Unique snapshot looks into a parasite paradise

A study of the wild horses of Nova Scotia’s Sable Island shows that worms may not be as fearful as once thought

A study of small, feral Sable Island horses is providing valuable information for domestic horse populations, said a veterinarian.

As a parasitologist in the Western College of Veterinary Medicine at the University of Saskatchewan, Emily Jenkins said she jumped at the chance in 2017 to travel to the small island, situated 300 kilometres southeast of Halifax, to study the herd.

She said the wild horse population, which numbers about 500, has been untouched by human influence and left to their own survival in the harsh and desolate environment for at least half a century.

In 2011, Sable Island officially became a national park reserve and as such visitor access is tightly controlled by Parks Canada. The unmanaged horses are protected from interference by humans.

“As hard as it is from a welfare perspective to see these horses, it is a really amazing opportunity for science and also for us to learn about what the natural condition of horses would have been. This is the natural condition. They’re not supplementary fed, they’re not dewormed, their feet aren’t trimmed and they’re making their own way in the world,” she said.

“It really is a parasitologist’s dream come true because there’s no deworming. It’s high densities. It’s high amounts of pasture contamination. And so there’s a lot to learn from this population,” she said during the Saskatchewan Equine Expo held in Saskatoon Feb. 17.

“They could have diseases that are no longer present in our companion horses and indeed there are some really unique parasites on this island.”

Jenkins, along with U of S PhD biology student Julie Colpitts, have travelled to the island several times and spoke about the ecology and health of the unique herd during the equine event.

Jenkins said hard winters increase the number of mortalities on Sable Island, which allowed her to conduct in-field necropsies on dead horses.

Based on the bone structure, it was determined that starvation, emaciation and hypothermia were the primary causes of death, particularly for yearlings.

“One thing that we use more in wildlife than in veterinary medicine in general is you can look at the amount of fat left in the bone marrow and that’s the last fat reservoir to go in a starving animals. So they’ll start with abdominal fat, they’ll move to subcutaneous fat. By the time they’re using their marrow fat, they’re on their last stores. So however much is left, is a good measure of body condition,” she said.

“Most horses should have fat in their marrows. This is by far the more common condition that we saw on the Sable horses. They had absolutely no fat left in their bone marrow, something like six percent fat. So this is what we call serious atrophy of the fat in the bone marrow indicating emaciation.”

Collected samples also revealed that selenium deficiency is another common mineral issue.

“There was not one horse that was considered within normal range. So that’s really common in the East Coast. It’s common in many places, but it’s a very known problem. People definitely supplement the horses and selenium at the East Coast,” she said.

Hoof care is another area that a number of horses have not managed to evolve for themselves and many front feet have curly toes.

“These horses aren’t selected out of the population. They would be if they were feral horses running around Alberta because coyotes, wolves, etc., would make short work of these horses that can’t run away. There are no predators on this island so these horses aren’t selected out of the population. And we think there is a strong genetic component to this hoof overgrowth because we even saw yearlings that already had their toes ready to come up, like little elf shoes,” she said.

Bad teeth caused by heavy sand content in the forage diet can also shorten the life of Sable horses.

“When we tried to age out these horses based on photographs of their dentition, they were aging out like 10 years older than we would normally see with companion horses, I think simply because the wear on those teeth is so dramatic with all the sand in their diet,” she said.

In fact, several post-mortems revealed sand blocking the entrance between the small and large intestines, which led to fatal colic.

The first necropsy Jenkins did was on a yearling they found after a severe snowstorm in 2017.

“It was honestly a gold mine. It had every single parasite that I’ve ever learned about that a horse could have in this one little yearling,” she said.

“In terms of parasites, it’s kind of a paradise for parasites.”

She said every horse on Sable Island they studied, particularly horses less then two years old, had parasites: mostly large and small strongyles, as well as dwarf tapeworms, and the large equine roundworm.

This foal was the first to be born on Sable Island in 2018. | Emily Jenkins photo

At least two species of bot larvae were discovered in every horse, which lines the stomach, but is fairly non-pathogenic.

During last year’s visit to the island, the researchers looked specifically for respiratory and reproductive pathogens in the horses.

Not surprisingly, they found that every horse has equine herpes virus, which is a common pathogen that can be associated with abortion, respiratory and neurological signs.

“It doesn’t seem like this is causing a lot of problems for the horses despite the fact that almost all of them are exposed,” she said.

Jenkins said strangles also exists among Sable Island horses but it does not appear to have ill effects on them.

“It really doesn’t seem to be causing a problem in this pasture population. It’s a big issue in stable horses, but doesn’t seem to be a huge pathogen on Sable,” she said.

The existence of lungworm throughout the population surprised Jenkins because she said it’s not supposed to survive in horses in the absence of donkeys.

“Donkeys are the main reservoir for this. That it’s doing so well on Sable Island is a little bit unusual and interesting,” she said.

The average strongyle egg count found in horse feces was 1,500 per gram, which she said is three times what is considered a high egg count in domestic horses.

Despite all the parasites she found, Jenkins said Sable horses are not dying from them.

“This gives us a unique snapshot. These parasites have never seen a dewormer. We can look at the genetics of these parasites and compare them to our increasingly resistant domestic horse populations to figure out what makes parasites resistant and what makes it susceptible. It’s also got an amazing reservoir of natural resistance because these horses are surviving despite incredible egg counts, really off the charts,” she said.

As a result, the parasitologist said current treatment guidelines for domestic horses should probably be reconfigured to look at natural populations of horses and determine what is the normal range.

“It tells us that maybe we can be a little less aggressive with our deworming policy and maybe rewrite the guidelines a little bit higher in terms of egg counts that we consider treating,” she said.

“The general wisdom with anti-parasite treatments has been to hit them with everything you’ve got. Unfortunately that’s created really a perfect storm for equine parasite resistance. We’re almost out of options for some of our pathogens,” she said.

“I think people were terrified by the large strongyles and so we motivated most of our deworming against them and we’ve pretty much eradicated them from our domestic horses,” she said.

“The Sable population is a reminder that maybe we don’t have to be quite so terrified of the large strongyles. Although every single horse had them, I don’t think they were killing the horses.”

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