Tuesday, 01 August 2017 10:53

Scientists Unravel Cause of Mysterious Horse Epidemic

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Published in mBiosphere

Sometimes, it pays to never throw things out. That is the lesson learned from researchers who identified the cause of a mysterious epidemic in horses in Iceland.

In 2010, a respiratory disease of unknown origin spread through almost the entire population of 77,000 native horses in Iceland. The disease involved coughing, nasal discharge, and high morbidity. Andrew Waller, PhD, head of bacteriology, Animal Health Trust, Suffolk, United Kingdom, was called in to help investigate the outbreak. "Iceland was so worried about what was causing it that they stopped exporting horses from Iceland to the rest of the world,” said Dr. Waller. “It had a big impact on the economy in Iceland, as they breed and sell an awful lot of horses each year.”

mBio: Genomic Dissection of an Icelandic Epidemic of Respiratory Disease in Horses and Associated Zoonotic Cases

The Icelandic horse population is geographically isolated, arising from animals introduced by settlers in the ninth and tenth centuries, with virtually no horses imported in the last thousand years. This isolation has meant that Icelandic horses have remained free from the most common contagious diseases of equine.

After performing microbiological investigations, the team of scientists at the University of Reykjavik ruled out known viral agents, but identified the gram-positive bacterium Streptococcus zooepidemicus from almost all of the nasal swabs taken from coughing horses and from the diseased tissues of occasional fatal cases. The bacteria is routinely isolated from healthy horses and widely considered to be commensal, but because it was so ubiquitous during the outbreak, the researchers began to think it could be the culprit.

"This was a fantastic team effort. I get in touch with vets around the world all the time trying to get isolates from cases of disease they have had. So many people throw these strains out and don’t store them. To their credit, the group in Iceland stored everything and that enabled us to go back and figure out what was going on," said Dr. Waller. "People thought it was a virus that was causing this, because it spread so quickly. It turns out it was probably a submerged water treadmill that was facilitating the rapid transmission of the bacteria."

Scientists at the Wellcome Trust Sanger Institute performed whole genome sequencing on 305 isolates of S. zooepidemicus, 257 from the epidemic including from 100 horses, two cats, one dog, and three people. They used ten archived Icelandic isolates of S. zooepidemicus from seven horses, two sheep and a dog to provide insight into the identity of historical isolates of S. zooepidemicus from Iceland, and 38 isolates, which represented the wider population diversity of the bacteria beyond Iceland.

The majority of S. zooepidemicus isolates recovered during the epidemic fell into four distinct clades. "ST209 popped out as likely to be responsible for the epidemic," said Dr. Waller. The epidemic ST209 strain was also recovered from a cat and the blood sample of an Icelandic woman who had suffered a miscarriage.

Network analysis of affected farms identified a single common training yard as a primary center of transmission and demonstrated how a novel strain can spread rapidly through a susceptible population devoid of sufficient cross-protective immunity, despite a background of concomitant colonization with endemic strains. The most likely route of transmission of the epidemic strain at this yard was a water treadmill, which horses used on a daily basis, did not contain disinfectant and was changed on a once- or twice-weekly basis. This provided ideal conditions for the transmission of S. zooepidemicus between visiting horses. Adding chlorine coupled with regular cleaning and disinfection of water treadmills may minimize or eliminate the transmission of S. zooepidemicus or other infectious agents via this route.

Previously, researchers have used whole genome sequencing to determine how germs spread through a hospital, but this is the first time the technology has been used to track the outbreak of a zoonotic disease. "This study enabled us to identify which strains were normally present in the Icelandic horse population and which was the epidemic strain that was causing the problem and that is very new," said Dr. Waller. "It was really great to be able to show that this particular strain had spread so quickly through the whole population, and as far as we are aware, that has not been done before using whole genome sequencing. Our study showed that you can use genomic sequencing to tell epidemic strains from endemic strains."

The research, published recently in the journal mBio, highlights the importance of national biosecurity to protect vulnerable populations of animals and the potential impact of S. zooepidemicus transmission to be transmitted to other animals, including humans.

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