Efficacy of a divalent and a multivalent water-in-oil formulated vaccine against a highly virulent strain of Flavobacterium psychrophilum after intramuscular challenge of rainbow trout (Oncorhynchus mykiss).

Flavobacterium psychrophilum is a well-known pathogen causing significant problems in aquaculture worldwide. In recent years an increasing number of disease outbreaks caused by F. psychrophilum has been reported on juvenile and post smolts of rainbow trout (Oncorhynchus mykiss) in Norway. The current study was performed to assess the efficacy of two autogenous water-in-oil formulated vaccines containing whole cell antigens of F. psychrophilum to induce protective immunity against challenge. The vaccines were formulated either as multivalent (FLAVO AVM6) or divalent (FLAVO IPN) and administered by the intraperitoneal route. Intramuscular challenge with a field strain of F. psychrophilum was carried out 552 day degrees post vaccination, at a time when the FLAVO AVM6 and FLAVO IPN vaccinated groups had significantly higher antibody responses compared to the negative control. Results from the challenge study showed that the multivalent and the divalent vaccines had capacity to induce significant protection, with RPS60>87% and RPSend>77.5% for both vaccines. The high level of protection seen in the vaccinated groups was also reflected in the reduced ulceration rates observed at the injection site. Combining our results demonstrate that vaccination with FLAVO AVM6 and FLAVO IPN induces responses capable of protecting rainbow trout against infections with F. psychrophilum.