History
There are several distinct strains of the virus, some are pathogenic and some are not. The most common are a European strain and a North American strain.
Genetic research into the ISA virus shows that the European and North American strains of the virus diverged from each other sometime around 1900. This research points out that starting in 1879 when rainbow trout were first brought to Europe from North America, there were many transfers of fish across the Atlantic ocean which may have carried the ISA virus. Some species were introduced to Europe from North America, and some species were introduced to North America from Europe. Given that the virus did not evolve into two separate strains until around 1900, and given how many transfers of fish there were, according to this research, "At present it is therefore not possible to suggest a geographical origin of the ISA virus."
Another study suggests that this virus was introduced into Norway between 1932 and 1959 and that the original strain was the European subtype found in North America. The strains found in Chile were transmitted from Norway between 1995 and 2007.
Read more about this topic: Infectious Salmon Anemia Virus
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