Using a Markov-Chain Monte-Carlo modelling approach to identify the relative risk to farmed Scottish Rainbow trout (Oncorhynchus mykiss) in a multi-sector industry of Viral Haemorrhagic Septicaemia Viruses from introduction and emergent sources

Abstract

The risks of introduction of Viral Haemorrhagic Septicaemia Virus (VHSV) into farmed Scottish Rainbow trout (Oncorhynchus mykiss) was simulated using a simple but robust Markov-Chain Monte-Carlo (MCMC) modelling approach. Outputs from the models were subjected to sensitivity analysis to investigate the contribution towards these risks to our parameter assumptions. The aim was to identify the factors whereby Viral Haemorrhagic Septicaemia (VHS) outbreaks are likely to be most sensitive to and thereby most likely susceptible to control against VHSV genotype 1a (G1a) from continental Europe versus the emergence of VHSV genotype 1b (G1b) or VHSV genotype 3 (G3) from within native marine fish populations. Seven scenarios were tested for the three VHSV genotypes that represent different assumptions as to the epidemiology of VHS, different environmental conditions and or possible future expansion of aquaculture. Results from the MCMC model outputs and sensitivity analysis confirm that the greatest risk to fresh water rainbow trout are from VHSV G1a and G3 for marine rainbow trout. Sensitivity analysis revealed that probabilities of introduction and persistence of VHSV G1a in the freshwater environment and freshwater trout were the parameters that had the most significant effect on the model outputs. Atlantic salmon (Salmo salar) was not significantly affected in any of the scenarios and so salmon-associated parameters play very little role in the risk to freshwater rainbow trout. The scenarios are not intended to be absolute but are the best we have for exploring a range of assumptions that may put Scottish freshwater rainbow trout at risk of VHS.