The role of fish movements and the spread of infectious salmon anemia virus (ISAV) in Chile, 2007–2009

Abstract

Infectious salmon anemia virus (ISAV) infection is a constant major threat to farmed and wild Atlantic salmon worldwide. Many epidemics have recently been reported in the most important salmon farming regions of the world, including Chile (2007–2009), where ISAV generated the most important disease and economic crisis in history of the salmon industry of the country. The spread of ISAV within a region is most likely by local or neighborhood spread from an infected farm; however, there is evidence that anthropogenic activities, such as movement of live or harvested fish or their byproduct, may have played a more important role than environmental or passive transmission in the 2007–2009 outbreak. Atlantic salmon farms (n=421) were retrospectively followed from stocking to harvesting in southern Chile at the time of the ISAV epidemic (2007–2009). The effect of husbandry and spatial risk factors, in addition to contact-network risk factors, which were obtained from the social network analyses, on time to first ISAV infection was estimated using a multivariable Cox proportional hazards model. Five variables were retained in the final fitted model: co-existing multiple generations on a farm (hazard ratio [HR]=2.585), mean smolt weight at stocking greater than 120g (HR=1.165), farm area (perkm2) (HR=1.005), and increased number of shipments entering a farm, i.e. the farm input degree (HR=1.876) were associated with reduced time to infection; whereas time-to-infection was longer for farms located farther from an ongoing ISAV outbreak (HR=0.943). It was demonstrated that movements of latently infected fish resulted in approximately 7 outbreaks, and potentially explain about 6% of the total number of cases during the epidemic. Results from this study provide new information about the mechanisms of spread of ISAV in one the largest documented ISAV epidemics in the world. Findings may be used to support the design and implementation of risk-based surveillance and control programs that may help to prevent, detect and control future ISAV outbreaks.