Abstract
A challenge model for pancreas disease in A tlantic salmon, S almo salar L . fry, was developed comparing two salmonid alphavirus ( SAV ) subtypes: SAV 1 and SAV 5. Viral doses of 3 × 10 5 TCID 50 mL −1 for SAV 1 and 3 × 10 4 for SAV 5 were tested in triplicate tanks, each containing 450 salmon fry. Cumulative mortalities of 1.2% were recorded. Titres of virus recovered from the mortalities ranged from 10 2 to 10 7 TCID 50 mL −1 . Fry were sampled at 3, 5 and 7.5 weeks post‐challenge. Sampling after 3 weeks revealed a high prevalence of infection in the absence of clinical signs, and infectious virus was recovered from 80% and 43% of sampled fry infected with SAV 1 and SAV 5, respectively. After 5 weeks pancreas, heart and red skeletal muscle lesions were generally observed, whilst degeneration in white skeletal muscle was observed only in fish infected with SAV 1. In situ hybridisation confirmed the presence of viral genome in infected pancreas, heart and muscle. After 7.5 weeks, infectious virus (both isolates) was recovered from 13.3% of the fish sampled, with a viral titre of 10 2 TCID 50 mL −1 . Clearly, salmon fry are susceptible to SAV infection and pancreas disease.