Efficacy of a Vaccine for Atlantic Salmon ( <i>Salmo salar</i> ) Using a <i>Tenacibaculum dicentrarchi</i> Strain Cultured Under Iron‐Limited Conditions

Tenacibaculosis, caused by T. dicentrarchi, results in skin lesions, ulceration, yellow plaques on teeth, and haemorrhaging in the operculum, peduncle and pectoral fins. It is the second leading cause of mortality in farmed Atlantic salmon (Salmo salar) in Chile. Currently, no vaccine is available to prevent the disease. In silico and in vivo studies have shown that iron plays a key role in T. dicentrarchi infection. We hypothesised that culturing the TdCh05 vaccine strain under iron-limiting conditions (i.e., with non-assimilable iron chelator 2.2'-dipyridyl [DIP]) would enhance Atlantic salmon protection against tenacibaculosis. Fish were vaccinated intraperitoneally with prototypes (A) TdCh05, (B) TdCh05 + DIP or (C) A + B (a 1:1 mixture of inactivated cultures), and a control group was challenged by bath immersion with a heterologous T. dicentrarchi strain. Each prototype was emulsified with a commercial adjuvant. At 14 days post-challenge, Atlantic salmon injected with FMM broth had a cumulative mortality of 73.3%, followed by the TdCh05 + DIP (66.7%) and A + B (54.8%) prototypes. The lowest cumulative mortality value (50%) was observed for the prototype containing T. dicentrarchi TdCh05 grown only in FMM. Moreover, a high proportion of the vaccinated fish that survived the challenge, regardless of the vaccine prototype, carried T. dicentrarchi in various internal organs, particularly in the spleen. Future studies should focus on identifying the most suitable antigens to develop an effective vaccine for the prevention of T. dicentrarchi-induced tenacibaculosis.