The Many Isolates of VHSV In Rainbow Trout

A review of: Campbell, S., Collet, B., Einer-Jensen, K., Secombes, C.J., Snow, M., (2009), Identifying potential virulence determinants in viral haemorrhagic septicaemia virus (VHSV) for rainbow trout, Diseases of Aquatic Organisms, 86, 205-212

Viral haemorrhagic septicaemia virus (VHSV) is virus responsible for the disease viral haemorrhagic septicaemia, a disease responsible for a large loss of the rainbow trout Oncorhynchus mykiss. So far, four main genotypes have been identified which allows for genomic regions to be made for analysis. Until recently the rainbow trout that were infected by VHSV were isolated to the geographic location of the Genotype I, but due to the sourcing of food fed to farmed fish, there are many contaminations that have occurred leading to infections of other genomic regions.

The aim of the test is to determine the mortality of the four different genotypes and to analyse the sequencing data. This was done by isolating 24 fish per study group then infected with the different strains of virus intraperitoneally. A control was set up containing an inoculum. RNA was then extracted using a viral RNA kit, cDNA was then synthesised from the viral RNA using a polymerase kit. Polymerase chain reactions were then conducted on each sample to give a consensus nucleotide sequence per isolate.

Results of the mortality test show that the SE-SVA-1033 (GIb) isolates had a mortality of 75% with the majority of fish dying at between 6 and 10 days after exposure. The other three strains resulted in mortalities of 34.6% and under. These deaths also occurred at the same point as those exposed to the SE-SVA-1033 strain but the mortality was reduced. There was no mortality in the control group allowing for the test to be validated. Results of a Chi-squared analysis revealed a statistically significant difference in the mortality rates between treatments. A complete coding region for VHSV strains SE-SVA-1033 and DK-4p37 were also determined.

The results of the infection trial confirm other studies carried out on the mortality of the trout when immersion tested with the SE-SVA-1033 isolate. Observations were made when looking into the lower mortality rate isolates when inoculated, whereby the fish had significant lower mortality rates which indicates that these isolates are less  virulent. 

Further studies can be based on the findings of this test, mainly further looking into the amino acid substitutions that were identified across the genome. This change clearly has an effect upon the virulence and mortality of the virus to the rainbow trout. The exact significance of these mutations can be studied in order to view possible causes to the virulence of the VHVS isolates and as more occur, these can be cross referenced with the genomic data found.