Vaccine-induced protection against furunculosis in Arctic charr (Salvelinus alpinus)

Arctic charr (Salvelinus alpinus) are a salmonid with potential economic growth in Canadian aquaculture. They are native to the Arctic, and have been an important food source for many years. Arctic charr can be sold for a higher price per kilogram than salmon, due to their delicacy status and niche market. For this reason, Arctic charr farming is becoming more popular. Unlike other salmonids, Arctic charr require a high stocking density to keep aggressive behaviours at bay, and thus many intensive culture-related diseases are easily spread among individuals. Currently, little information is available on the Arctic charr immune system, and their disease susceptibility to common culture-related diseases, such as furunculosis. Furunculosis is a septicemic disease caused by Aeromonas salmonicida, which has two forms: typical and atypical. Both of these diseases have the potential to cause mortality and spread quickly within a population. These infections tend to have acute onsets, with chronic conditions only occurring in adult fish. Previous research has been conducted on culturerelated diseases in Arctic charr, although more information is needed to determine what prophylactic measures would work best to prevent such diseases. The objectives of the current thesis were to: (1) determine the efficacy of two commercial salmonids vaccines against furunculosis infections in Arctic charr; (2) to determine the length of protection acquired by these vaccines; and (3) to characterize the immune responses to vaccination and subsequent A. salmonicida infection.

To answer these questions, an infection trial was designed to determine the Arctic charr immune response to a typical A. salmonicida infection using two different vaccine regimes. Vaccination with the vaccine Forte Micro® (Micro®) showed 72% survival, while vaccination with Forte Micro® + Renogen® (MicroRenogen) showed 83% survival. The presence of A. salmonicida was detected, and results showed significant differences between both vaccinates and a control (Sham -PBS vaccinated) group at 8 days post-infection (dpi) with A. salmonicida. Molecular techniques such as ELISA (Enzyme Linked lmmunosorbent Assay) and RT-qPCR (Reverse Transcriptase quantitative PCR) were used to examine the immune responses to vaccination, and following challenge with a typical A. salmonicida at 8 and 29 dpi. Strong IgM responses, specific to typical A. salmonicida, were detected at 8 dpi in vaccinated fish only. Reverse transcriptase quantitative PCR revealed an emphasis on the acute phase response of innate immunity, and some intracellular responses to vaccination and infection. Co-administering of the vaccines did not negatively impact vaccine efficacy, and could provide a viable option for protection against typical A. salmonicida infections in Arctic charr in aquaculture settings.

During atypical A. salmonicida infections, Arctic charr were either challenged approximately 2-months or 1- year post-vaccination. The Micro® vaccine provided good protection (88% survival) at 2-months post-vaccination in Arctic charr challenged with atypical A. salmonicida, but only 63% survival when challenged 1-year post-vaccination. The MicroRenogen vaccinated fish showed approximately 80% survival in both challenge trials. When Renogen® was administered alone it provided similar protection as the sham control. Presence of infection was evaluated by the presence/absence of A. salmonicida-specific bacterial growth on blood agar. Positive bacterial growth was found in 25% of fish per treatment group at 10 dpi and 13 dpi (short-term vaccine efficacy and long-term vaccine efficacy, respectively), while no detectable levels of bacterial growth were found at 28 dpi in the long-term vaccine efficacy study, and only one Renogen® fish at 23 dpi in the short-term vaccine efficacy study. Circulating IgM antibody responses to A. salmonicida infection were strongest in Micro® vaccinated fish when challenged 2-months post-vaccination, while overall muted IgM responses were observed when challenged 1-year post-vaccination for all groups. Similar to the typical A. salmonicida infection, Arctic charr showed similar results to the circulating IgM antibody responses, and acute phase and intracellular responses to infection. The Micro® vaccine appeared to provide significant protection when fish were challenged 2- months post-vaccination, but not at 1-year post-vaccination and no apparent negative effects on vaccine efficacy were observed when the Micro® and Renogen® were co-administered.

Overall, these data demonstrate the protective effects of two vaccination regimes (Micro® and Micro®+ Renogen ) on Arctic charr at different sizes/age classes to challenge with A. salmonicida subspecies. This has provided a better understanding of the Arctic charr immune response to vaccination and A. salmonicida infection. More work should, however, be done to concentrate on the ideal timing for vaccination, to provide the most protection for these fish. Finally, these vaccines can currently be used to manage A. salmonicida infections in Arctic charr culture situations, but further research is required to determine if protection can be enhanced through different vaccination strategies, such as targeting more specific A. salmonicida antigens, or through coordinating vaccination and harvest, to find the ideal time to vaccinate Arctic charr.