A comparative immunopathological study of injection site reactions in salmonids following intraperitoneal injection with oil-adjuvanted vaccines

An adequate understanding of the inflammatory mechanisms following intraperitoneally-injected oil-adjuvanted vaccines in fish is required for the improvement of the quality of vaccines. We report the findings of a study in which the importance of neutrophils, macrophages, lymphocytes and mast cells (MCs) in early inflammatory reactions of Atlantic salmon were examined in comparison with those of rainbow trout and grayling. The inflamed areas in the pylorus/pancreas area (injection site) were sequentially measured using computer-assisted microscopy, parallel with the quantification of leucocytes from 2 to 16 weeks post-vaccination. Antigen retention was also evaluated. Results show that inflammation in Atlantic salmon was slow to develop and was associated with a prolonged initial dominance of neutrophils. In contrast, the onset in rainbow trout was rapid and strong with early macrophage domination. The inflammatory and cellular response in grayling was initially in-between but progressed to be the strongest at 16 weeks. The results suggest that slow development of inflammation in Atlantic salmon and grayling leads to delayed sequestration of antigens resulting in chronically active inflammation that attempts to remove the antigens from the injection site. The strong onset in rainbow trout leads to rapid breakdown and removal of antigens resulting in early resolution.