Vaccination of rainbow trout against infectious hematopoietic necrosis (IHN) by using attenuated mutants selected by neutralizing monoclonal antibodies

A neutralizing monoclonal antibody against infectious hematopoietic necrosis virus (IHNV) was used to select neutralization-resistant mutants from isolates of virus obtained from adult: steelhead Oncorhynchus mykiss returning to the Round Butte Hatchery (RB mutants) on the Deschutes River in Oregon, USA, and from rainbow trout (nonanadromous O. mykiss) at a commercial hatchery in the Hagerman Valley of Idaho, USA (193-110 mutants). Two of the mutants, RB-1 and 193-110-4, were significantly (P < 0.001) attenuated compared with parental strains. Vaccination of rainbow trout by waterborne exposure to the mutants conferred solid protection against challenge with wild-type virus. In some trials, fish vaccinated with the RB-1 mutant at 50% tissue culture infectious doses (TCID50) of 1 x 10(4)-1 x 10(5) TCID50/mL or with the 193-110-4 mutant at 1 x 10(2)-1 x 10(3) TCID50/mL, hell for 14 d, then challenged with the homologous wild-type strain at 1 x 10(5) TCID50/mL showed relative percent survival of 95-100% (P < 0.005). There was no significant difference (P > 0.05) in protection among fish exposed to the RB-1 vaccine strain at a dose of 1 x 105 TCID50/mL for periods of either 1, 12, or 24 h, held for 14 d, and then challenged with the wild-type RB isolate, although the 1-h exposure seemed to be somewhat less effective. Fish were vaccinated with the RB-1 strain at 1 x 10(3)-1 x 10(5) TCID50/mL for 24 h then challenged after 1, 7, 14, or 21 d with the wildtype RB isolate. No significant (P > 0.1) protection was observed at 1 d postvaccination, but the relative percent survival increased progressively at each subsequent challenge period, becoming statistically significant by day 7 (P < 0.001) and beyond. These results suggested that: resistance to challenge with wild-type virus resulted from development of IHNV-specific immunity and not from viral interference or interferon induction, and they reinforce the potential elf an attenuated vaccine to control this important disease.