Pathogenicity of <em>Aeromonas veronii</em> from Nile Tilapia (<em>Oreochromis niloticus</em>) and Efficacy of Fish Oral Vaccine against Motile Aeromonad Septicemia in Tank Trials

Abstract

The Motile Aeromonad Septicemia (MAS) caused by Aeromonas species has been a serious problem in fish health management particularly in Nile tilapia (Orechromis niloticus). This study characterized an Aeromonas species isolated from farmed tilapia fingerlings in Binangonan, Rizal, Philippines, and tested for its pathogenicity in tank trials. We designated this isolate as Aeromonas veronii DFR01 (Diseased Fish Rizal) based on 16S rRNA phylogenetic analysis, 16S rRNA homology and MALDI-TOF MS identification with its biochemical profile generated from API and BiOLOG Gen III systems. A median lethal dose of A. veronii DFR01 was determined at 107 CFU/ml in tank trials and was utilized as whole cell inactivated antigen for oral vaccine development. The immunized tilapia fingerlings produced elevated levels of IgM in the blood as determined by enzyme-linked immunosorbent assay (ELISA). The increase in IgM levels is significantly higher 14 days post vaccination. The relative IgM gene expression as determined by quantitative polymerase chain reaction (qPCR) showed increasing levels after vaccination until 38 days of culture. The vaccinated fish showed 25-35% cumulative mortalities compared to the non-vaccinated-challenged group. Results showing improved relative percent survival (RPS) rate in tilapia fingerlings and induced immune response after vaccination indicate the potential use of the oral vaccine in cultured tilapia.