Francisella orientalis is an emerging pathogen responsible for outbreaks of high mortality in Nile tilapia (Oreochromis niloticus) in Brazil and other countries, causing significant losses in aquaculture. Considering the facultative intracellular nature of this bacterium, traditional inactivated vaccines tend to show lower efficacy, whereas technologies such as DNA vaccines have the potential to induce more effective 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.
A total of 300 fish (44.01 ± 0.71 g) were used in a completely randomized design with five treatments and five replicates, housed in 60 L aquaria with a recirculation system and water quality control. The treatments consisted of a negative control (uninfected), a positive control (infected without vaccination), an inactivated vaccine (T1), and three doses of the DNA vaccine: 50 µL (T2), 70 µL (T3), and 100 µL (T4). Twenty-one days after immunization, the fish were challenged via intraperitoneal injection with 890 CFU/fish of F. orientalis and monitored for 15 days.
Relative Percent Survival (RPS) values (Figure 1) were 32% for the inactivated vaccine, 0% for the 50 µL DNA vaccine, and 50% for the 70 µL and 100 µL DNA vaccines. It is concluded that, although the inactivated vaccine provided partial protection, the DNA vaccines at doses of 70 µL and 100 µL were more effective, reducing mortality by half. These results indicate the potential of DNA vaccines as a promising tool for controlling francisellosis in tilapia and reducing antibiotic use in aquaculture.