Infectious hematopoietic necrosis (IHN) and viral hemorrhagic septicemia (VHS): Detection of the trout antibodies to the causative viruses by means of plaque neutralization, immunofluorescence, and enzyme-linked immunosorbent assay

Sera collected from cultured rainbow trout Oncorhynchus mykiss surviving outbreaks of infectious hematopoietic necrosis (IHN) or viral hemorrhagic septicemia (VHS) were examined for the presence of antibodies to both of the causative viruses, infectious hematopoietic necrosis virus (IHNV) and Egtved virus (viral hemorrhagic septicemia virus: VHSV). Sera were screened with three serological tests: 50% plaque neutralization test (PNT), immunofluorescence (IF), and enzyme-linked immunosorbent assay (ELISA). In sera from 20 rainbow trout surviving IHN, antibodies to IHNV were detected in 9 fish by PNT, in 12 fish by IF, and in 9 fish by ELISA. In these sera, antibodies cross-reacting with VHSV were rare (detected in 0 fish by PNT, in 1 by IF, and in 1 by ELISA). In sera from 20 rainbow trout surviving VHS, antibodies to VHSV were detected in 9 fish by PNT, in 16 fish by IF, and in 18 fish by ELISA. A considerable percentage of the VHS-survivor sera contained antibodies that cross-reacted with IHNV, as detected by ELISA (16 fish) and 1F (7 fish) but not by PNT (0 fish). The three serological tests appear to be useful tools for IHNV and VHSV epidemiology; however, the presence of cross-reacting antibodies in some sera suggests caution when farms require specific pathogen-free certification for one of the viruses in the presence of the other.