Genomic dissection of an Icelandic epidemic of equine respiratory disease

The native horse population of Iceland has remained free of major infectious diseases. Between May and July 2010 an epidemic of respiratory disease swept through the population. Initial microbiological investigations ruled out known equine viral agents as the cause of the infections, but identified the opportunistic pathogen Streptococcus zooepidemicus as being frequently isolated from diseased animals. This diverse bacterial species has a broad host range and is usually regarded as a commensal of horses. By genome sequencing S. zooepidemicus recovered from horses during the epidemic we show that although multiple clones of S. zooepidemicus were present in the population, one particular clone, ST209, was responsible for the epidemic. Concurrent with the epidemic, ST209 caused zoonotic infections, highlighting the pathogenic potential of this clone. Phylogenetic analysis suggests that the original ST209 strain entered Iceland in late 2008 or early 2009. Epidemiological investigation revealed that the incursion of this strain into a training yard that utilized a submerged treadmill between the 5th and 19th of February 2010 was a critical trigger for the ensuing epidemic of disease, provided a nidus for the infection of multiple horses, and subsequent distribution of these animals to multiple sites in Iceland.