Two isolates of sea bass, Dicentrarchus labrax L., nervous necrosis virus with distinct genomes

Diagnosis by reverse-transcriptase polymerase chain reaction (RT-PCR) amplification using previously published primers specific for the capsid protein gene of fish nodaviruses detected an isolate of sea bass, Dicentrarchus labrax L., nervous necrosis (SBNNV) from the French Mediterranean coast, but did not detect an isolate from the Atlantic coast. The capsid protein coding sequence of the nodavirus of both isolates was cloned. Sequence analysis revealed that the Mediterranean isolate was identical to the sequence previously published for SBNNV, while the Atlantic isolate is related, but carried numerous substitutions, in particular in the region used for RT-PCR diagnosis. A new primer set was tested which detected the viral genome of the Atlantic isolate. Using the new primer set, PCR amplification of a range of 10-fold dilutions of a plasmid containing the capsid protein gene of the Atlantic isolate showed that the limit of detection of the assay was between 10 and 100 copies of plasmid.