DEVELOPMENT OF PASSIVE IMMUNOTHERAPY FOR CONTROL OF INFECTIOUS HEMATOPOIETIC NECROSIS

When rainbow trout Oncorhynchus mykiss (mean weight, 4.3 g) were injected with 100 mul of anti-infectious hematopoietic necrosis virus (IHNV) rainbow trout immune serum (IHNV neutralizing titer 640) immediately following waterborne exposure to 10(5) plaque-forming units ml-1 of IHNV, relative protection was 91% compared to fish injected with normal serum (titer <20). Fish injected with immune serum at 24 and 48 h post-exposure to virus exhibited relative protection of 88 and 75%, respectively. Less protection was observed in fish injected 3 or 4 post-exposure. Endogenously produced virus neutralizing titers in surviving fish injected at 0 and 24 h post-exposure were not detected but fish injected at 48 h produced a significant humoral response. Juvenile rainbow trout (mean weight 1 g) receiving injections of 50 mul of serum having a low plaque neutralization titer of 20 to 40 resulted in relative protection of 67 to 82% compared to fish injected with saline. When the same volume of a higher-titered serum was injected, greater protection was generally observed. These results suggested that low neutralization titers could be sufficient for significant protection against IHN disease. In 1 field trial, mortality due to IHN was lower in the group that received the passive immunotherapy compared to a group that was left untreated.