CHINCOTEAGUE NATIONAL WILDLIFE REFUGE, Va. — More than three years ago, members of the Chincoteague Volunteer Fire Company noticed unusual lesions on the legs of one of the shaggy, stocky wild ponies that have made this barrier island a year-round tourist destination.

The bright coral wound was treated and it seemed to be healing, but in a little over a week the mare became too weak to stand and had to be euthanized. Seven more ponies followed in the coming years — the majority during the very wet 2018 — all succumbing to a deadly disease called swamp cancer.

Spread by a fungus-like microorganism that lives in stagnant water such as swamps, ponds and lakes, swamp cancer has been found worldwide. It frequently infects horses and dogs and thrives in tropical and subtropical regions.

Pythium insidiosum was first documented in the United States in Texas and Florida about 60 years ago (under a different name). Scientists are still learning about how it spreads and infects. But one thing scientists do know is that with a warming climate, it is moving north — including to the island of Assateague, where the wild ponies live.

“Preliminary results show it’s fairly ubiquitous across the refuge,” said Nancy Finley, refuge manager.

Even so, there may be some good news this year.

Facing a threat to the herd of about 160 ponies and foals, the fire department, which owns the animals, last year turned to an experimental three-phase vaccine to inoculate the animals.

And while it is too early to declare victory, officials say the treatment may have paid off. During the recent annual spring pony roundup for health checks on April 18, all 148 ponies and 12 foals showed no sign of the disease, said Charles Cameron, the veterinarian who treated the ponies for 30 years and still helps during the checkups.

Richard Hansen, the research veterinarian whose small Oklahoma-based company created the vaccine, is “cautiously optimistic” the vaccine is working.

The Chincoteague refuge sits within Assateague Island National Seashore, a 37-mile-long barrier island that runs from Ocean City, Md., south to about six miles beyond the town of Chincoteague, Va.

There are woodlands, marshes and miles of pristine beach and dunes that bring wildlife and tourists — especially each July when the ponies are herded across the channel between Assateague and Chincoteague islands and the foals are auctioned off to support the fire department and keep the herd size at about 150 ponies. Puddles of fresh water that attract migratory species of birds can get as hot as 100 degrees during the summer.

That’s a perfect environment for equine pythiosis, said Erica Goss, a scientist from the University of Florida, who specializes in plant pathogens, or oomycetes, such as this one.

Last year, with the pony swim as a backdrop, Goss, refuge wildlife biologist Kevin Holcomb and Gustavo Machado, a scientist from North Carolina State University, began taking water samples throughout the refuge. Machado, who studies the evolution and spread of infectious diseases in swine, poultry and horses, is using the data to help map the spread of the organism at the refuge. Goss took water samples using horse hair as bait to confirm the presence of pythium.

On the Maryland side of Assateague, the National Park Service maintains a herd of 73 ponies but that group has not had any swamp cancer cases. Fresh water on that side is often mixed with ocean water, said Allison Turner, a biological technician there. But she said they don’t know why their ponies haven’t gotten the disease.

Goss is also investigating the tolerance of the microorganism to salt water.

She said there’s a possibility pythium might not release its infectious spores in salt water. She and her colleagues are hoping to find out whether salt water actually kills the microorganism, but their research has been interrupted by covid-19 precautions that shut down their universities and travel plans.

It’s a really unusual organism. They swim around and look like they should be animals,” Hansen said. “Pythium is a disease of plants, too. This organism is similar to the organism that caused the potato blight in Ireland in the 1800s.”

In the environment, he said, they form a spore with a gluelike substance that adheres to the damaged parts of a plant or, in the case of horses, a wound or insect bite. Like a science-fiction monster, the organism shoots in a tube, which creates fingerlike projections inside the tissue, then eats its way through the tissue.

“The way that these fingerlike projections move around in the tissue is they secrete an enzyme that helps clear the path for them to grow,” he said.

Once a horse is infected, masses caused by the disease spread quickly on the legs and abdomen, eventually disabling the animal. Hansen said one way the disease can spread into the environment is when ponies move around and the masses shed.

Last year, Hansen was given permission by the Food and Drug Administration for compassionate use of his vaccine treatments for three human patients diagnosed with swamp cancer in Texas, Illinois and Georgia.

Human infection is considered rare; all three had preexisting health conditions. Hansen said the hospitals arranged the emergency FDA approvals. Two of the three patients were cured, he said, but the third “succumbed to leukemia, a condition she had when also diagnosed with swamp cancer.”

With positive results from the spring health check, the fire department is now making plans for the July pony swim and auction.

“Knock on wood, counting our blessings, so far it seems to be doing really well,” fire department spokeswoman Denise Bowden said.

It may be a much quieter pony swim this year, however.

Given the novel coronavirus, it is unclear how many tourists will make it. This month’s pony roundup and health check normally would have attracted many tourists, as well. But with a nod from the fire department, the refuge closed the event to tourists as part of the effort to stop the spread of covid-19.

CORRECTION: This story was updated to correct the spelling of Gustavo Machado’s name.