Passive immunization with anti-virulent Aeromonas hydrophila whole serum protects Channel Catfish against motile Aeromonas septicemia.

OBJECTIVE

Recently, we demonstrated the efficacy of oral bacterin virulent Aeromonas hydrophila (vAh) vaccines (S14-452 and ML09-119) in Channel Catfish Ictalurus punctatus, but gaps in knowledge exist with respect to protective immunity induced by vAh bacterins. Specific antibodies were proposed as a potential mechanism of protection; thus, the aim of the study was to assess whether passive immunization with anti-vAh whole serum provides protection upon vAh challenge.

METHODS

The protective role of anti-vAh whole serum was determined by passive immunization of Channel Catfish, followed by vAh challenge. Antiserum was generated by active immunization of Channel Catfish with three inactivated vAh strains (ALG-15-097, S14-452, and ML09-119). Thereafter, antibody presence was confirmed and quantified via enzyme-linked immunosorbent assay. Three replicated passive immunization experiments were completed using 15-g Channel Catfish with various treatment groups, including control sera, heat-inactivated control sera, anti-vAh sera, heat-inactivated anti-vAh sera, and fetal bovine serum. A western blot analysis was conducted postchallenge to determine whether antibodies generated against one isolate of vAh cross-reacted with antigens of other vAh isolates and a typical A. hydrophila strain, thus providing insights into the cross-protective nature of the vaccine.

RESULTS

All fish immunized with anti-vAh serum or heat-inactivated anti-vAh serum demonstrated 100% protection and cross-protection upon vAh challenge. Fish that were passively immunized with control serum, heat-inactivated control serum, or fetal bovine serum exhibited 30-50% survival depending on the experiment. The antiserum exhibited antibodies that cross-reacted with some proteins and the lipopolysaccharide of the homologous and heterologous isolates, including a typical A. hydrophila isolate.

CONCLUSIONS

Passive immunization with anti-vAh sera provided 100% protection in all trials, demonstrating strong and consistent efficacy even at reduced antibody levels. Despite variability caused by heat inactivation, complete protection was maintained, highlighting the potency of the antiserum. Survival outcomes and western blot cross-reactivity further supported the protective and cross-protective capability of whole serum against diverse vAh strains. Collectively, these findings indicate that vAh-derived antibodies are key mediators of protection. Our findings point to mechanisms of immunity against vAh and guide vaccine development, ultimately preventing large-scale mortalities caused by vAh.