In vitro assay to select rainbow trout with variable resistance/susceptibility to viral haemorrhagic septicaemia virus.

The merit of a candidate criterion of resistance to viral haemorrhagic septicaemia virus (VHSV) was tested with the view of producing experimental trout progeny with a predictable level of resistance. The criterion, the measure of in vitro viral replication in excised fin tissue (VREFT) was previously developed. Three experiments were performed, using both ordinary and homozygous doubled-haploid breeders. A set of 48 progeny was tested. Breeders were individually scored for repeated measures of VREFT, and the progeny were tested against VHSV (strain 07-71, serotype 1) through a waterborne challenge (5 x 10(4) pfu ml(-1) during 2 h). Analysis of repeated measures of VREFT revealed the risk of identifying 'false' resistant individuals. The highest value should be considered the most predictive of the resistance status. Survival of progeny ranged from 0 to 100% according to the group and the experiment. The survival was correlated to the mean VREFT value of the breeders in Expts 1 and 2 (R = 0.96 and 0.61 respectively), but not in Expt 3 (R = 0.36, ns) where all tested progeny were highly susceptible. Results thus indicate that viral growth in fin tissue is genetically correlated to resistance to waterborne disease and may be used to produce selected progeny, at least at the experimental scale. Possible implications of the relationship between VREFT and resistance for the study of resistance mechanisms are discussed.