Introduction
The Atlantic salmon winter-ulcer disease generates significant volume and value losses across the industry. Functional feed specifically targeting increased skin robustness and recovery from this pathology must be developed.
Prior studies have documented and part-deciphered the distinctive immune functionalities of contrasted inactivated yeast cell walls (YCW) products of different origins (e.g. species, strain, production process), biochemistry and morpho-functionality. In the Atlantic salmon, some YCW typologies promote, e.g., systemic immune-stimulation while others favor the skin immune barrier (Leclercq et al., 2020) or mechanical wound-healing (Leclercq et al., 2021). Based on this prior knowledge, a novel YCW product was pre-selected for its putative mitigation of infectious skin ulceration.
This study tested the capacity of a novel YCW product (nYCW) to mitigate and resolve a realistic Atlantic salmon winter-ulcer challenge combining exposure to a mild pathogen-load by immersion then to aggravating abiotic factors.
Materials and Methods
The trial lasted 7.5 weeks (102 days) using Atlantic salmon post-smolts (93 ± 4 g; flow-through tank; 80 fish/tank; 10.5°C, 22 to 28 ppt) challenged with the “typical” winter-ulcer agent (Moritella viscosa) by immersion at day 39. Abiotic manipulations were applied post-challenge to increase the disease severity or impact (27-day-post-challenge (dpc), temp. drop to 9.5°C; 36-dpc, salinity increase to 30-35 ppt; 63-dpc, 1 min. crowding) then to promote wound-recovery (63-dpc, temp. increase to 11°C).
Two test diets were tested in quadruplicate (n = 4) consisting of a base recipe (15% FM, 7% FO) either non-supplemented (Control) or supplemented pre-extrusion with the test ingredient (nYCW, Lallemand SAS, France) at 4 kg/T. Feed were hand-fed to apparent appetite 4 times daily over the trial’s duration.
Survival profiles post-challenge were compared using a Mantel-Cox log-rank test. At 37, 50 and 63-dpc, all fish were anaesthetized for semi-quantitative wound-scoring based on worst wound-score (0: no wound; 1: focal scale-loss; 2: cutaneous lesion; 3: muscle exposed; 4: muscle exposed over Ø ≥ 1cm; score 2 to 4 marked either “a" (active) or “r” (recovering) based on wound-edge appearance; R: fully recovered wound(s) only).
Proportion of wound-categories were compared between diets within time-points (Kruskal-Wallis with Dunn’s post-hoc tests). Gut and skin histomorphometry, and skin mucus functionality were assessed pre-challenge and at end-point (5 fish/tank/time-point). Data shown as mean ± SEM; significance accepted at P < 0.05.
Results and Discussion
Survival post-challenge (Fig 1) was significantly higher in the nYCW group showing, in particular, higher resilience following abiotic manipulation and crowding. Population mean-wound score was significantly improved in the nYCW group at all time-points (28%, 21%, 34% improvement compared to Control at 37, 50, 63-dpc respectively). At 37-dpc, 37% of fish had at least one open-wound (score 2 or 3) in the nYCW compared to
50% in the Control-group; followed by 28% and 41% respectively at 50-dpc when score 4 were first observed. Recovering wound were also first observed at 50-dpc and were significantly more prevalent in the nYCW compared to the Control (14.5 ± 0.6 % and 7.4 ± 0.8% respectively, not shown) at that time-point.
At the end of the trial (63-dpc), the proportion of fish showing no or only fully recovered wounds (scores 0 and R) was significantly higher in the nYCW compared to the Control (44.9 ± 6.8% and 20.9 ± 2.0% respectively; Fig 2a). Amongst wounded specimens, a higher proportion of fish had at least one recovering wound in the nYCW compared to Control group (51.9 ± 7.2% and 36.6 ± 7.6% respectively; trend, P < 0.10; Fig 2b).
Further data on gut and skin cytoarchitecture and skin mucus properties will be presented.
Conclusion
Combining a mild M. viscosa exposure by immersion with abiotic manipulations induced low mortalities but severe wound development and subsequent wound-recovery, together providing a realistic, commercially relevant winter-ulcer challenge.
Prolonged supplementation with a novel functional ingredient, i.e. 5 weeks prior and across the disease period, mitigated the severity of the winter ulcer outbreak and promoted healing of infectious skin ulcers. This ingredient has the potential to contribute to the preventive management of infectious skin diseases in the aquaculture industry.
References
Leclercq et al., 2020. Dietary supplementation with a specific mannan-rich yeast parietal fraction enhances the gut and skin mucosal barriers of Atlantic salmon (Salmo salar) and reduces its susceptibility to sea lice (Lepeophtheirus salmonis). Aquaculture 495, 1-12.
Leclercq et al., 2020. Yeast-based ingredients as early-promoters of mechanical skin wound healing, a comparative study using a zebrafish model. Poster presentation. Aquaculture Europe 2020; Funchal, Madeira.