What is killing Sable Island's horses?

A team of researchers that has travelled to Sable Island for the past two years to find out what causes the horses to die is finding evidence of starvation in some of the feral animals as well as unusual levels of parasites.

Not many people would be grinning ear to ear as they gaze down at the corpse of a Sable Island horse, but Emily Jenkins, part of the team doing necropsies on the horses, regards it as a rare opportunity.

"It's like a kid in a candy shop," says Jenkins, an associate professor in the department of veterinary microbiology at the University of Saskatchewan.

"A dead horse is a mystery to solve. What's inside it? What killed it? It's a scientific puzzle."

Jenkins acknowledges her gleeful reaction is "kind of weird."

But she's quick to redeem herself: "I would like to also point out that I was very excited to see the first foal on the island in 2018 and that live horses are also pretty darn amazing."

Located about 300 kilometres off the southeast coast of Nova Scotia, Sable Island is a 42-kilometre sandbar that has supported a population of feral horses since the 18th century.

Jenkins said when she first touched down on the island in April 2017, she was crossing her fingers that there would be a few carcasses to examine. She got lucky.

The team examined 30 dead horses, just over half of the 50 or so estimated to have died that winter. Those mortalities represent 10 per cent of the population of about 500 — a percentage that "isn't off the charts" for wildlife, Jenkins said, but is considered high by Sable Island standards. The following year, she studied just five.

The last time a scientist studied the horses from a veterinary perspective was in the 1970s, so "there was really very, very little that we knew about why horses would die on the island," Jenkins said.

And since the population is protected under federal laws, getting samples from them is difficult, which is why dead horses are such a "treasure trove," Jenkins said. Researchers can examine the organs, bones and internal parasites to find out what makes them tick — or doesn't.

'Not a scrap of vegetation'

Starvation was one of the key factors in the deaths of the animals, especially for yearlings, who have lower social status and less access to prime grazing territory.

That did not come as a surprise to Jenkins, who said the lush photos her colleagues took on the island in July and August were a far cry from what she captured at the end of a rough winter.

"I'm like, 'Did you guys Photoshop this?' Because it's green, totally green. And when I go out it is totally brown. There is not a scrap of vegetation."

That lack of forage is reflected in the horses' bones. Jenkins said healthy horses will have fat everywhere — under their skin, around their organs and in their bone marrow. When food is lacking, the fat in the marrow is the last to get depleted.

Some dead Sable Island horses were on their last fat reserves, with just six per cent of fat left in their marrow.

It's not just the lack of food that affects the horses' nutrition. Their bad teeth, worn down by the large quantities of sand in their diet, mean they can't chew their food well and don't get as many nutrients as they would otherwise.

The ever-present sand on the island also contributes more directly to the demise of some horses, as it can block their gastrointestinal tract.

The researchers found the bacteria that causes strangles, a virus that causes respiratory disease and abortion, and the parasite lungworm — a surprise, Jenkins said, since lungworm is usually associated with the presence of donkeys.

Fecal egg counts — measurements that reflect the number of worms in a horse's stomach and intestines — were 1,500 eggs per gram of feces, or three times higher than what would be considered high for a domestic horse.

"I think if our domestic horses had fecal egg counts as high as the Sable horses, they would just drop dead," Jenkins said.

Since parasites in Sable Island horses have never been exposed to dewormers, the genetics of both the parasites and the horses could hold clues for researchers tackling increased resistance to dewormers in domestic horses.

A perfect storm

There's likely no single cause of death for most of Sable Island's horses, Jenkins said, but rather a "perfect storm" of challenges that causes their demise.

The population's tenacity to survive in such harsh conditions is remarkable, Jenkins said.

"I just couldn't get over the fact that they were eking out this existence on a sandbar in the middle of Atlantic," she said. "I've just total respect for how tough they are."