A novel totivirus and piscine reovirus (PRV) in Atlantic salmon (Salmo salar) with cardiomyopathy syndrome (CMS)

Abstract

BackgroundCardiomyopathy syndrome (CMS) is a severe disease affecting large farmed Atlantic salmon. Mortality often appears without prior clinical signs, typically shortly prior to slaughter. We recently reported the finding and the complete genomic sequence of a novel piscine reovirus (PRV), which is associated with another cardiac disease in Atlantic salmon; heart and skeletal muscle inflammation (HSMI). In the present work we have studied whether PRV or other infectious agents may be involved in the etiology of CMS.ResultsUsing high throughput sequencing on heart samples from natural outbreaks of CMS and from fish experimentally challenged with material from fish diagnosed with CMS a high number of sequence reads identical to the PRV genome were identified. In addition, a sequence contig from a novel totivirus could also be constructed. Using RT-qPCR, levels of PRV in tissue samples were quantified and the totivirus was detected in all samples tested from CMS fish but not in controls. In situ hybridization supported this pattern indicating a possible association between CMS and the novel piscine totivirus.ConclusionsAlthough causality for CMS in Atlantic salmon could not be proven for either of the two viruses, our results are compatible with a hypothesis where, in the experimental challenge studied, PRV behaves as an opportunist whereas the totivirus might be more directly linked with the development of CMS.

An Overview of the Immunological Defenses in Fish Skin

“…NGS is revolutionizing the approaches taken to study both transcriptomics and genomics, due to the massive amount of sequence information that can be generated in a relatively short length of time [363]. There are some reports describing NGS on salmonid fish, both wild (Salvelinus namaycush and Coregonus clupeaformis) [364,365] and farmed (S. salar and Oncorhynchus mykiss) [366,367].…”

Piscine myocarditis virus (PMCV) in wild Atlantic salmon Salmo salar

“…This was based on the presence of eosinophilic inclusions in nuclei of myocardial fibres in the vicinity of degenerated areas. Experimental transmission studies have supported this hypothesis (Bruno & Noguera 2009, Fritsvold et al 2009, and in 2010 a doublestranded RNA virus tentatively of the Totiviridae family was described as the causative agent of CMS (Løvoll et al 2010, Haugland et al 2011.…”

“…Studies of farmed salmon indicate that viral RNA from PMCV can be present several months before a CMS outbreak (Wiik-Nielsen et al 2012) but also that presence of PMCV is closely linked to development of disease (Løvoll et al 2010, Haugland et al 2011. CMS is a severe condition affecting the myocardium and one would expect wild fish with CMS to either die or to have reduced cardiovascular capacity, thus being less capable of returning to rivers where the sampling was conducted.…”

“…1) The tissue distribution of PMCV in infected salmon is not completely mapped. Thus, other organs than head-kidney could be more suitable for screening (Løvoll et al 2010, Wiik-Nielsen et al 2012 and give different results. In addition, most of the sampled fish were returning spawners, and the virus may be more prevalent in other life history stages of wild salmon.…”

“…RNA was isolated from head kidney tissue and analysed by use of a PMCV-specific real-time RT-PCR assay (Løvoll et al 2010). RNA was extracted as described in Garseth et al (2012 To perform the magnetic based separation, a KingFisher (Labsystems Oy) was used.…”