A mysterious microorganism that was once only found in tropical and subtropical regions is living in fresh water ponds at Chincoteague National Wildlife Refuge, which sits within the Assateague Island National Seashore.
The organism causes a disease in horses called pythiosis or "swamp cancer." It seems to enter through a cut or bug bite and manifests in coral-like lesions called kunkers. Scientists still don't know much about the disease, which first occurred in the U.S. about 30 years ago.
It has led to the death of eight female ponies that belong to part of the herd owned by the Chincoteague Volunteer Fire Department. The U.S. Fish & Wildlife, which manages the refuge and allows the ponies to live in large penned-in areas there, has brought in scientists and veterinarians to try to eliminate the culprit and cure the disease.
The disease caused great concern at the annual Pony Penning, when the wild ponies from Assateague Island are herded across the channel to Chincoteague Island and then auctioned off.
Fans crowded around the holding pens. This was the closest they will ever get to their passion – the scruffy, feral, and sometimes laser-blue-eyed Chincoteague Pony, some whose lineage goes back 400 years. People come from all over the world. Some watch over the internet. Jeannie Zurich moved to Hebron, Maryland from New Jersey this year just to be closer to the ponies.
“So, now I'm only an hour away from here,” said Zurich. “That was the whole plan.”
Like Zurich, most here are hyper aware of the mysterious disease that has infected only fillies and mares. It was a park visitor who, last month, reported seeing an injured filly, the most recent victim of pythiosis or swamp cancer. Like seven other ponies, she didn't survive even though she received a series of a new vaccination.
“We're getting back some of the data on it,” said Richard Hansen, a research veterinarian from Oklahoma who for the last seven years has been developing a vaccination and treatments. “That [horse] received the three vaccinations but developed the characteristic pythium lesions very quickly after the third vaccination." Hansen said that after doing some of the blood work, he found she had had anemia. Now he's wondering if perhaps the horse may not have been able to respond to the vaccinations as well as had she not been anemic.
Nearly all the herd has received three vaccinations since April. Hansen is 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 develops the disease because of some reason that we're still investigating.”
Erica Goss is a scientist from the University of Florida where she specializes in plant pathogens like this one called an "oomycete." It's basically a water mold but has behaviors like a fungus. It lurks in the refuge's shallow fresh water ponds and standing water used by migrating birds and the ponies.
To hear her describe the micro-organism, pythium insidiosum, 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.”
Pythium insidiosum lives all over the world in tropical and subtropical regions. In the U.S. it lives mostly in Gulf Coast but has been found in California, Wisconsin and Washington state. What makes the organism unusual 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.”
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 are cured, he said, but a 14-year-old girl is still being treated.
At the refuge, the horses all check out healthy, but it's still too soon to declare the vaccine a success.
As crowds watched the ponies, across the way, people waded in canals along the access roads catching crabs. Others headed to the marshes across from the beach to fish. 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.”
As the fire department prepared for the next day's auction, Goss along with Fish and Wildlife biologist and a scientist from North Carolina State University travel the refuge taking water samples. They will use the data to build a map of where pythium insidium has taken up residence in the refuge.
This story was updated on August 5 to correct details about the people who have been treated for pythiosis.
