Salmon rickettsial septicaemia (SRS), caused by the aetiological agent Piscirickettsia salmonsis, has been the most important infectious disease in the Chilean salmon industry since the 1980’s, costing >3 00 million USD per annum . Despite several initiatives, the industry is still heavily reliant on antibiotic usage to treat SRS. For example, i n 2022, 458.6 g of antibiotics were consumed for every ton of Atlantic salmon produced, although this was slightly down compared to 2021. This study was set out t o evaluate the efficacy of an enhanced acidifier against P. salmonsis (EM-90 Like/ geno group A) in post–smolt Atlantic salmon (Salmo salar), in a cohabitation challenge . Three hundred and thirty-six Atlantic s almon were equally distributed into eight tanks (42 fish/ tank), and randomly split into two dietary treatments (n = 4); Diet 1 (control) and Diet 2 ( control + enhanced acidifier at 2 g kg-1 ). Fish were acclimated for 18 days before an eight week feeding period where growth performance parameters (feed intake, SFR, weight gain, SGR, TGC) were assessed . After the feeding period, 15 fish per tank were randomly selected as ‘shedder’ fish, tagged and inoculated with a P. salmonsis challenge via intraperitoneal injection (0.2 ml of defined dose, 8.6 x 107 CFU ml-1), before reintroduction into their original tanks. Afterwards, all fish received control feed for the first 10 days to avoid shedder fish access to the feed additive, before reverting to their original feeding regime (Diet 1 or Diet 2), for 55 additional days. After 65 days post infection, the mortality of fish receiving the enhanced acidifier (Diet 2) was significantly reduced (55.0 ± 7.9% vs 72.2 ± 13.9%, respectively ; P = 0.0064 ). The relative percentage of survival (RPS) at the end of the challenge was 23.8 and the hazard ratio (Mantel-Haenszel) of Diet 2 was 0.604 , compared to Diet 1. These data indicated that the probability for a mortality event in salmon is 39.6% lower in fish fed with the enhanced acidifier, compared to those fed diets without the additive. The benefits observed were not at the expense of growth performance, which remained unchanged during the feeding trial ( P > 0.05). Compared with commercial trials, t hese results demonstrate that this enhanced acidifier is not only a useful nutritional tool to improve salmon health and welfare , whilst reducing the reliance on antibiotics.
Keywords: Piscirickettsia salmonis , SRS , salmon , enhanced acidifier, sustainability
Topic : Feed additives and functional feeds