Use of Biomarkers in Plasma and Faeces to Evaluate Physiological Status of Atlantic Salmon (<i>Salmo salar</i> 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.