Genetic stability of the VHSV consensus sequence of G-gene in diagnostic samples from an acute outbreak

The negative stranded RNA virus viral haemorrhagic septicaemia virus (VHSV) is an important disease-causing agent in aquacultured fish and internationally harmonized diagnostic procedures are continuously under development. The present study concerns the suitability of genotyping by sequencing of RT-PCR products for epidemiological analysis.

Focus was put on a specific case story involving an acute outbreak of VHS in a Danish rainbow trout farm which otherwise had been free of VHSV during the previous 5 years. Tissue materials from individual fish were collected during routine inspection and the initial diagnosis was based on isolation of the virus by cell cultivation and subsequent identification by ELISA.

Additional tissue samples were collected 25 days after the initial sampling. RT-PCR amplification and sequencing of the entire glycoprotein-gene (1524 nt) was performed on RNA purified from collected tissue material as well as from inoculated cell culture. No nucleotide substitutions where identified when aligning the obtained sequence data for the two sample types.

The presented data indicate that the overall consensus sequence of the virus outbreak was stable during the survey, and that initial passage of the virus on BF-2 cells did not result in changes within the G-gene at a detectable level. The results suggest that genotyping of VHSV isolates based on RT-PCR products amplified from infected primary tissues material is a reliable tool for epidemiological studies.