ANTIGENIC VARIANTS OF INFECTIOUS HEMATOPOIETIC NECROSIS VIRUS AND IMPLICATIONS FOR VACCINE DEVELOPMENT

Two monoclonal antibodies and 2 polyclonal antisera were used in cross -neutralization tests of 106 isolates of infectious hematopoietic necr osis virus (IHNV) obtained from acutely infected rainbow trout Oncorhy nchus mykiss at 4 locations in a 12 mile section of the Snake River, I daho, USA, during 1990 to 1992. Ten antigenic groups exhibiting differ ent neutralization profiles were identified. Approximately 91% of thes e isolates fell into 3 of the groups. Each location exhibited a range of antigenic variants from 1 to 7. Prevalence of isolates within a gro up also varied. The frequency of different antigenic groups and preval ence of isolates within each group differed over time. Virulence compa risons using representative isolates from 7 of the antigenic groups to challenge rainbow trout at 2 different mean body weights resulted in average cumulative mortalities that ranged from 14 to 92%. Passive imm unization of rainbow trout with convalescent (immune) serum from fish exposed to 1 antigenic variant provided significant protection after c hallenge with representative isolates from other variant groups. Relat ive protection of fish injected with immune serum compared to normal s erum ranged from 91 to 100%. The results indicated that neutralizing a ctivity produced against 1 antigenic variant of IHNV would cross-prote ct against other antigenic variants. A vaccine against a single type o f IHNV may be efficacious against all IHNV strains.