Phil, colder climates initiate cluster and clustering would promote Nosema. I am not sure if cold directly affects or not. High moisture would enhance as well as confinement. Buckfast apirary, a cold climate by USA standards was wiped out due to trachea mite, not Nosema. The original black Buckfast bee was eliminated, completely wipe out. Only the itialians survived the outbreak. Brother Adam searched the world over looking for the original survivors, he never found any.
Blessings
Had to edit trachea mite, not Nosema, at Buckfast..