Tom and his team at Washington State University have been studying the effects of this respiratory pathogen and its impact on wild sheep.
'The NBR sheep were documented to be free of the causative bacteria the year before their outbreak, so something happened that year.
We think based on all we know about the animals and the disease, the most likely scenario is that one of the young rams from NBR jumped the fence, went on a foray looking for breed-able sheep, got infected and brought it back to MBR and there it spread like wildfire to cause the outbreak.'
When European settlers moved out west, they had no idea the sheep they brought with them were carrying an old-world bacterium that would prove to be deadly in the new world.
Big horns, which are only found in North America, have no immunity to the diseases they carry, and when they come into contact with domestic sheep, the outcome can be fatal.