Rainbow Trout Orthomyxovirus

We know that orthomyxoviruses mutate and evolve quite regularly. Most of these viruses infect mammals, insects and birds, but only rarely the infect fishes.

Infectious Salmon Anemia Virus was once the only orthomyxovirus that had its eyes on fish and we wrote that it was 'currently the only known species in the newly formed genus Isaviruses within the family Orthomyxoviridae'. Tilapia Lake Virus (TiLV) was recently discovered in Israel and Ecuador.
A novel virus, tentatively called Rainbow Trout Orthomyxovirus (RbtOV), was isolated in 1997 and again in 2000 from commercially-reared rainbow trout (Oncorhynchus mykiss) in Idaho (USA)[1]. In the laboratory, the virus produced a diffuse cytopathic effect. However, juvenile rainbow trout exposed to cell culture-grown virus showed no mortality or gross pathology.

Another virus isolated in 2014 from steelhead trout (also Oncorhynchus mykiss) in Wisconsin (USA) and designated Steelhead Trout Orthomyxovirus (SttOV) was found to be so similar to Rainbow Trout Orthomyxovirus (RbtOV), that it is suggested that these new viruses are isolates of the same virus species and may be more widespread than currently realized. Steelhead trout and rainbow trout are the same genus and species of trout, simply the steelhead are the form that migrate out to saltwater for the adult stage of the fish, while the rainbow trout remain landlocked.

The new isolates had the same genome segment order and the closest pairwise amino acid sequence identities of 16-42% with Infectious Salmon Anemia Virus. However, further analysis showed that, while Rainbow Trout Orthomyxovirus and Steelhead Trout Orthomyxovirus clustered most closely with Infectious Salmon Anemia Virus, they diverged sufficiently to merit consideration as representatives of a novel genus.

While all this may be seem confusing, William Batts, the lead author, explained his choice of naming via personal communication: The Rainbow Trout Orthomyxovirus is the virus name, while the isolates are: Rainbow Trout Orthomyxovirus-1, Rainbow Trout Orthomyxovirus-2, Steelhead Trout Orthomyxovirus-1, etc. If another species of fish is found to have the same virus (close enough by sequence identity), the new virus isolates can be called a Bluegill Orthomyxovirus-1 or Largemouth Bass orthomyxovirus-1. The virus name is different from the isolate name. 

[1] Batts et al: Molecular characterization of a novel orthomyxovirus from rainbow and steelhead trout (Oncorhynchus mykiss) in Virus Research – 2017

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