Variability among Vibrio anguillarum O2 isolates from Atlantic cod (Gadus morhua L.): Characterisation and vaccination studies

Farmed Atlantic cod, Gadus morhua L., in Norway are vaccinated against Vibrio anguillarum serotypes O1, O2a and O2b, but outbreaks of classical vibriosis remain relatively common. In the present study, V. anguillarum isolated from vaccinated and unvaccinated cod between 2002 and 2005, were shown to differ in biochemical, serological and genetic characteristics compared to V. anguillarum serotypes O2a and O2b. They did not utilize sorbitol and some were shown to be lysine decarboxylase positive or did not produce gelatinase. Furthermore, they displayed varying reaction patterns with monoclonal antibodies, and appeared in a separate cluster in a similarity dendrogram based on amplified fragment length polymorphisms (AFLP) analysis, distant from the cluster containing both serotypes O2a and O2b. The results indicate that these V. anguillarum isolates belong to a sero-subtype other than O2a or O2b.The efficacy of ALPHA MARINE™ Vibrio vaccine in cod varied depending on the V. anguillarum O2 isolate used for challenge. The vaccine elicited good protection against bath challenge with isolates of V. anguillarum O2b and O2a (RPS > 80) comparable to the vaccine strains, but protected poorly (RPS of 32) or not at all when challenged with two recent and differing isolates from diseased vaccinated cod.