Rainbow trout Oncorhynchus mykiss were injected intraperitoneally with a bacterin containing killed Renibacterium salmoninarum cells delivered alone or in an oil‐based adjuvant. We evaluated the relative abilities of the bacterins to prevent the initiation or progression of infection in fish challenged by waterborne exposure to R. salmoninarum. Sixty‐one days after vaccination, fish were held for 24 h in water containing either no bacteria or approximately 1.7 × 103, 1.7 × 105, or 5.3 × 106 live R. salmoninarum cells/mL. An enzyme‐linked immunosorbent assay (ELISA) was used to monitor changes in the levels of R. salmoninarum antigen in live fish before and after the immersion challenges. High levels of R. salmoninarum antigens were detected by ELISA in kidney–spleen tissue homogenates from vaccinated fish immediately before the challenges. Levels of those antigens remained high in the tissues of unchallenged fish throughout the study. We found that the ELISA used in this study may be unsuitable for evaluating the efficacy of bacterins because it did not distinguish antigens produced by the challenge bacteria during an infection from those of the bacterins. Groups of control and vaccinated fish also were injected with either L7 × 104 or l.7 × 106 R. salmoninarum cells and served as R. salmoninarum virulence controls. Relative survival among the various subgroups in the injection challenge suggests that adverse effects might have been associated with the adjuvant used in this study. The lowest survival at both injection challenge levels was among fish vaccinated with bacteria in adjuvant.

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Coldwater Vibrio species isolated from Atlantic salmon, Salmo salar L., during winter ulcer disease outbreaks at saltwater sites in Norway and Iceland were characterized phenotypically, tested for virulence, and used to evaluate the efficacy of multivalent, oil‐adjuvanted vaccines. The intraperitoneal (i.p.) injection of rainbow trout, Oncorhynchus mykiss (Walbaum), in fresh water with one bacteria species isolated during winter ulcer outbreaks, V. ‘viscosus’, produced rapid mortality and disease signs which resembled those observed during natural outbreaks [105 colony‐forming units (cfu) fish−−1]. Another species, V. ‘wodanis’, was not virulent to rainbow trout (103–106 cfu fish−−1). Although vaccination of rainbow trout with a mineral‐oil‐adjuvanted, injectable vaccine containing V. anguillarum (serotypes 01 and 02), V. salmonicida and Aeromonas salmonicida did not provide protection against injection challenge with V. viscosus, vaccines which included V. viscosus produced significant protection in Atlantic salmon and rainbow trout. Atlantic salmon vaccinated with an oil‐adjuvanted vaccine containing V. viscosus, V. wodanis and atypical A. salmonicida produced a relative percentage survival (RPS) of 97% when challenged i.p. with V. viscosus, demonstrating cross‐protection between strains from Iceland and Norway. Short‐term efficacy was demonstrated in rainbow trout by injection challenge at 21 and 43 days post‐vaccination with an oil‐adjuvanted vaccine containing V. viscosus, V. anguillarum (01/02), V. salmonicida and A. salmonicida, which produced an RPS of 96–99%. Rainbow trout challenged with V. viscosus at 52 and 362 days post‐vaccination produced an RPS of 93% and 79%, indicating that vaccination provided long‐term protection. In a similar manner, rainbow trout injected i.p. with 0.2 mL of a vaccine containing the five bacteria species and infectious pancreatic necrosis virus produced a 90% RPS when challenged with V. viscosus 66 days later. The high RPS under a severe challenge burden, along with disease signs in experimental freshwater challenges which resembled the saltwater disease condition, indicated that V. viscosus is a contributing factor to winter ulcer and that vaccination will protect against the disease.

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