At the end of March 1987 in a trout farm situated in the centre of Italy, a mortality close to 100% occurred in rainbow trout yolk sac fry, apparently due to a haemorrhagic syndrome.
Virological examination using RTG-2 and EPC cell cultures led to isolation of a virus which induced a cytopathic effect (CPE) in 48-72 hours. Particularly in EPC cells the CPE resembled the one induced by Egtved virus, the causative agent of VHS (Jensen, 1965).
However, immunofluorescence- and serum neutralization tests utilizing antisera to Egtved virus, strain F1, indicated a lack of relatedness with that virus. Further serological tests clearly show that the virus in question is IHN virus.