IMPACT OF WATER TEMPERATURE ON THE SEA BASS (D. LABRAX) RESPONSIVENESS TO IN FIELD VACCINATION AGAINST VIBRIOSIS/PHOTOBACTERIOSIS

Routinely sea bass vaccination programs against Vibriosis/Photobacteriosis include the immersion of fish at 1–2 g in aqueous vaccine prior to transportation to floating cages, followed by an intraperitoneal (ip) administration of water-oil based vaccine at 20–25 g, which usually coincides with fish sizing. The most suitable period for the latter treatment would range from spring to summer, considering that the best antibody response both in terms of level and rapidity is detectable in sea bass immunized at 24°C (Cecchini & Saroglia, 2002). Still the time required to vaccinate all the farmed stocks sometimes exceeds the optimal period, and vaccination has to be carried even during winter season. For this reason, the objective of the present investigation was to explore the opportunity to get effective immune responses after a vaccination at sub-optimal temperatures (14°-16°C). The efficacy of the bivalent (Vibrio-Photobacterium) commercial vaccine AlphaJect 2000™ (Pharmaq AS) was assessed in the field (CROMARIS, Mala Lamjana Croatia), by submitting cage reared sea bass (size range 80-145 g) to sedation and ip vaccination (0.1 ml/fish) at different temperature ranges during autumn/winter/summer 2017-2018. Serum sampling was performed in each cage at T0 (intended as pre-vaccination control) and at 500° day post vaccination (this could vary between 32 and 38 days post vaccination, depending on the water T°). Parallel samplings were performed also in cages not submitted to vaccination. In detail, 30 randomly selected individuals/cage/sampling time were submitted to sedation and then sampled in order to obtain the serum. The specific antibodies (IgM) raised against one of the two target antigens (V. anguillarum O1), being part of the formulation, were determined by E.L.I.S.A. accordingly to the protocol of Galeotti et al., (2013). As far as we know there are not information on the vaccination effectiveness in sea bass against bacterial diseases (such as Vibriosis or Photobacteriosis) at different temperatures, and the present investigation represented a preliminary approach to this issue. The specific IgM titration performed on pre-vaccination samples showed their negativity (O.D. values often lower than those recorded in the “blank” wells, or negligible). The i.p. vaccination performed in winter months at temperatures near to 14-16°C triggered a synthesis of specific IgM to V. anguillarum O1 only in a limited number of individuals among those vaccinated, highlighting that there is an evident individual variability in terms of response to vaccination. If we exclude the limited role ascribable to non-appropriate vaccine provision, based on our findings we can assert that there is a relevant difference in the individual “immune efficiency” at low temperatures, possibly limiting the uniformity of fish batches responsiveness after the treatment. On the contrary the ip vaccination performed in summer, at temperatures ranging between 21,4-23,8 (average 22,6) promoted a remarkable synthesis of specific IgM to V. anguillarum O1 in almost all the fish under study, suggesting the requirement of specific (optimal) temperatures enabling fish to benefit from the immunization treatment