Abstract
ABSTRACTIceland is free of the major infectious diseases of horses. However, in 2010 an epidemic of respiratory disease of unknown cause spread through the country’s native horse population of 77,000. Microbiological investigations ruled out known viral agents but identified the opportunistic pathogen Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) in diseased animals. We sequenced the genomes of 257 isolates of S. zooepidemicus to differentiate epidemic from endemic strains. We found that although multiple endemic clones of S. zooepidemicus were present, one particular clone, sequence type 209 (ST209), was likely to have been responsible for the epidemic. Concurrent with the epidemic, ST209 was also recovered from a human case of septicemia, highlighting the pathogenic potential of this strain. Epidemiological investigation revealed that the incursion of this strain into one training yard during February 2010 provided a nidus for the infection of multiple horses that then transmitted the strain to farms throughout Iceland. This study represents the first time that whole-genome sequencing has been used to investigate an epidemic on a national scale to identify the likely causative agent and the link to an associated zoonotic infection. Our data highlight the importance of national biosecurity to protect vulnerable populations of animals and also demonstrate the potential impact of S. zooepidemicus transmission to other animals, including humans.
Highlights
Iceland is free of the major infectious diseases of horses
Our data fully support the epidemiological analysis of this epidemic and point to the incursion of a novel strain of S. zooepidemicus, sequence type 209 (ST209), into the Icelandic horse population
The endemic strains of S. zooepidemicus recovered from Icelandic horses in this study are likely to have caused clinical signs of disease
Summary
Iceland is free of the major infectious diseases of horses. in 2010 an epidemic of respiratory disease of unknown cause spread through the country’s native horse population of 77,000. This study represents the first time that whole-genome sequencing has been used to investigate an epidemic on a national scale to identify the likely causative agent and the link to an associated zoonotic infection. IMPORTANCE An epidemic of respiratory disease affected almost the entire native Icelandic horse population of 77,000 animals in 2010, resulting in a self-imposed ban on the export of horses and significant economic costs to associated industries. The resident horse population of Iceland is geographically isolated; it arose from animals introduced by settlers in the 9th and 10th centuries, with virtually no import of horses for the last 1,000 years This isolation of the population has meant that Icelandic horses have remained free from most common contagious diseases of Equidae, including equine influenza, equine rhinopneumonitis, equine viral arteritis, Rhodococcus equi infection, and strangles We demonstrate that this clone was responsible for zoonotic infections during the course of the epidemic and that this strain has become endemic within the Icelandic horse population
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