A vaccine based on biodegradable microspheres induces protective immunity against scuticociliatosis without producing side effects in turbot

The histiophagous scuticociliate parasite Philasterides dicentrarchi is an emergent pathogen in aquaculture and causes significant economic losses on turbot (Scophthalmus maximus) farms. In this study, the surface antigens (Ag) of the parasite were encapsulated and covalently linked to a polymeric microparticle formulation composed of two biodegradable polymers (chitosan and Gantrez). The antigenicity of the formulation and the protection provided were compared in mice and turbot. This formulation induced a higher antibody (Ab) response in mice at doses of 5 mg of microspheres (MS) conjugated with approximately 230 μg of Ag (MS-Agc). However, Ab levels were significantly lower than in mice vaccinated with the same concentration of Ag in complete Freund’s adjuvant (FCA). In turbot, the MS-Agc formulation induced a higher level of Abs than that induced by the same vaccine emulsified in FCA. The challenge experiments performed with P. dicentrarchi and vaccinated turbot also showed a clear correlation between Ab levels and survival levels. Growth was significantly affected in fish vaccinated with FCA, but not in fish vaccinated with MS. The high adjuvant capacity of MS, together with its biodegradability and low toxicity to fish, makes this new vaccine an economical, effective and safe alternative to oil-based adjuvants for the immunoprophylaxis of scuticociliatosis in turbot.