In vitro viral haemorrhagic septicaemia virus replication in excised fins of rainbow trout: correlation with resistance to waterborne challenge and genetic variation.

In vitro viral haemorrhagic septicaemia virus replication in excised fin tissue (VREFT) was investigated as a possible criterion to predict the resistance of groups or individuals to viral haemorrhagic septicaemia virus (VHSV) in rainbow trout. Adipose and rayed fins were compared for VREFT response, and a statistically significant correlation was found. Correlation between VREFT and survival after waterborne viral challenge was estimated on a set of 27 groups of trout, and was highly significant (R = 0.72). A further experiment with fish individually tagged and challenged some time after fin clipping for determination of VREFT confirmed that the mean value of resistant (surviving) fish was significantly lower than the mean value of susceptible (dead) ones, but there was a wide variation within each of these groups. In particular, a large proportion of fish expected to be resistant based on VREFT values died all the same. Using clones, we showed that the correlation between VREFT and survival was dramatically high (R = 0.96). Genetic analyses of the data from the different groups available in the experiment consistently indicated a large amount of genetic determination of VREFT, an encouraging result for selection purposes. Though these results were obtained in experimentally controlled conditions not identical to those in the field, they shed new light on the analysis of defence mechanisms against the virus and on the possibility of performing indirect selection for resistance, using VREFT as the secondary character.