Epidemiological Investigation of Infectious Salmon Anaemia (ISA) Outbreaks in Norway 2003-2005

Epidemiological information was summarized from 32 outbreaks of infectious salmon anaemia (ISA) in Norway 2003-2005.

Details about ISA outbreaks in 2005 are presented in appendix I in this report. The National Veterinary Institute has earlier summarised case reports about ISA outbreaks in 2003 and 2004 (Lyngstad TM et al., 2005a; Lyngstad TM et al., 2005b).

Virus isolates from the outbreaks in 2003-2005 were genotyped, and postulated associations between outbreaks due to risk factors were assessed. The ISA outbreaks were distributed along most of the Norwegian coast and showed a variable clinical picture. The virus genotypes clustered into three genogroups, and tended to scatter in time and along the coast. ISA outbreaks matched for the risk factors ”proximity” or ”contact”, shared a significantly higher number of mutual genogroups than expected. For the risk factor ”smolt supplier”, corresponding genogroups appeared in seven out of 12 matched pairs, which was not significant.

In conclusion, genotyping of virus isolates from ISA outbreaks supports associations between adjacent outbreaks. This is consistent with horizontal transmission. The present study failed to find patterns of genogroups related to smolt suppliers or brood fish companies.