On the basis of limited experimental evidence, brown trout have generally been considered to be almost refractory to natural infection with Egtved virus. Nevertheless VHS in a few rare cases has been observed in brown trout under trout farm conditions. The present experiments were carried out to make clear whether the pathogenicity of strain 23/75 to brown trout was a unique feature of that strain or whether other Egtved virus isolates were also brown-trout-pathogenic when tested by means of bath infection. A total of eight virus isolates was examined, among others the reference strain of each of the three preliminary identified serotypes, strain F1 (serotype 1), strain He (serotype 2) and strain 23/75 (serotype 3). Among the eight virus isolates three were found to be pathogenic to brown trout as well as to rainbow trout. The remaining five were pathogenic only to rainbow trout. Strain Anso, which caused the highest mortality in brown trout, was, like strain 23/75, originally isolated from brown trout, whereas strain No was isolated from rainbow trout.
The symptoms of the brown trout which died during the experiments were clearly those of VHS and Egtved virus was readily isolated from affected individual fish.