Vaccinated Atlantic salmon ( Salmo salar L.) maintain a specific antibody response throughout the seasonal fluctuations of a full commercial production cycle in sea: a case study

Vaccination is a critical prophylactic measure to prevent disease outbreaks in Atlantic salmon (Salmo salar L.) and multivalent oil-adjuvanted vaccines administered intraperitoneally (IP) are widely used for this purpose. The Atlantic salmon in this observational study were sampled during commercial production in a salmon farm in Northern Norway. Fish were sampled in the hatchery prior to vaccination, and further every 6 weeks after sea transfer up until slaughter. The key findings revealed that Atlantic salmon vaccinated IP with a commercial multivalent oil-based vaccine developed specific and long-lasting antibody responses against A. salmonicida, V. anguillarum, Y. ruckeri, and IPNV for up to 20 months post-vaccination. However, there was variation in the measured specific antibody response to the different antigens. Notably, the antibody responses against A. salmonicida and V. anguillarum antigens were higher and more prolonged compared to the antibody responses against Y. ruckeri and IPNV antigens. Histological examination showed presence of vaccine-induced granulomas in the pancreas at the injection site for up to 50 weeks post-vaccination and V. anguillarum antigens were found in the granulomas up to 44 weeks post-vaccination. There were also individual IgM-positive cells present in these vaccine-induced granulomas, and qPCR confirmed increased gene expression of membrane-bound and secretory IgM up to 37 weeks post-vaccination. Altogether the qPCR data of membrane and secretory IgM suggests local production of IgM in the vaccine-induced granulomas, while the head kidney might be responsible for the longterm secretion of IgM. Neither the low temperature the fish experienced for two winter seasons (below 5 degrees C) nor the seasonal changes in photoperiod affected the vaccine-induced specific antibody responses.