Virulence and nucleotide sequence analysis of marine viral haemorrhagic septicaemia virus following in vivo passage in rainbow trout Onchorhynchus mykiss

A marine isolate of viral haemorrhagic septicaemia virus (VHSV) (860/94) wa s passaged in triplicate through sequential batches of rainbow trout via an intra-peritoneal infection route, without amplification in tissue culture. Following 5 passages, the VHSV glycoprotein gene was amplified directly fr om fish tissue homogenates and the consensus sequence compared to that of t he original tissue culture isolate. Virus was also recovered directly from pools of kidney and spleen material after 5 passage events, and its virulen ce compared to that of unpassaged material by intra-peritoneal infection. F ollowing passage in rainbow trout, isolate 860/94 exhibited a higher virule nce for rainbow trout than unpassaged material. Sequence comparisons identi fied no difference in the consensus sequence of the glycoprotein gene follo wing in vivo passage. The mechanisms responsible for the observed increase in virulence of isolate 860/94 following passage in rainbow trout thus rema in unknown. The possibility that viral isolates may exhibit an increased vi rulence following passage in novel host species does, however, have importa nt implications with regard to the epidemiology of this important fish path ogen.