Immune Response of Nile Tilapia (<scp><i>Oreochromis niloticus</i></scp>) Vaccinated With Diatom‐Based Oral Vaccines Against Piscine Francisellosis

Piscine francisellosis is a highly infectious and economically significant disease caused by Francisella orientalis in tilapia (Oreochromis spp.). There are currently no approved treatments or commercial vaccines for this disease in cultured fish. Injectable vaccines using diatoms as antigen expression vectors have demonstrated efficacy in tilapia models; however, no oral vaccine trials have been performed. We hypothesised that fusion proteins consisting of F. orientalis IglC and flagellin expressed in Thalassiosira pseudonana diatoms will act as self-adjuvanting antigen delivery systems to confer a protective immune response against F. orientalis in tilapia when administered as top-coated feed. Different treatments were immunised and subsequently provided with one or two boosters prior to challenge. Fish were challenged with virulent F. orientalis via immersion thirty days post initial immunisation. Tilapia immune response was assessed 24 h post-challenge by quantifying il-12, il-10, ifn-γ and tgf-β gene expression in gills and internal organs. Morbidity and mortality were monitored for 21 days after challenge and bacterial load was assessed in survivors. Findings indicate significant changes in the expression of ifn-γ and tgf-β in immunised fish, but similar mortality rates and bacterial load across all exposed groups.