Studies on the possibility of vertical transmission of Norwegian salmonid Alphavirus in production of Atlantic salmon in Norway.

Disease associated with salmonid Alphavirus (SAV) infection is a significant problem for farm production of salmonids in Europe. The SAV subtype 3 (SAV3) is a Norwegian subtype present exclusively in production systems for Atlantic salmon Salmo salar and rainbow trout Oncorhynchus mykiss in western Norway. It has been suggested that SAV3 is transmitted through smolt transport from the main area for SAV disease in western Norway to as far as northern Norway. One explanation for this type of spread is that SAV is present at freshwater production sites for Atlantic salmon smolts. The present study confirms this, showing that SAV3 is present at smolt production sites in Norway. At two sites in northern Norway that had received eggs from broodfish companies in Hordaland County, western Norway, 2-4-g fry were positive for SAV3. Hence, it cannot be excluded that vertical transmission could have contributed to the presence of SAV3 in northern Norway. In the present study, we followed the normal production cycle for Atlantic salmon in a fish farming company in Hordaland County. Twelve of 353 broodfish in study 1 and 28 of 31 broodfish in study 2 were found to be carriers of SAV3. In the same two studies, SAV was also detected in eggs (1 of 220), eyed eggs (3 of 270), and fry (6 of 600). The SAV was not detected in parr, smolts, or postsmolts, but after a year at sea the fish developed SAV disease. Given the difficulties in tracing the virus through the production cycle until development of SAV disease in the marine farm, we cannot draw any firm conclusions about whether vertical transmission occurs in Norwegian salmon production, and we cannot exclude the possibility that the development of SAV after 1 year at sea was caused by horizontal transmission rather than vertical transmission.