Unusual parasite burdens of horses on Canadian island revealed

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Sable Island horses. Photo: HiFlyChick via English Wikipedia CC BY 3.0

An island population of horses in Canada never exposed to deworming drugs has been found to have an unusual predominance of large strongyles in their feces.

This is in contrast to most domestic horse populations in North America, which are regularly exposed to anthelmintic drugs.

The research focused on the horses of Sable Island, off the coast of Nova Scotia, about 275km from Halifax.

The first horses were released on the 34 square kilometer island in the mid-1700s. There were additional introductions to improve breeding stock until 1935.

The horses, which today number about 500, have been legally protected from human interference since 1961, with the island becoming a national park reserve in 2013. The horses, the only terrestrial mammals on the island, are now classified as “naturalized wildlife”.

For their research, Emily Jenkins and her colleagues examined 35 horses on the island found dead in spring 2017 and spring 2018.

The bodies, which were all fresh, were cut open and examined for lungworms. The intestines were also investigated, and dung samples taken for analysis.

They also collected fresh dung deposited from horses in 2018, in spring and summer, within minutes of the horses defecating. In all, 281 fecal samples from live horses underwent analysis.

The study team, writing in the International Journal for Parasitology: Parasites and Wildlife, found unusual patterns in the parasite loads of the horses.

They found the horses harbored parasites no longer commonly found in domestic horses.

Analysis of the manure samples revealed cyathostomes, Strongylus equinus, S. edentatus, and S. vulgaris, with S. equinus (unusually) dominating in adult horses and cyathostomes dominating in horses less than three years of age.

Dictyocaulus-arnfieldi. © https://doi.org/10.1016/j.ijppaw.2020.02.002

The Sable Island were found to tolerate strongyle fecal egg counts much higher than domestic horses.

Average strongyle fecal egg counts were 666 eggs per gram in the dead horses, 689 eggs per gram in the live horses in spring, and 1105 eggs per gram in summer.

Domestic horses are usually treated at egg counts exceeding 200 per gram, the authors noted.

Unusually, adult Sable Island horses have evidence of ascarid (Parascaris spp.) and lungworm (Dictyocaulus arnfieldi) infections.

In spring, dead horses had five times higher odds of having showing evidence of ascarid infections than live horses, likely due to malnutrition and corresponding immunodeficiency.

The fecal prevalence and intensity of D. arnfieldi and Parascaris spp. were significantly higher in young horses, and in spring versus summer.

Eggs of the tapeworm Paranoplocephala mamillana were present in fecal samples from 28% of the live horses, and 42% of the dead horses, in spring.

Discussing their findings, the authors said the large migratory strongyles and lungworms found in the horses are now considered highly unusual in domestic horses in North America.

There was, they said, an unusual predominance of large strongyles over small strongyles.

“While cyathostomes (small strongyles) dominated in young horses (less than 3 years of age) on Sable Island, the large strongyle S. equinus dominated in larval cultures from adult horses, and all three large strongyles (S. equinus, S. edentatus, and S. vulgaris) were present in horses over a year of age.”

They said while the average strongyle fecal egg count in Sable Island horses (around 650 eggs per gram in spring, 1100 in summer) was considered high by domestic horse standards, such levels were not unusual compared to averages seen in other feral horse populations using the same method (McMaster).

Detection of the lungworm D. arnfieldi in the dead horses was unexpected, they said, as this parasite is generally thought to be reservoired in donkeys and “spills over” into horses sharing the same pasture, although horse-to-horse transmission has been documented.

“Donkeys have not been present on Sable Island for at least 60 years, making this finding enigmatic.”

Similar to lungworm, it was unexpected to find that some adult horses had ascarid infections. Such infections are rare in domestic horses over one year of age.

“It is likely,” they said, “that ascarid parasites in adult horses on Sable Island are a symptom of other problems, such as malnutrition and corresponding immunodeficiency.”

The authors said their findings emphasize the need to develop population-specific treatment guidelines for domestic horses based on proportions of high shedders rather than absolute fecal eggs counts, and that feral horses can tolerate higher levels of parasitism (based on egg counts) than generally accepted in managed horses.

“Sable Island horses harbor parasites no longer commonly found in domestic horses in Europe and North America, such as S. equinus and D. arnfeldi, and their cyathostome and ascarid populations represent a critical refugium that has never experienced selection pressure from anthelmintic use.

“Our findings support biosecurity measures currently in place to prevent the introduction of parasites and pathogens onto the island,” they said. These include boot and hand-washing, toileting restrictions, and quarantine of people experiencing gastrointestinal illness.

“Consideration should be given to implementing exit biosecurity measures as well, to prevent transmission from horses on Sable Island to companion horses.

“Both the horse and parasite populations on Sable Island are of conservation and scientific significance, in which future work will explore genetic markers of parasite resistance and the effects of parasitism and nutrition interactions on host survival and fitness.”

The full study team comprised Jenkins, Amber-Lynn Backwell, Jennifer Bellaw, Julie Colpitts, Alice Liboiron, David McRuer, Sarah Medill, Sarah Parker, Todd Shury, Martha Smith, Christina Tschritter, Brent Wagner, Jocelyn Poissante and Philip McLoughlin, variously affiliated with the University of Saskatchewan, the University of Kentucky, Parks Canada, and the University of Calgary.

Not playing by the rules: Unusual patterns in the epidemiology of parasites in a natural population of feral horses (Equus caballus) on Sable Island, Canada
Emily Jenkins et al. International Journal for Parasitology: Parasites and Wildlife
Volume 11, April 2020, Pages 183-190 https://doi.org/10.1016/j.ijppaw.2020.02.002

The study, published under a Creative Commons License, can be read here

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