Use of Biomarkers in Plasma and Faeces to Evaluate Physiological Status of Atlantic Salmon (Salmo salar L.) During Salmonid Alphavirus Infection.

Salmonid alphavirus (SAV) is the causative agent of pancreas disease (PD), a disease that can cause severe implications for marine farming of Atlantic salmon. This study examines physiological changes in Atlantic salmon during SAV infection through a controlled trial and two field trials. In the controlled trial, plasma creatine kinase (CK), alanine aminotransferase (ALAT) and aspartate aminotransferase (ASAT) levels increased significantly 4 weeks post challenge, peaked at 8 weeks and by 12 weeks, ALAT levels returned to pre-challenge levels, while CK and ASAT remained elevated. Weekly mortality peaked at 4.1% in week 5, and there was an 89% reduction in appetite in week 4 post challenge. In two field trials in southern Norway, SAV3 was detected in spring/early summer by reverse transcription quantitative polymerase chain reaction (RT-qPCR). As SAV prevalence increased, plasma CK, ALAT, ASAT and lactate dehydrogenase (LDH) levels peaked, along with the stress markers cortisol and tetrahydrocortisone in faeces. Despite low mortality (< 0.2% weekly), a decrease in appetite was observed in both field trials. Understanding the physiological status of the fish through monitoring tools like RT-qPCR, blood chemistry and stress markers provides a foundation for implementing mitigation measures such as stress reduction, timed harvest or clinical nutrition at the onset of pancreas disease.