

Local veterinarians helped fire company volunteers treat the sick Chincoteague ponies and thought treatments were working for a while. Dr. George Marble of Eastern Shore Animal Hospital told CNN he had some success with an antifungal treatment, but the pythiosis “came back with a vengeance.”

Prevention is key

The key to dealing with this devastating illness is prevention, rather than treatment, say those who have worked on the front lines. A preventative vaccine is being developed, but it hasn’t been approved by the U.S. Department of Agriculture yet.

“Cross your fingers that it gets approval and we can work to vaccinate our ponies very soon,” the Chincoteague Volunteer Fire Department posted on Facebook. “Also, Fish & Wildlife service is taking a good hard look at this fungus and will hopefully be doing everything they can to eradicate it from the refuge. It isn’t only a pony problem. It can affect any animal that comes in contact with it. This has been a learning experience, an exhausting experience and a life experience.”

In the meantime, the disease is on the radar of other wild pony protectors, as they hope it doesn’t impact their herds. The outbreak has people also concerned about the ponies that roam North Carolina’s Outer Banks.

“Are the horses here in Corolla at risk? The short answer is yes, they are,” the Corolla Wild Horse Fund wrote in a Facebook post. Volunteers and people who care for the horses are asking people to help clean up the area to reduce the risk of injury so the fungus would be less likely to cause an infection.

“We will also be distributing examples of what to look for in case of an outbreak (like pictures of Pythiosis lesions) and ask people to call and report any horse they see with open cuts and scrapes, or suspicious wounds … We will continue to stay on top of the latest research and developments, and we will be ready to act should we see an outbreak here. Hopefully it will not come to that, but we’d certainly rather be safe than sorry.”

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