Research on Pancreas Disease in Irish Farmed Salmon 2004/2005 – Current and Future Initiatives

Pancreas Disease is the most significant single infectious disease affecting marine salmon farms in Ireland. The first epidemiological studies of Pancreas Disease (PD) in Ireland in the early 1990’s indicated that significant losses occurred in farmed Atlantic salmon in their first year at sea on some individual farms (Menzies et al., 1996). A serological survey for the presence of Salmon Pancreas Disease Virus (SPDV) antibody in 1996 revealed that 53% of Irish sites were positive, but that not all positive sites had recognised clinical signs of PD. This indicated a relatively low incidence and severity of PD at that time (McLoughlin et al., 1998) and this pattern persisted throughout the remainder of the 1990’s. In 2002, a serious increase in both the incidence and severity of PD was reported on farmed Atlantic salmon marine sites in Ireland. An epidemiological survey of Irish sites for 2002 revealed that 59% of sites reported an outbreak of PD, with mortalities averaging 12% (range 1 – 42%; McLoughlin et al., 2003). A recent study using data collected for 2003 and 2004 indicated that PD occurred in 62% and 86% of sites respectively. The average mortality due to PD on affected farms was 18.8% in 2003 and 14.8% in 2004. In response to these significant losses due to PD a range of research initiatives was launched in Ireland and was supported by the Marine Institute. This document gives an overview of the most recent findings from the current projects, carried out in 2004/2005 and also provides an overview of the various actions in Scotland and Norway, where PD has also become a disease of economic significance in salmon aquaculture.Funder: Marine Institut

Virological, serological and histopathological evaluation of fish strain susceptibility to experimental infection with salmonid alphavirus

Pancreas disease (PD) of farmed Atlantic salmon Salmo salar L., which is caused by an alphavirus known as salmon pancreas disease virus (SPDV), can have serious economic consequences. An epidemiological survey carried out in Ireland in 2003 indicated that within individual farms there were significant differences in the susceptibility of different strains of farmed Atlantic salmon to infection with SPDV, as measured by levels of clinical disease and mortality. The aim of this preliminary study was to investigate this field observation by comparing lesion development, viraemia and serological responses of 3 commercial strains of Atlantic salmon (A, B and C) experimentally infected with SPDV. Highly significant differences in the severity of lesions in the pancreas at Day 21 post-infection (pi) were detected (p < 0.01), with Group B being more severely affected. There were also significant differences in the prevalence and severity of lesions in heart and skeletal muscle at Day 21 and 35 pi respectively, with Group B results again significantly higher than those from both Groups A and C (p < 0.05). There was no overlap between viraemia and the presence of specific SPDV antibody. Some fish in all groups had no viraemia, lesions or evidence of seroconversion. There were no significant differences seen between the challenged groups in relation to the percentage of viraemic fish at each time point. Viral loads were not determined. Differences between the number of antibody-positive fish in each challenge group were found at Days 28 and 35 pi (p < 0.1). Highly significant differences (p < 0.01) in the geometric mean titres of seropositive fish were detected at Day 28. These results, obtained using a challenge model, confirm that there are strain differences in the susceptibility to experimental SPDV infection in commercial farmed Atlantic salmon