Scottish Marine and Freshwater Science Vol 1 No 4

An outbreak of infectious salmon anaemia (ISA) in the Scottish Shetland Islands during 2008/9 is described during which six sites were confirmed ISAV positive. Spread of the virus via movement of fish between marine sites, harvest vessels, movements of smolts and wild fish appear to have been of little or no importance. The spread is associated with hydrodynamic currents, although local intra-company activity may have caused some spread. The application of a statutory control strategy by Marine Scotland, based on the use of its established model (Anon 2000) has apparently limited the occurrence and economic impact of ISA to management area 3a; however spread within this area has been extensive. This localised waterborne spread is in contradiction to a previous outbreak in 1998/9 which was spread over a wide geographic area by transport of fish and harvest vessels. The development of industry codes of practice and good biosecurity procedures, following the 1998/9 outbreak, that limited marine site-to-site movement of live fish and improved disinfection of vessels and processing plant waste, may explain why the 2008/9 spread of infection was localised. Depopulation of confirmed sites is a key element of eradication and this was achieved within 7 weeks of confirmation, although the last confirmed case suggests subclinical infection may persist undetected for months. The potential sources of ISAV infection that were investigated did not determine the origin of the 2008/9 outbreak. Local evolution from an avirulent strain of ISAV; importation of ova; or association with movement of equipment could have caused the outbreak. The virus responsible for the 2008/9 outbreak belongs to a different genogroup (group 1) to the 1998/9 virus (group 3). The intensive cultivation of salmon farming, close proximity of sites and historic absence of synchronous fallowing of management areas is considered to have increased the risk of disease outbreaks and their re-emergence, such as ISA in the Shetland Islands. A policy of synchronous fallowing and stocking of sites within management area 3a is being considered by industry in consultation with Marine Scotland to address this risk. Where movement of fish occurs between sites in 1 different management areas, this represents the greatest risk of regional-scale spread of diseases such as ISA. Controls appear to have been effective in minimising that risk.