Infectious salmon anaemia (ISA) risk factors in sea-cultured Atlantic salmon Salmo salar

A matched case-control study of infectious salmon anaemia (ISA) risk factors in Norwegian salmonid sea sites was performed in 1993. The distribution of potential exposure factors associated with the site management and location was compared in 2 paired groups of sea sites, one group comprising 37 ISA-positive sites and the other 37 ISA-negative sites. The risk of ISA was found to be significantly associated with the location of the site. Location within 5 km from a salmonid slaughterhouse 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 the 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 with 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 through sea water. Further disease control measures should concentrate on minimising the risk of transmission through sea water by shortening the time period between the diagnosis of ISA and the elimination of positive sites, and should work towards the establishment of 5 km as a minimum distance between sea sites. In addition, decontamination systems must be systematically introduced into the fish processing industry. The implementation of good sanitary practices by fish farmers may also reduce the risk of ISA.