Immunogenicity and Protective Efficacy of the First Autogenous Aeromonas hydrophila Vaccine for Piaractus brachypomus.

Aeromonas hydrophila is a major zoonotic bacterial pathogen responsible for haemorrhagic septicemia, substantial economic losses in aquaculture and increasing antimicrobial resistance among field isolates. It is also one of the main bacterial species associated with mortality outbreaks in pirapitinga, Piaractus brachypomus, culture in the Amazon region of Latin America. Therefore, this study evaluated the immunogenicity and protective efficacy of the first autogenous vaccine against A. hydrophila developed for P. brachypomus, formulated with or without a Montanide adjuvant. A total of 150 fish were vaccinated by intraperitoneal injection with a formalin-inactivated homologous bacterin and, at 35 days post-vaccination, challenged with a homologous strain of A. hydrophila at the median lethal dose (LD50). Following bacterial challenge, biological samples were collected at 24 h post-challenge, and survival was monitored for 15 days (50-day trial period). The vaccinated groups showed enhanced immune responses, characterized by increased ROS levels, lysozyme activity, agglutinating antibody titers and IgM levels, particularly in the fish vaccinated with the adjuvant. Haematological evaluation revealed lymphocytosis and, morphologically, lower lesion severity and preservation of renal architecture in vaccinated groups. In contrast, infected non-vaccinated fish exhibited leukopenia, tubular disorganization, intense mononuclear inflammatory infiltrates, glomerular alterations and lesions suggestive of tissue necrosis. These findings were reflected in enhanced protection following the bacterial challenge, with relative percent survival (RPS) values of 76.19% for fish vaccinated without the adjuvant and 85.71% for those vaccinated with the adjuvant. Overall, the autogenous vaccine was safe, immunogenic and provided effective protection against Aeromonas hydrophila infection in Piaractus brachypomus.