Quantitative genetics of vaccine-induced side effects in farmed Atlantic salmon (Salmo salar)

Vaccine-induced side effects (adhesions and melanin deposits) were subjectively evaluated in this study for 150 full-sib families of Atlantic salmon from Salmo Breed AS (year-class 2007). There were four experimental fish groups of which three received a commercial six-component vaccine with an injection volume of 0.1 ml per fish. The groups are described as follows: 1) Pre-smolts kept in freshwater at high temperature (17 °C) and evaluated three months after vaccination (FW); 2) Post-smolts evaluated six months after sea transfer (SW6); 3) Post-smolts evaluated 12 months after sea transfer (SW12); 4) Post-smolts that received an experimental vaccine with a reduced injection volume of 0.05 ml per fish and evaluated six months after sea transfer (SW6-RV). Adhesions between intraperitoneal organs and melanin deposits on internal organs were scored. The FW group had the largest adhesion scores (2.06, SD = 0.49) (P < 0.05) but the smallest melanin scores (0.95, SD = 0.44) (P < 0.05). The SW6 group had greater adhesion scores (1.68, SD = 0.65) than the SW12 group (1.46, SD = 0.56) (P < 0.05), but similar melanin scores (1.49 vs. 1.45; P = 0.12). The SW6-RV group had smaller adhesion scores (0.93 vs. 1.68) and melanin scores (1.04 vs. 1.49) (P < 0.05) than the parallel SW6-group. For the fish groups given the commercial vaccine, the heritability estimates for adhesion scores were 0.31 ± 0.05 (FW), 0.19 ± 0.04 (SW6) and 0.16 ± 0.05 (SW12), and for melanin scores 0.27 ± 0.05 (FW), 0.28 ± 0.05 (SW6) and 0.30 ± 0.05 (SW12). For the SW6-RV group the heritabilities were smaller; 0.08 ± 0.03 (SW6-RV) for adhesion score and 0.11 ± 0.03 (SW6-RV) for melanin score. The genetic correlation between adhesion scores and melanin scores within groups was intermediate for the FW group (0.52 ± 0.11) but greater for the SW6 (0.89 ± 0.06) and SW12 groups (0.87 ± 0.06). The genetic correlation between the SW6 and SW12 groups was large for both adhesion (0.89 ± 0.07) and melanin (0.92 ± 0.11) scores. However, the genetic correlations between FW group and the SW6 and SW12 groups were smaller for adhesions (0.62 ± 0.12 and 0.48 ± 0.14, respectively) and for melanin (0.84 ± 0.08 and 0.61 ± 0.11, respectively). These results show that vaccine-induced side effects can be reduced through selective breeding, but that a reduction can also be achieved by other factors such as improvement of the vaccine.