Study of virulence markers in Viral Haemorrhagic Septicaemia Virus (VHSV)

Viral haemorrhagic septicaemia virus (VHSV) is the causative agent of a very serious and threatening disease affecting rainbow trout (Oncorhynchus mykiss) aquaculture. It causes a systemic disease with various mortality rates in rainbow trout. VHSV has been isolated from several species of fish, including marine species which can be infected and carry the virus without clinical symptoms.

It has been previously found that VHSV isolated from marine species causes low to no mortality in rainbow trout, while VHSV isolated from rainbow trout has similar results in challenges with marine species. Many earlier studies have tried to identify genetic molecular markers of VHSV to rainbow trout with little success.

The development of new technologies for DNA sequencing such as Next Generation Sequencing (NGS) has recently supported the full genome sequencing of a large number of VHSV field isolates while reverse genetics technique has allowed the manipulation of the entire genome of VHSV. In this project, we assessed the in vivo virulence by immersion challenges in rainbow trout of several VHSV isolates obtained from the DTU Aqua VHSV repository.

These isolates were also subjected to full genome sequencing by NGS and analyzed together with other mixed virulence isolates generating the largest VHSV full genome sequence dataset up to date. Based on earlier studies and on these results we hypothesized that virulence markers of VHSV to rainbow trout were located in the nucleoprotein gene.

By the establishment of a new reverse genetics system based on the low virulent VHSV isolate SE SVA 1033-9C as a backbone, we were able to rescue three mutant recombinant VHSVs (rVHSVs) with specific amino acid residue changes in the nucleoprotein and two chimeric rVHSVs with parts containing the nucleoprotein gene exchanged from the high virulent VHSV DK-3592B to the low virulent SE SVA 1033-9C.

The virulence of these rVHSVs was evaluated by in vivo challenges in rainbow trout by intraperitoneal injection (IP) and by immersion. Results indicated that both strategies (amino acid residue changes and chimeras in the low virulent VHSV backbone) caused gain-of-function in IP injected rainbow trout but not in immersion infected.

We show evidence that the nucleoprotein is implicated in VHSV virulence to rainbow trout however, the results obtained also show that these alterations are probably not acting alone and therefore further investigation is necessary to look into the role of other genes and intergenic regions that could be associated with virulence to rainbow trout.