Enhanced tilapia mucosal immunity by nano-immersion vaccine against columnaris disease  

Summary

Aquaculture is the fastest growing animal food production sector globally and very important to food security. Reaching harvest size in just 6 months, tilapia is a very important aquaculture species for many low to middle income countries (LIMCs), since they provide an important source of protein and essential revenue for many low-income families. Intensification of tilapia farming has promoted severe disease outbreaks, resulting in high mortalities and economic hardship for tilapia farmers. Columnaris disease, caused by Flavobacterium columnare, has emerged as one of most important bacterial diseases for farmed tilapia, for which there is no commercial vaccine, and farmers resort to using antibiotics to control this disease.

With increasing concerns about using antibiotics in aquaculture, attention had focused on vaccination for disease control. Many tilapia farmers will not vaccinate by injection once the fish have been moved onto the farm and would prefer fish to be vaccinated in the hatchery before they are moved to their grow-out site; however, injection of small fish is difficult. Alternative vaccine delivery methods are needed, such as oral or immersion delivery, but vaccines delivered by these routes tend to provide low efficacy and short duration of protection. We will investigate a novel and innovative immersion vaccine for columnaris disease in tilapia. We will deliver inactivated Flavobacterium columnare to tilapia in nanoparticles, coated with a mucoadhesive biopolymer, to provide "pathogen-like" properties that enhance binding of the nanoparticles to fish mucosal membranes and we will assess the immune response and level of protection elicited by these in vaccinated tilapia.

Project Outcomes

Columnaris, a highly contagious bacterial disease caused by Flavobacterium columnare, is recognized as one of the most important infectious diseases in farmed tilapia, especially during the fry and fingerling stages of production. The disease is associated with characteristic lesions in the mucosa of affected fish, particularly their skin and gills. Vaccines delivered via the mucosa are therefore of great interest to scientists developing vaccines for this disease. In the present study, we characterized field isolates of F. columnare obtained from clinical columnaris outbreaks in red tilapia to select an isolate to use as a candidate for our vaccine study. This included characterizing its colony morphology, genotype and virulence status. The isolate was incorporated into a mucoadhesive polymer chitosan-complexed nanovaccine (CS-NE), the efficacy of which was determined by experimentally infecting red tilapia that had been vaccinated with the nanoparticles by immersion. At 30 days post-vaccination, the fish were infected F. columnare. The relative percentage of protection was 78% in vaccinated fish relative to control fish. Histology of the mucosal associated lymphoid tissue (MALT) showed a significantly higher presence of leucocytes and a greater antigen uptake by the mucosal epithelium in CS-NE vaccinated fish compared to control fish and whole cell vaccinated fish, respectively, and there was statistically significant up-regulation of IgT, IgM, TNF α, IL1-β and MHC-1 genes in the gill of the CS-NE vaccinated group. Overall, the results of our study confirmed that the CS-NE particles achieved better adsorption onto the mucosal surfaces of the fish, elicited great vaccine efficacy and modulated the MALT immune response better than the conventional whole cell-killed vaccine, demonstrating the feasibility of the mucoadhesive nano-immersion vaccine as an effective delivery system for the induction of a mucosal immune response against columnaris disease in tilapia.