SEROLOGICAL DIFFERENCES AMONG ISOLATES OF VIRAL HEMORRHAGIC SEPTICEMIA VIRUS DETECTED BY NEUTRALIZING MONOCLONAL AND POLYCLONAL ANTIBODIES

The serological variation among 127 isolates of viral hemorrhagic septicaemia virus (VHSV) was examined by plaque neutralization tests (PNT) using a panel of 4 neutralizing monoclonal antibodies (MAbs) and 1 neutralizing rabbit antiserum (PAb). Three distinct neutralization patterns were observed. All isolates in the largest group (I) were neutralized at high titres by the MAbs and the PAb. This group included isolates representing the previously described 'Serotypes' 1 and 2. The isolates in Group II were efficiently neutralized by 1 of the MAbs and the PAb but not, unless at low titres, by the remaining 3 MAbs. Isolates represented by previously described 'serotype' 3 were in this group. The isolates in Group III were not neutralized at all by any of the MAbs and not, or only moderately, neutralized by the PAb. Isolates with this reaction pattern have become the most frequent ones from VHS outbreaks in Denmark in recent years. Pronounced crossreaction between Groups I, II & III in Western blotting using polyclonal rabbit antibodies combined with the ability of 1 antiserum to neutralize 120 out of the 127 isolates indicate that the polyclonal antibodies used define only 1 serotype of VHSV. Groups I to III should be regarded as subtypes. The present panel of antibodies did not allow a distinction between pathogenic and nonpathogenic isolates. No relationship was observed between host species and the reaction pattern as all 3 reaction patterns were present among the 5 isolates originating from fish species other than rainbow trout Oncorhynchus mykiss.