Piscirickettsia salmonis , the etiological agent of Salmonid Rickettsial Septicaemia (SRS), poses a major threat to salmonid aquaculture. This study aimed to evaluate the effects of different dietary inclusion levels of a thymol-based additive (TBA) on mortality rates and disease progression in post-smolt salmon following a cohabitation challenge with P. salmonis.
One hundred and two Atlantic salmon ( Salmo salar) post-smolts ( 141.47 ± 26.28 g) were divided into nine tanks assigned to three experimental groups: 1) Basal diet (CTR); 2) Basal diet + 0.10% TBA (Diet 0.1); and 3) Basal diet + 0.20% TBA (Diet 0.2) for a 31-day feeding period at a daily ration of 1.15% biomass (17 ind/tank, n = 3) . Then at the end of the feeding trial, salmons were individually marked with colored elastomer for identification, and transferred to a different facility where cohabitation challenge to P. salmonis was carried out. Seeder fish were intraperitoneally infected with 0.1 ml of solution containing 6x107 field LF-89 inoculum and released in the tanks . Sampling was carried out to assess performance parameters, feed utilization, mortality and clinical signs. Environmental parameters were kept in the optimum levels for this species (O 2 = 7.96 – 8. 69 mg/L, dissolved O2 85.6 – 97.7 % and Temp. 14.2 – 14.6 °C).
The challenge lasted 55 days and cumulative mortality of shedders fish reached 100% within 13 days post-inoculation, confirming the high virulence of the field LF-89 bacterial inoculum . Clinical signs of SRS in death fish were monitored along the trial : gills pallor, ventral ecchymosis, hemorrhagic anus, and symptoms correlated to systemic septicemia as presence hepatomegaly, splenomegaly, and granulomas in liver and kidney. Among cohabitant fish, cumulative survival was lowest in CTR group at the end of the trial , followed by Diet 0.1 and Diet 0.2 having the highest average survival performance with +2.83% and +16.90% increase in R elative P ercent Survival (RPS) .
The lower mortality rates and presence of survivors of the fish fed with diets enriched with TBA indicate that dietary inclusion of functional ingredients may enhance resilience and support survival to P. salmonis infection in Atlantic salmon under controlled challenge conditions. Further studies are recommended to elucidate underlying immunomodulatory mechanisms and assess performance under field conditions.