Duration of protective antibodies, and the correlation with protection in Atlantic salmon (Salmo salarL.), following vaccination with anAeromonas salmonicidavaccine containing iron-regulated outer membrane proteins and secretory polysaccharide

Atlantic salmon (n=82) were individually marked and injected intraperitoneally with an unadjuvanted A-layer negative (A)Aeromonas salmonicidabacterin which had been grown under iron-restricted conditions to induce expression of iron-regulated outer membrane proteins (IROMPs). The bacterin was supplemented with the extracellular polysaccharide (PS) antigen which had been extracted using hot phenol from the supernatant of a normal broth culture ofA. salmonicida. Eight weeks after vaccination, the fish were bath-challenged with a virulentA. salmonicidastrain. Specific antibody titres to IROMPs and PS were determined by ELISA. All 20 unvaccinated control fish died. In the vaccinated fish 1 week prior to challenge, seven of the eight fish which died had antibody titres to IROMPs of log21 or 2; the eighth fish had log2titre of 5. With the exception of this individual, all fish with log2titres greater than 4 survived the challenge (66 out of 67) while 47% of fish with titres of log22 or below died (7 out of 15). Similar correlations of anti-PS antibody titres with protection were observed. Of those fish with antibody responses to neither, only one or both antigens, 100% of non-responding fish died, 7·2% of responders to only one antigen, and 1·2% of responders to both antigens died. One month following challenge the antibody titres to IROMPs in surviving fish decreased by about 30% and thereafter a monthly decrease of about 10% was observed. In contrast, the antibody titres in an identical but unchallenged vaccinated group continued to rise to a peak at week 12 and thereafter decreased by about 10% per month. Nine months after vaccination both groups were challenged. Similar relative percent survival was recorded in both groups (over 50%) and once again individual survival correlated strongly with anti-IROMP antibody titres before challenge. Titres above log25 correlated with protection. This is the first report of specific antibody responses having a strong correlation with protection against furunculosis.