Epidemiological aspects of infectious salmon anemia

Sammanfattning: Infectious salmon anemia is an orthomyxyviral disease posing a serious challenge to the aquaculture of atlantic salmon (Salmo salar). The virus causes severe disease resulting in high moratlity and significant costs to the industry. Several factors, natural and human induced, have been studied as to their proposed role in the spread of the ISA-virus and its impact on salmon farming. Spread by equipment and fish movement is well substantiated. Equally substantial is the impact of geographical relations between farms, processing boat lanes, processing plants and other farms. However, the evidence for the proposed direct waterborne spread, that this often is attributed to, is inconclusive and perhaps even negligible in a natural setting. This is because the virion’s ability to survive in biologically active and UV-radiated water might not allow waterborne spread over long distances. There is also evidence suggesting that the general health of a population of salmon affects the risk of outbreaks and that immunosuppression due to chronic stress induced by high stocking densities, routine vaccination via injection and major sea lice infestation might increase susceptibility to the disease. Much like mammalian and avian influenza (other orthomyxoviral agents) there is a substantial difference in the pathogenic potential between different strains, with some producing no clinical disease while others are deadly due to minor changes in the viral genome. The non-pathogenic strains, causing no clinical disease, was in one study found to be highly prevalent and have been shown to be potential ancestors to pathogenic strains, and as such might be central to the disease’s epidemiology. Wild salmonids have been shown to be potential symptomless carriers of the virus and sea lice have under experimental conditions acted as mechanical vectors; suggesting a complex epidemiology that might be dependent on the combined prevalence of pathogenic and non-pathogenic strains as well as an unknown role of natural reservoirs and vectors in the spread of the disease.