The evolution of ISA

ISA virus relationship tree
If you can interpret this, we would love to have you blog for us!

We won’t pretend to understand all of this paper, it’s a doozy with lots of technical talk that would require expertise in genetics and cutting-edge molecular tools to fully interpret (not the field of today’s authors).

But the conclusions of “Evolution of infectious salmon anaemia virus (ISA virus)” are sure interesting, and come from some of the top scientists in the world studying ISA.

It appears to suggest that ISA virus is exclusively a biosecurity problem for salmon farmers, and nothing to do with wild fish.

The scientists studied 100 different ISA viruses to see how they evolve and mutate, and using that information turned back the “genetic clock” to see when the viruses would have forked in the past to form the distinct strains we know today. Here’s some key points.

  • The study looked at ISA viruses collected from 1987-2011.
  • There are two major genotypes of ISA virus: North American and European.
  • There is also a subtype of the European genotype in North America.
  • This paper only looked at viruses from the European genotype.
  • The viruses were collected from Norway (89), Scotland (2), Faroe Islands (1), North America (2, one from New Brunswick and one from USA east coast), and Chile (5).
  • The study showed that reassortment commonly happens in ISA viruses in farmed Atlantic salmon. If a fish is infected with several different strains of the ISA virus, it’s common for the viruses to swap genetic material.
  • It’s possible for such a process to allow an avirulent (low ability to infect) strain of ISA virus to evolve into a pathogenic strain.
  • However, the study says, the movement and risks from ISA appear to be contained entirely within farmed salmon:

“The natural reservoirs for ISA viruses (wild Atlantic salmon and trout) seem to be of little or no importance for the spreading and emergence of ISA viruses in farmed Atlantic salmon, i.e. no new ISA viruses have been detected in Norway during the last 20 years.” 

The virus can evolve from a harmless strain to a dangerous strain, but because there are very few hosts, confined in farms, if the virus moves it’s most likely because of a gap in farming biosecurity practices.

“Transmission of the avirulent strains of ISA viruses may occur through vertical (transgenerational) transmission followed by horizontal short- and long-distance movement of embryos and infected smolt from freshwater sites. Virulent strains probably emerge in farmed Atlantic salmon after mutations… These strains can be transmitted horizontally in connection with movement of smolt and possibility also between farms, in connection with human activity.”

This is why it is important for farmers in B.C. to maintain high levels of biosecurity, keep testing for all known ISA viruses regularly and cease egg imports. They haven’t happened for three years, but in the recent past, farmers have imported eggs from Iceland, an ISA-free zone. These eggs were only used to supplement production (they were not used in broodstock programs to enhance stock genetics).

However, in our opinion, even this is too risky and should cease altogether. Salmon farmers need to show people that their farming practices are safe and make every effort to avoid importing ISA to Canada’s west coast, and stopping all egg imports is a great way to do it.

Since this genetic research shows it’s pretty unlikely there is any sort of “sleeper” ISA virus reservoir out there in wild fish populations, the best way to keep it out of BC and to assure the public farmers are doing the right thing is to maintain vigiliance, and commit to a halt to any egg imports which could be seen as risky.



5 thoughts on “The evolution of ISA”

  1. If you don’t “understand all of this paper, [that contains] lots of technical talk that would require expertise in genetics and cutting-edge molecular tools to fully interpret” it is wise to stay out of the debate and offer your conclusion that it “appears to suggest that ISA virus is exclusively a biosecurity problem for salmon farmers, and nothing to do with wild fish.”

  2. Sports fisherman imported millions of Atlantic salmon eggs to the North Pacific last century. Could this have been the source of a “subtype of the European genotype in North America”?

    1. The subtype is actually only identified in eastern North America. But you’re right, it’s quite possible virus came along in those early imports, which went to eastern North America as well. And there were a lot of fish egg exports to Europe during that time. It’s even possible that the virus is originally from North America, was exported to Europe where it evolved, then transported back in later transfers. Hopefully more genetic studies will be able to unravel the mystery.

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