Potential origins of fish toti-like viruses in invertebrates

The virus family Totiviridae had originally been considered to include only viruses which infected fungal and protist hosts, but since 2006 a growing number of viruses found in invertebrates and fish have been shown to cluster phylogenetically within this family. These Totiviridae-like, or toti-like, viruses do not appear to belong within any existing genera of Totiviridae, and whilst a number of new genus names have been suggested, none has yet been universally accepted. Within this growing number of toti-like viruses from animal hosts, there exists emerging viral threats particularly to aquaculture, namely Infectious myonecrosis virus in whiteleg shrimp and Piscine myocarditis virus (PMCV) in Atlantic salmon (Salmo salar). PMCV in particular continues to be an issue in salmon aquaculture as a number of questions remain unanswered about how the virus is transmitted and the route of entry into host fish. Using a phylogenetic approach, this study shows how PMCV and the other fish toti-like viruses probably have deeper origins in an arthropod host. Based on this, it is hypothesized that sea lice could be acting as a vector for PMCV, as seen with other RNA viruses in Atlantic salmon aquaculture and in the toti-like Cucurbit yellows-associated virus which is spread by the greenhouse whitefly Trialeurodes vaporariorum.