A method to measure an indicator of viraemia in Atlantic salmon using a reporter cell line

•Luciferase is induced in RTG-P1 cells when infected with salmonid alphavirus.•Viable infectious particles can be quantified in the serum collected from Atlantic salmon infected by cohabitation with salmonid alphavirus.•This novel method designed to measure viraemia in salmonids fish is not dependent on development of CPE.

RTG-P1 is a transgenic fish cell line producing luciferase under the control of the IFN-induced Mx rainbow trout gene promoter. This cell line was used to measure viraemia of Salmonid alphavirus (SAV), the cause of Salmon Pancreas Disease (SPD), a serious disease in farmed Atlantic salmon. Two SAV genotype 1 (SAV1) isolates were used in this study, F93-125 (tissue-culture adapted, from Ireland) and 4640 (from a field case in Scotland). The kinetics and magnitude of luciferase activity were monitored versus the time of infection. During a direct infection experiment, the induction of luciferase significantly increased 16- and 4-fold after incubation for 6 days with F93-125 at 15 and 20 °C, respectively. Filtration and heat treatment experiments demonstrated that the luciferase induction in RTG-P1 was dependent on viral replication. Unlike many cell lines used in fish viral diagnostic, RTG-P1 is not sensitive to salmonid serum, therefore, viraemia could be successfully monitored on serum collected from fish during a cohabitation challenge with 4640 isolate. A peak of viraemia could be detected 16 days post IP inoculation of the shedders. This novel cost-effective method to measure viraemia does not rely on development of cytopathic effect (CPE) in culture, is compatible with non-lethal blood collections in fish and can be used to assign emerging diseases to a viral aetiology.