Simulated waterborne transmission of infectious hematopoietic necrosis virus among farmed salmon populations in British Columbia, Canada following a hypothetical virus incursion

Abstract

Infectious hematopoietic necrosis (IHN) virus is a rhabdovirus of significant concern to the salmon aquaculture industry in British Columbia (BC). IHN epidemic events in BC have historically resulted in high mortalities and economic losses among farmed salmon populations. A viral disease management plan (VDMP) was developed by salmon companies in BC in, 2010 and implemented during the 2012 IHN outbreak in BC with a positive outcome; however, to date the effectiveness of VDMP practices in managing IHN epidemics on a coast-wide scale has not been formally evaluated. Therefore, we developed a model to simulate the waterborne incursion and spread of IHN virus into salmon farms across the BC coast. The model was developed using the DTU-DADS-Aqua modelling framework and model processes were designed to reflect the epidemiology of IHN, along with farm management and disease mitigation protocols currently implemented in BC. The model was used to assess effectiveness of the VDMP compared with alternative mitigation approaches. Model results quantified the combined and individual effects of disease surveillance, detection efforts, depopulation measures, and vaccine efficacy in reducing IHN virus transmission. Model outputs indicated the need for use of multiple mitigation measures in combination for a highly effective reduction in spread. Furthermore, the use of an IHN vaccine with high population coverage and efficacy, and the implementation of surveillance zones and pre-emptive depopulation of all net-pens in IHN-infected farms are crucial to contain IHN epidemics. A comparison between simulated model scenarios showed that current VDMP practices were effective in both limiting the spread and mitigating IHN epidemics across the BC coast. The model can be used to inform policy decisions in aquaculture and siting of marine farms, and provides insights to limit the magnitude of IHN epidemics in farmed salmon populations.