Isolation and characterisation of a variant strain of infectious haematopoietic necrosis (IHN) virus

An unusual strain of IHNV was isolated after a clinical outbreak of the dis ease in rainbow trout fry. The causative agent tested negative for VHSV and IHNV by immunofluorescence performed by a regional diagnostic laboratory u sing commercially available monoclonal antibodies. The isolate, named DF04/ 99, was however identified as IHNV by EU-authorised diagnostic methods such as indirect immunofluorescence and virus neutralisation performed at the G erman National Reference Laboratory for Fish Diseases. Moreover, the virus was also identified as IHNV by immunoelectron microscopy and by reverse tra nscriptase-dependent polymerase chain reaction. This unusual isolate of IHN V differs in the amino acid sequence of the G- and NV-proteins from the com mon IHNV types RE, WRAC, and SRCV. A mutation in the G protein might be res ponsible for the missing reaction in immunofluorescence using standard comm ercially available MAbs. It is concluded that a continuous monitoring of th e reaction pattern of MAbs with current isolates as well as the generation of new diagnostics which match the epizootiological situation are important tasks for a fast and reliable diagnosis of fish diseases.