NEW YORK (GenomeWeb) – An international team has identified an epidemic strain of Streptococcus bacteria that swept through Iceland's genetically isolated horse population in 2010, causing respiratory disease and symptoms that included a dry, persistent cough and nasal discharge.
As they reported in mBio today, the researchers from Iceland, UK, and Denmark performed whole-genome sequencing on more than 250 S. zooepidemicus samples from Icelandic horses, using subtle sequence differences to distinguish forms of the bug that don't normally cause problems from those involved in the outbreak. Their comparative analysis led to a clone known as ST209 that also turned up in a human suffering from septicemia.
"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," senior author Andrew Waller, head of bacteriology at the UK's Animal Health Trust, said in a statement. "It was 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."
To get to the root of the respiratory disease, which spread rapidly through Iceland's 77,000 horses, the team first ruled out a viral culprit using PCR-based nasal swab sample testing and testing for telltale blood antibodies in infected animals. When that search reached a dead end, the group turned its attention to possible bacterial players, looking for species that showed up reliably in horses with the condition.
"Although the speed of transmission suggested that a viral pathogen was responsible, only the presence of the opportunistic pathogen Streptococcus zooepidemicus was consistent with the observed clinical symptoms," the authors wrote.
Using Illumina GAII, HiSeq, or MiSeq instruments, the team did pooled whole-genome sequencing on 257 samples collected at 31 sites in Iceland in 2010, including nasal swabs from 100 horses, two cats, a dog, and three people.
When they compared the S. zooepidemicus genomes to one another and to dozens of S. zooepidemicus isolates from Iceland and beyond that had been sequenced previously, the researchers identified isolates from four clades and four multilocus sequence types (MLSTs) in the infected horses.
Their subsequent experiments indicated that at least one of these, ST209, produced respiratory infection systems in three healthy horses within 10 days of their initial exposure. That strain appeared to have muted genetic diversity compared to some of the other S. zooepidemicus strains found in horses, the team reported, and appears to share a common ancestor with S. zooepidemicus strains going back to the summer of 2008.
New horse imports to Iceland have not been permitted for more than a century, the authors noted. They speculated that the ST209 strain may have moved from one horse to another within the country on contaminated riding equipment, though they noted that ST209's apparent ability to infect humans may have contributed to the epidemic as well.