Risk factors for pancreas disease (PD) outbreaks in farmed Atlantic salmon and rainbow trout in Norway during 2003-2007.

Pancreas disease (PD) is an emerging infectious disease in farmed Atlantic salmon (Salmo salar L.) and rainbow trout (Oncorhynchus mykiss Walbaum) caused by salmonid alphavirus (SAV). The present study is a large scale study aiming at quantifying the probability of contracting PD in farmed salmonid cohorts in Norway due to exposure to risk factors that may be associated with specific transmission pathways for SAV, or may increase a cohort's susceptibility to PD. Monthly reports of numbers of fish and mean fish weight from all marine salmonid farm sites in Norway were used to identify cohorts of farmed salmonids. Only cohorts that were initiated and terminated during 2003-2007 were assembled for the study. Records of clinical diagnosis of PD on marine farm sites were used to identify PD case cohorts. In PD case cohorts, PD-outbreaks were defined to start the month the diagnosis was recorded and last until the cohort was terminated. All cohorts in which PD was not recorded were assigned to the control-class. In total 143 PD case cohorts and 1079 control cohorts were assembled. Risk factors were assigned to the cohorts and analysed using logistic regression by generalized additive models (GAM). We find that infection pressure, a variable designed to capture the potential for local disease spread, has a strong effect on the probability of recording a PD-outbreak in a cohort. The function describing the effect of infection pressure increased steeply as infection pressure increased from 0 to moderate values corresponding to having a mean sized neighbouring fish stock with PD at a distance of 2 km, after which the function levelled off. The study emphasises horizontal transmission pathways as important for the spread of PD in Norwegian salmon farming, and accordingly that bio-security measures aimed at controlling horizontal transmission are necessary in order to reduce the number of outbreaks of PD.