Increased mortalities in halibut (Hippoglossus hippoglossus), Atlantic cod (Gadus morhua), and Dover sole (Solea solea) were reported on two fish farms situated on the West coast of Scotland. Samples were taken from affected fish and processed for histopathology and virus isolation in cell culture. Microscopic examination of haematoxylin and eosin stained sections revealed the presence of varying dgrees of neuronal necrosis and vacuolation. This was most obvious in the brainstem, but was also present in spinal cord. The retina was less affected. Histopathological changes were similar in halibut, cod, and Dover sole, varying only in their extent and severity. Infected SSN-1 cells exhibited cytopathic effects characteristic of nodavirus that developed as areas of rounded refractile cells, which eventually spread throughout the cell sheet and resulted in destruction of the monolayer.
RNA was extracted from tissue samples and used in RT-PCR with primers corresponding to nucleotides 604-1030 of the capsid protein gene of striped jack nervous necrosis virus. Reverse transcription and thermal cycling were performed at 42°C for 30 min, 95°C for 5 min, followed by 32 cycles of 95° for 1 min, 55°C for 1 min and 72° for 1 min. Amplification products were analysed in 1% agarose gels, stained with ethidium bromide and visualised under UV illumination. An amplification product of Mr approximately 420bp was evident in all tissue samples tested from infected fish. Amplification products were processed using PCR clean up and their nucleotide sequence determined using dye terminator chemistry. The nucleotide sequences of the amplification products from halibut, Atlantic cod, and Dover sole were all strongly homologous (>98%) to the corresponding region of the published sequence of Atlantic halibut nervous necrosis virus (Grotmol et al., 2000). In phylogenetic comparison with the corresponding nucleotide sequences of viruses representing previously characterised nodavirus genotypes, all UK nodavirus isolates studies in this report clustered with Atlantic halibut nervous necrosis virus (Grotmol et al., 2000).
d To our knowledge, this is the first description of nodavirus infection in Atlantic cod and Dover sole, and the first report of nodavirus infection in the UK.