Detection of Specific Antibodies against Infectious Hematopoietic Necrosis Virus from Rainbow Trout Sera by ELISA using Two Novirhabdoviruses

An enzyme-linked immunosorbent assay (ELISA) with viral culture fluids as capture antigens was performed to detect specific antibodies against infectious hematopoietic necrosis virus (IHNV) from rainbow trout Oncorhynchus mykiss sera. When using IHNV antigen (IHNV-Ag), ELISA OD values for IHN-survived (IHN-Surv) rainbow trout sera were relatively higher than those for specific pathogen free (SPF) fish sera. However, some of the SPF sera were diagnosed as positive due to high OD values (> 0.2). To clarify reasons for these high OD values, each of five IHN-Surv and SPF sera was subjected to ELISA with viral hemorrhagic septicemia virus (VHSV) and hirame rhabdovirus (HIRRV) antigens (VHSV-Ag and HIRRV-Ag). Some of the IHN-Surv and SPF sera showed high OD values in both VHSV-Ag and HIRRV-Ag plates even though there was no possibility that those sera contained antibodies against VHSV or HIRRV antigen. These results suggest that some of the sera contained antibodies against impurities in viral culture fluids such as FBS and cell debris, and caused the false-positive reactions. The corrected OD values, subtracted the OD values in VHSV-Ag plates from those in IHNV-Ag plates, were all < 0.2, in SPF sera (n = 148) while those in IHN-Surv sera (n = 238) were randomly distributed from 0 to 0.7. It was considered that the corrected OD values may represent true values recognized by IHNV-specific antibodies.