Eliciting expert judgements to estimate risk and protective factors for Piscirickettsiosis in Chilean salmon farming

Global production of farmed salmon is increasingly threatened by emerging infectious diseases. Piscirickettsiosis or Salmonid Rickettsial Septicemia (SRS), a disease caused by the bacterium Piscirickettsia salmonis, is the major responsible for the overall disease-specific mortalities in the Chilean salmon industry. In this study, we applied a structured expert elicitation process to identify risk and protective factors associated with severe outbreaks of SRS during a production cycle. We used a qualitative based-expert approach to calculate risk estimators for ten risk factors and seven protective factors. In the expert elicitation process, each participant independently estimated factors in two rounds. Between the first and second round, we facilitated a workshop among experts to discuss preliminary results. As a result, the inter-expert variation of the experts' estimates was systematically reduced. Our results are concordant with preliminary studies about risk factors for SRS. Importantly, we identified novel factors that may be associated to an increased risk for severe SRS outbreaks, such as fouling of cages, density of farms in neighborhoods, presence of sea lions and early mature salmonids. Novel factors that appear to reduce the risk of severe outbreaks of SRS were synchronized farm rest periods, opportune diagnosis and necropsy training. Using these results, we propose an intervention model to provide better information for strategic decision making.