Using expert opinion to identify risk factors important to infectious salmon-anemia (ISA) outbreaks on salmon farms in Maine, USA and New Brunswick, Canada

Thirty industry or regulatory professionals, with extensive experience in the local infectious salmon-anemia (ISA) epidemic, were queried on their opinions regarding the spread and impact of ISA in Maine, USA and New Brunswick, Canada. Subjective probability-estimation techniques were used to elicit likelihood ratios (LR) for risk factors of potential relevance to the epidemic. Experts were asked to answer questions based on their direct and local experience with ISA, rather than through knowledge gained from scientific references or experience in other regions. The results found the strongest independent predictors of ISA infection to include (1) a site's proximity to other farms with clinically infected fish, (2) a previous history of ISA on the site, (3) whether a site fallows for a month or more between year classes and (4) whether the site employs harvest vessels practicing full containment of blood and stun water. The strongest predictors of ISA severity included (1) stocking density, (2) the length of time between infection and removal of infected fish, (3) whether fish are moved between pens (after infection) and (4) a farm's sea-lice (Lepeophtheirus salmonis) status. Experts believed that transmission of ISA virus during the local epidemic was influenced by proximity (spatial and temporal) to activities resulting in large-scale shedding of virus into a shared water column, and that severity of infection corresponded more to infected-fish removal practices and certain husbandry decisions. Personnel and equipment biosecurity measures were not seen as strong predictors of either infection or severity in this analysis, though their perceived level of importance was greater among government than industry experts.