ISAv can be introduced by the shipment of infected salmon tissue and eggs; water containing the virus could also be infectious.  Since it emerged in virulent form from a native strain found in Norway, ISAv has been introduced by the salmon aquaculture industry into Ireland, Scotland, the Faroe Islands and Chile.  The detection of the European strain in eastern Canada is likely also due to the movement of infected fish or eggs, although it is possible that is was brought to eastern Canada by migrating Atlantic salmon. The primary method of ISAv transmission is horizontal (fish-to-fish) because infected fish shed the virus in mucous, urine, feces and ovarian fluid.  Sea lice have been implicated as a vector for ISAv, and the wounds they make on a salmon’s skin may allow the virus to infect new hosts.  It also appears to spread vertically (from an infected female to her eggs), although it is unclear if it is simply found on the outside of eggs or whether it manages to infect the young, unhatched offspring.