New Vaccine Offers Hope in Chincoteague Ponies’ Battle Against Swamp Cancer

Over the past three years, the disease has claimed the lives of seven of the famously resilient ponies

Chincoteague ponies
Chincoteague ponies take a moment to graze after swimming across the Assateague Channel in 2015. Photo by Brittany Greeson / The Washington Post via Getty Images

A herd of shaggy wild ponies has gallivanted around the Chincoteague National Wildlife Refuge on Assateague Island for roughly 400 years. But while the famously hardy ponies have survived centuries of nor’easters and hurricanes, a new threat has the herd’s fans and custodians worried.

A strange, deadly ailment called swamp cancer started ravaging the barrier island’s pony population three years ago, reports Pamela A. D’Angelo for the Washington Post. Swamp cancer tends to infect cuts and abrasions, turning them into open lesions that deepen and spread across the body.

Since 2017, seven ponies have been laid low by the disease, caused by a fungus-like microorganism called Pythium insidiosum. An eighth pony thought to be infected with the disease was euthanized in July 2019, but the diagnosis has yet to be confirmed, according to Julia Rentsch of Delmarva Now.

Last year, the Chincoteague Volunteer Fire Company—the organization that owns and cares for the ponies—started testing a vaccine against swamp cancer in hopes of safeguarding the roughly 160 ponies living at the refuge. The treatment is still in early stages, but it appears to be working, spokeswoman Denise Bowden tells D’Angelo for a separate WVTF radio story.

“We need to continue to vaccinate over the next [two to three] years to tell better if this vaccine was/is an effective tool in this herd along with environmental management practices,” wrote Bowden in a January Facebook post. “We, along with the team of vets[,] are cautiously optimistic.”

Swamp cancer, first described in 1884, mainly infects horses, dogs and humans. Infection usually occurs via contact with water that contains the swimming zoospores of P. insidiosum, which are drawn to open wounds. Once inside a host, the microbe spreads via snaking filaments it uses to feed on the tissues of the unlucky animal. P. insidiosum behaves somewhat like a fungus but is part of a separate group called oomycetes, or “water molds,” that includes devastating plant pathogens such as potato blight, downy mildew of grapes and sudden oak death.

P. insidiosum is mostly found in tropical climates. It was first documented in the United States—specifically Texas and Florida—some 60 years ago. But as climate change heats up the globe, reports the Post, cases of swamp cancer have started to creep north.

The Chincoteague National Wildlife Refuge has conditions that are close to ideal for the parasite—wetlands with stagnant water and summer temperatures topping 100 degrees—and preliminary tests have shown it is “fairly ubiquitous across the refuge,” manager Nancy Finley tells the Post.

The refuge is situated on Assateague Island, a 37-mile-long barrier island that straddles the boundaries of Maryland and Virginia. The island actually hosts two separate pony herds: the 160-strong group on the Virginia side and, to the north, another group of 73 ponies managed by the National Park Service. Interestingly, the scourge of swamp cancer has—to date—completely spared the Maryland herd.

Much of the standing water on the Maryland side is brackish with ocean brine, spurring scientists to investigate whether salt water might kill the disease-causing microbe, according to the Post.

As of the ponies’ spring health checkup on April 18, all 160 Virginia ponies were free of swamp cancer, Charles Cameron, a veterinarian who has worked with the herd for 30 years, tells the Post.

The pony roundup that accompanies the annual checkup features “Saltwater Cowboys” who wrangle the herd across the refuge. Though the event usually draws visiting onlookers, this year, the COVID-19 pandemic forced the fire company to ask the public to stay away. The nearby town of Chincoteague shuttered its hotels and restaurants in late March, per WVTF.

This year’s roundup targeted roughly a dozen ponies that had yet to receive the experimental three-stage vaccine, according to WVTF. The vaccine’s creator, Richard Hansen, a research veterinarian based in Oklahoma, tells the Post that he is “cautiously optimistic” about its potential for keeping the ponies healthy.

“So far, it seems to be doing really well,” says Bowden to WVTF. “We didn’t have an extremely wet, rainy end of summer last year. I’m thinking that between that and the vaccine, we’re on the right path here.”

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