INFECTIOUS SALMON ANEMIA (ISA) RISK-FACTORS IN SEA-CULTURED ATLANTIC SALMON SALMO-SALAR

A matched case-control study of infectious salmon anaemia (ISA) risk f actors in Norwegian salmonid sea sites was performed in 1993. The dist ribution of potential exposure factors associated with the site manage ment and location was compared in 2 paired groups of sea sites, one gr oup comprising 37 ISA-positive sites and the other 37 ISA-negative sit es. The risk of ISA was found to be significantly associated with the location of the site. Location within 5 km from a salmonid slaughterho use gave an ISA odds ratio of 13.0 compared to location further away. The risk of infection increased by 8.0 if the site was situated closer than 5 km to another ISA-positive site as compared to the risk when t he site was more than 5 km away. Disinfecting the waste water from the slaughtering and processing plants seemed to prevent transmission of ISA. The density of fish markets for sea-caught fish was higher in the vicinity of cases than of controls. The risk of ISA was associated wi th the number of hatcheries delivering smelt to the sea sites, and the risk increased if the hatcheries were located outside the site's home county. The overall results from the present study indicate that ISA is mainly transmitted from infected salmonid sources to clean sites th rough sea water. Further disease control measures should concentrate o n minimising the risk of transmission through sea water by shortening the time period between the diagnosis of ISA and the elimination of po sitive sites, and should work towards the establishment of 5 km as a m inimum distance between sea sites. In addition, decontamination system s must be systematically introduced into the fish processing industry. The implementation of good sanitary practices by fish farmers may als o reduce the risk of ISA.