Solved: Scientists identify mystery bug that infected Iceland’s horses

Icelandic Horses in Iceland.
Icelandic Horses in a field in Iceland. Horses have not been imported into the country since 1882. © Nick Hodgson

Researchers have solved a seven-year mystery, identifying the cause of a disease outbreak that affected Iceland’s horses.

The mystery bug that infected horses in Iceland has been identified.
The mystery bug that infected horses in Iceland has been identified. © Nick Hodgson

Iceland suffered a nationwide epidemic of a mysterious respiratory disease in 2010. It infected not only Iceland’s native horse population, but also dogs, cats and humans.

Now, researchers with the Animal Health Trust, in partnership with British and Icelandic research and veterinary institutions, have pinpointed the cause, identifying a particular strain of a common bacteria.

Researchers in recent years have looked at a wide range of disease-causing bugs that could have been the culprit, with the only potential suspect being the bacteria Streptococcus zooepidemicus. It was recovered from coughing horses and rare fatal cases of infection.

However, this bug is also often found in healthy horses, which adds to the mystery.

The Animal Health Trust and Britain’s Wellcome Trust Sanger Institute were brought in to investigate bacteria recovered from horses, humans, dogs and cats throughout Iceland.

“To identify the culprit, we sequenced the DNA from 257 samples of bacteria in diseased animals,” said the institute’s Dr Simon Harris.

“This showed that one specific strain of Streptococcus zooepidemicus, called ST209, was likely to be the guilty bacteria, and we also found this strain in a human case of blood poisoning.

“This study,” he said, “highlights how DNA sequencing can be used to identify the cause of an epidemic infection.”

The disease had spread rapidly in Iceland’s population of 77,000 horses, causing significant economic cost to the country.

Streptococcus zooepidemicus
Streptococcus zooepidemicus.
An Icelandic pony showing a nasal discharge.
An Icelandic pony showing a nasal discharge.

Iceland is free of all major equine infectious diseases thanks to the imposition of a ban on importing horses into the country in 1882.

Enjoying a relatively disease-free environment for the last 1000 years, Icelandic horses are particularly susceptible to any new bacteria or virus that crosses the border, and so strict biosecurity regulations are in place to help protect them.

Unfortunately, the bug in question did manage to enter the country and in 2010 it wasted no time in taking advantage of the vulnerable horse population, spreading across the country within weeks.

Not only did the disease affect horses, but dogs, cats and humans also fell ill. The disease was found in an Icelandic woman, who contracted septicaemia and suffered a miscarriage.

Dr Sigríður Björnsdóttir of the MAST Icelandic Food and Veterinary Authority, used information from owners and vets to build an epidemiological network.

A blue-eyed Icelandic horse. Photo: Anjali Kiggal CC BY-SA 4.0 via Wikimedia Commons
A blue-eyed Icelandic horse. Photo: Anjali Kiggal CC BY-SA 4.0 via Wikimedia Commons

This enabled her to retrace the journey of the bug back to an equine rehabilitation centre where horses exercised in a communal water treadmill.

The water is thought to have provided the perfect breeding ground for the disease; allowing the bug to be easily transmitted to the next unfortunate animal if water was splashed up and ingested.

Map disease traced back to water treadmill -red; primary cases = light blue; secondary cases = dark blue.
Map disease traced back to the water treadmill (red); primary cases = light blue; secondary cases = dark blue.

Horses would complete their rehabilitation and return home, taking the infection with them.

The bacteria that infected Iceland's equine population may have arrived in the country on equipment or even a person, experts say.
The bacteria that infected Iceland’s equine population may have arrived in the country on equipment or even a person, experts say. © Nick Hodgson

This particular strain of S. zooepidemicus has also been recovered from a coughing horse in Sweden and an abdominal abscess in a Finnish horse trainer.

Dr Andrew Waller, who is head of bacteriology at the Animal Health Trust, says there are a couple of theories as to how the disease entered Iceland in the first place.

“The offending pathogen is able to survive outside a horse for a week or so, which means the import of contaminated equipment or clothing could have transmitted the disease to horses in Iceland. However, this particular strain may have even travelled to Iceland in a person, before transferring back to a horse and sweeping nationwide.

“This investigation not only highlights the importance of biosecurity in protecting and preventing spread amongst animals, but to humans too,” he says.

“This particular strain was able to cross species boundaries, jumping from a coughing horse to a person.

“We are pleased to have helped uncover the identity of this mystery disease. We hope that raising awareness of the cause of this epidemic will encourage people to improve disease control precautions worldwide.”

Icelandic ponies graze in Iceland. Photo: Thomas Quine (originally posted to Flickr as Icelandic ponies) CC BY-SA 2.0 via Wikimedia Commons
Icelandic ponies graze in Iceland. Photo: Thomas Quine CC BY-SA 2.0 via Wikimedia Commons

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