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 disease in rainbow trout fry. The causative agent tested negative for VHSV and IHNV by immunofluorescence performed by a regional diagnostic laboratory using 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 German National Reference Laboratory for Fish Diseases. Moreover, the virus was also identified as IHNV by immunoelectron microscopy and by reverse transcriptase-dependent polymerase chain reaction. This unusual isolate of IHNV differs in the amino acid sequence of the G- and NV-proteins from the common IHNV types RE, WRAC, and SRCV. A mutation in the G protein might be responsible for the missing reaction in immunofluorescence using standard commercially available MAbs. It is concluded that a continuous monitoring of the 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