Francisella orientalis is a pathogen associated with high-mortality outbreaks in Nile tilapia (Oreochromis niloticus ) in Brazil and other countries, causing significant economic losses in aquaculture. Due to its facultative intracellular nature, traditional inactivated vaccines often show limited efficacy, whereas DNA vaccines have demonstrated potential to induce more robust adaptive immune responses. The aim of this study was to evaluate and compare the efficacy of an inactivated vaccine and DNA vaccines against F. orientalis in Nile tilapia, focusing on the humoral immune response measured by specific IgM production.
A total of 300 fish (44.01 ± 0.71 g) were used in a completely randomized design, with five treatments and five replicates in 60 L aquaria with recirculating water and quality control. Treatments included negative control (uninfected), positive control (infected, unvaccinated), inactivated vaccine (T1), and three DNA vaccine doses: 50 µL (T2), 70 µL (T3), and 100 µL (T4). Twenty-one days post-immunization, blood samples were collected from 10 fish per treatment to obtain serum for quantification of F. orientalis-specific IgM.
ELISA assays showed that fish vaccinated with the inactivated vaccine (T1) and DNA vaccines at 70 and 100 µL doses (T3 and T4) exhibited significantly elevated levels of F. orientalis-specific IgM, indicating a robust adaptive immune response. These results suggest that, while the inactivated vaccine provides partial protection, DNA vaccines at adequate doses are more effective, reducing mortality by half and representing a promising strategy for controlling francisellosis and reducing antibiotic use in aquaculture.