Whole cell inactivated autogenous vaccine effectively protects red Nile tilapia (Oreochromis niloticus) against francisellosis via intraperitoneal injection

Abstract

Francisella noatunensis subsp. orientalis is a pathogen of tilapia and other warm‐water fish for which no vaccines are commercially available. In this study, a whole cell formalin‐inactivated vaccine was developed for the first time using the highly virulent isolate STIR‐GUS‐F2f7 and the oil‐based adjuvant Montanide™ ISA 763A VG. The efficacy of the vaccine was assessed in red Nile tilapia via intraperitoneal (i.p.) injection using homologous experimental infection and correlates of protection such as seral antibody production and bacterial loads in the spleen. For immunization, fish were i.p. injected with 0.1 ml of the vaccine, the adjuvant alone or PBS. At 840 degree days post‐vaccination, all fish were i.p. injected with 4.0 × 103 CFU/fish of pathogenic bacteria. The RPS at the end of the trial was 100% in the vaccinated group with significantly higher survival than in the adjuvant and control groups. The RPS in the adjuvant group was 42%, and no significant difference was seen in survival between this and the PBS group. Moreover, significantly higher antibody titres in the serum and significantly lower bacterial loads in the spleen were detected in the vaccinated fish by ELISA and qPCR, respectively. These findings highlight the potential of autogenous vaccines for controlling francisellosis in tilapia.

Immune Responses and Protective Efficacy of a Formalin-Killed Francisella Noatunensis Subsp. Orientalis Vaccine Evaluated through Intraperitoneal and Immersion Challenge Methods in Oreochromis Niloticus

“…The specific Fno antibody titers and the RPS values obtained in this study correlated with those in previous studies that also used a whole-cell inactivated vaccine made from the highly virulent strain STIR-GUS-F2f7 isolated from red Nile tilapia mixed with the oil-based adjuvant Montanide TM ISA 763 AVG [12,14]. In the study by Ramírez-Paredes et al (2019), the strain STIR-GUS-F2f7 was used as the vaccine strain, as well as the challenge strain, in red Nile tilapia, wherein a high protection of 100% was obtained [14]. Later, Shahin et al (2019) used the same vaccine, but challenged Nile tilapia with two more heterologous strains in addition to the homologous strain STIR-GUS-F2f7.…”

“…However, with live vaccines, there is always a risk of pathogens reverting to their virulent state. Therefore, an injectable inactivated vaccine with a commercial adjuvant (the oil-based adjuvant Montanide TM ISA 763 AVG) is a better choice against fish bacterial diseases [12,14,22]. Moreover, the efficacy of immersion vaccines is low or moderate when compared to that of injection vaccines.…”

“…Moreover, the emergence of antibiotic-resistant bacterial strains is a severe consequence of the overuse or misuse of antibiotics. Therefore, vaccination is a more effective strategy for controlling and preventing tilapia francisellosis [12][13][14][15]. Currently, approved commercial vaccines against tilapia francisellosis are not available.…”

“…However, concrete and extensive evidence of the safe use of a live attenuated vaccine is urgently needed. The utilization of Genetically Modified Organism (GMO)-based vaccination for fish followed by their release into open water fish farms is highly limited and is still not allowed in aquaculture worldwide [14,16,17]. An inactivated vaccine would overcome this problem and would also be economically feasible for use in a cheap fish like tilapia.…”

“…Two recent studies investigated the effect of an inactivated vaccine on tilapia francisellosis. A whole cell formalin-inactivated vaccine was first developed using the highly virulent isolate STIR-GUS-F2f7 and the oil-based adjuvant Montanide™ ISA 763 AVG [14]. This vaccine elicited good protective efficacy (relative percent survival (RPS) 100%) against francisellosis in red Nile tilapia.…”