High prevalence of viral haemorrhagic septicaemia virus (VHSV) in Norwegian spring-spawning herring

Viral haemorrhagic septicaemia (VHS) is a viral disease known to cause high mortal- ity in many teleost species, both in wild and farmed populations. The effects of this virus infection on the population dynamics of Atlantic herring Clupea harengus are still unknown; however, high mortalities have been reported in infected Pacific herring C. pallasii. We investigated Atlantic her- ring from the Norwegian spring-spawning (NSS) stock during the spawning season and found a much higher prevalence of VHS virus (VHSV) than had been identified in previous surveillance studies. Positive results were found in 19% of the samples analysed using viral cell culture on pooled brain, spleen and kidney tissue samples from 5 fish. Real-time RT-PCR performed on the same pooled samples revealed 33% positive samples, while analysis of individual organs resulted in a prevalence of 12�13%. Gills, which were analysed only by real-time RT-PCR, displayed a prevalence of 69%. It is not clear whether the virus detected in the gills reflects an infection or a passive carrier status. Sequencing of positive samples from gills and internal organs revealed high identity levels with VHSV of genotype Ib detected previously in Norwegian herring. The high prevalence identified in our study indicates a need for further research into the significance of VHSV infection on the herring population.