CHINCOTEAGUE—Every summer, tourists visit the Chincoteague National Wildlife Refuge to see the famous wild ponies and spend the day at the beach. Some fish and crab the marshes and canals.

But this year, as crowds thronged to the Chincoteague Volunteer Fire Department’s Pony Penning Day, scientist Erica Goss was here trying to catch a deadly pathogen. Since the disease first appeared at the refuge three years ago, eight female ponies have been put down.

The mysterious microorganism that causes the disease called swamp cancer thrives in wet, warm regions. Like its name implies, pythium insidiosum can be found all over the world in tropical and subtropical regions. In the U.S., it lives mostly on the Gulf Coast. But as the climate has changed, it has moved north and is now on Virginia’s Eastern Shore at Chincoteague.

Local veterinarian Dr. Charles Cameron has been treating the ponies for 30 years. This year, there are 142 total. He said another mare began exhibiting the disease’s tell-tale lesions in early July.

“This was unusual. Normally it’s the latter part of August and September in the past that we’ve seen the lesions,” he said. “It’s a factor of temperature, warm weather and stagnant water. The last few years, we’ve had pretty wet springs, a lot of standing water and warm weather.”

Goss, a scientist from the University of Florida, is taking water samples throughout the refuge to help map the spread of the organism.

“We got some samples from Chincoteague in June, and some of the water temperatures where those water samples were taken were up to 100 degrees,” she said. “And this pathogen thrives in the wet. It can only reproduce and spread when there’s available water. So, as the summers become hotter and wetter, we do expect it to be more prevalent in the environment.”

The organism is likely living in some of the freshwater ponds at Chincoteague National Wildlife Refuge. The refuge uses dams to increase or decrease the water in 14 pools on over 2,600 acres.

Once a horse is infected, the disease manifests in coral-like lesions called kunkers that spread quickly on the legs and abdomen, eventually disabling the horses to the point where they have to be put down.

Scientists still don’t know much about the organism that causes the disease. It was first documented in the U.S. in Texas and Florida about 60 years ago under a different name.

Gustavo Machado, a scientist from North Carolina State University, studies the evolution and spread of infectious diseases in swine, poultry and horses. He’s using data to map out locations of pythium. He says the microorganisms living in the refuge could be infecting ponies through a bug bite or a cut.

“It’s super hard to tell, exactly. There’s not much research on that,” he said. “That’s one of the reasons why we’re here trying to understand more or less how this is actually spreading in this area.”

Goss specializes in plant pathogens such as this one. It’s basically a water mold, but behaves like a fungus. It lurks in the refuge’s shallow freshwater ponds and standing water used by migrating birds and the ponies.

To hear her describe it is like listening to a science-fiction horror story.

“It has these long filaments, so almost like these fingers that reach out into the tissue it infects to pull in nutrients, like plant roots,” she said. “That’s how it feeds itself. When it’s reproducing, it produces spores that swim in water. And so they swim until they find a host, and then they infect that host.”

When Goss collects specimens, she uses dog and human hair to bait them.

“The swimming spores swim to the hair and attach to the hair like they’re going to infect it, but they can’t because it’s just hair. Then we pull the hair out of the water and put it on growth media. That’s how we culture the pathogen from water,” she explained.

Refuge Manager Nancy Finley is working with scientists and vets and the local fire department, which owns the ponies living in the refuge, auctioning foals each year to raise money and keep the size of the herd in check.

“There’s two fronts. We’re trying to find out where the organism is, what habitat it might hang out in,” she said. “And then there’s the other front the fire company is taking the lead on, which is to find out what’s going on with the health of the horses and what we can do to prevent the disease through vaccines or treatments.”

Scientists are also sampling freshwater reservoirs the refuge built to provide a habitat for migratory waterfowl.

Machado, Goss and Finley believe the organism may be found naturally throughout the Eastern Shore.


Chincoteague sits within Assateague Island National Seashore, a 37-mile-long barrier island that runs from about six miles south of Chincoteague in Virginia almost to Ocean City, Md. Allison Turner is a biological technician who helps maintain the herd of wild ponies that live on the Maryland side of Assateague.

On Assateague, no one owns the herd. Ponies are left to nature as with the rest of the wildlife. There are no health checks, no auctioning off foals to control the population as with the Chincoteague herd. The only human intervention is darting the herd with birth control or when a ranger has to wave ponies away from the road.

Turner said there have been no cases of equine pythiosis on Assateague.

“We do monitor the horses. We do a complete survey six times a year. And we have so many people out there that are very interested in them and they’re looking at them all the time,” she said. “So if this was to show up in our population, we would see it.”

Turner also said the habitat is different on Assateague. Water holes are small and widely spaced, and salt water washes in. Pythium can’t live in salt water.

“So that would prevent the spread of it,” Turner said, before quickly adding, “That’s speculation. We don’t know that for a fact, but that could be the reason we don’t have it up here. We don’t know; we really don’t know.”

What makes pythium dangerous is it can infect mammals. So far, it’s been horses, dogs and humans. But human cases are rare, with only about 10–15 recognized in the U.S. during the last 30 years.

“Some of those cases have been in the eye because the eye is a very conducive environment for organisms to grow,” said Goss. “The other cases have largely been in people who have some underlying health problem that seems to make them susceptible to the infection.”


Richard Hansen is a research veterinarian from Oklahoma who for the last seven years has been developing a vaccination and treatments. He was brought in by the fire department to try out a new vaccine. Since April, most of the herd has received three vaccinations, including the mare that contracted swamp cancer in July.

“That one received the three vaccinations, but developed the characteristic pythium lesions very quickly after the third vaccination,” he said.

He said blood work indicated the mare was anemic, so “she may not have been able to respond as well as she would have normally.”

He’s trying to figure out why some horses don’t get the disease.

“They seem to be, need to be, immunocompromised in some way for them to be susceptible,” he said. “You could have two horses that may have wounds on them walk in the same pond and only one develop the disease because of some reason that we’re still investigating.”

Hansen has been using the vaccine to treat three human cases in Texas, Illinois and Georgia, A 3-year-old girl and a 26-year-old man appear to be cured, he said. But a third case, a 14-year-old girl, is still being treated. As with the horses, all were immunocompromised.

“What you’re trying to do is reverse the immune system so that it recognizes the infection,” he explained.

As adoring fans squeeze along the fencing to watch the ponies, across the way people wade in canals along the access roads catching crabs. So far, scientists are not concerned about people being in the water at the refuge.

“What we found in Florida is that this pathogen is really common in the environment,” said Goss. “It’s in most of the lakes and ponds we sampled and I think that is probably true across the Gulf Coast. People use the lakes for recreation in Florida and it’s not something we worry about.”

Denise Bowman is one of the firefighters who has been trying to maintain a positive attitude as the vets and scientists do their detective work.

She watched as Cameron and Hansen vaccinated more ponies and give them a regular health check before the next day’s swim and auction. They all checked out healthy, but it’s still too soon to declare the vaccine a success.

There is discussion of possibly moving the herds elsewhere within the refuge once scientists determine where the organism is living.

“Last year was rough on us, she said. “As we say here on Chincoteague, we’re just going to crab along the bank.”

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