The effects of intraperitoneal administration of Francisella noatunensis subsp. noatunensis on hepatic intermediary metabolism and indicators of stress in Patagonian blennie Eleginops maclovinus

Francisellosis is a disease produced by Francisella spp. which affects farmed fish. Eleginops maclovinus specimens can be caught close to salmon farming centers, feeding on un-consumed pellet, making the transmission of pathogens such as Francisella noatunensis possible. The aim of this study was to evaluate the effect of F. noatunensis on liver intermediary metabolism in E. maclovinus. 144 fish were injected intraperitoneally with F. noatunensis at a low dose LD (1 × 10¹ cells/μL), medium dose MD (1 × 10⁵ cells/μL), high dose HD (1 × 10¹⁰ cells/μL), or with culture medium C (control), and sampled at 1, 3, 7, 14, 21 and 28 days post injection (dpi). No mortality was recorded during the experimental period, but there was a marked metabolic response in fish injected with high doses. Metabolites in plasma were lowest in the high bacterial dose. Cortisol levels were highest at day 7 in the high dose and then decreased from day 14 until the end of the study. Liver enzymes showed a similar pattern to plasma metabolites, with decreased enzymatic activity, mostly with the high bacteria dose. PK was the exception, with increased enzymatic activity in a dose-dependent manner over time. Liver metabolites were highly variable, except in the high bacterial dose where variability and total levels decreased significantly. Our results show that fish infection with F. noatunensis induces a clear stress response, especially with at the highest dose, shifting intermediary metabolism towards mobilization of energy and suggesting that E. maclovinus detects experimental infection of F. noatunensis as a stressor, which it is dependent on the bacterial dose.