Salmon pancreas disease (SPD) is one of the most significant diseases affecting marine salmon farming in Europe. This study describes development of a bath challenge model with SPD virus in Atlantic salmon at the fry stage (average weight of 1.2g). Two salmonid alphavirus (SAV) genogroups were used: SAV 1 and SAV 5 at two viral doses: 106 and 105 TCID50/ml for SAV 1 and 104 and 103 for SAV 5 in duplicate tanks. Tanks of salmon fry challenged with either genogroup showed very low cumulative percent mortalities of 1.2% or less. Virus was recovered from the majority of mortalities (24 of 30 deaths) which took place between 11 to 34 days post challenge. Titres of virus recovered from the mortalities ranged from 102 to 107 TCID50/ml. Histopathological changes consistent with pancreas disease pathology were observed in moribund fish sacrificed and analysed.
Fry were sampled at 3, 5 and 7.5 weeks post challenge from one of the higher dose replicate tanks for each isolate tested. Between 15 and 30 fish on each sampling date were processed for virus isolation by inoculation onto CHSE-214 cells followed by specific RT-PCR confirmation of cells showing cytophatic effect. Between 15 and 20 fish of each sample were also assessed for histopathology and localisation of virus by in situ hybridisation. We observed a high prevelance of infection in fry in the absence of clinical signs and a general decrease in the proportion of positive fish within the samples as time post challenge increased. For SAV1, 80% of sampled fish were virus positive at 3 weeks post challenge reducing to 60% at 5 weeks and 13.3% for at 7.5 weeks post challenge.
The presence and localisation of viral genome was confirmed in sampled fish by in situ hybridisation (ISH) Presence was most easily identified in skeletal muscle. Heart showed a low level of labelling. Pancreas was difficult to study by ISH due to the infected fish showing dramatic pathology and loss of pancreatic tissues. Liver showed strong labelling but with high background in negative controls and strong labelling was also detected in the lamina propria of the intestine.
