EPIDEMIOLOGICAL MODELLING OF WATERBORNE SPREAD OF INFECTIOUS HEMATOPOIETIC NECROSIS VIRUS IN FARMED SALMON POPULATIONS

Introduction

 Three  high-mortality epidemics of infectious he matopoietic necrosis (IHN)  occurred  in  farmed  salmon populations in British Columbia (BC), Canada from 1992 to 1996 (St-Hilaire et al., 2002), 2001 to 2003 (Saksida, 2006), and in 2012 (Garver & Wade, 2017) .  Wild Pacific salmon were hypothesized to be the source of  IHN virus to farmed populations (Saksida, 2006; Garver & Wade, 2017). Since the 2012 epidemic, IHN has been well controlled  through vaccination on salmon farms . In 2010, a V iral D isease Management (VDMP) was developed  by BC salmon companies to rapidly and effectively minimize infection transmission and prevent spread to more distant  Management Zones (MZ);  however, to date the  effectiveness of VDMP practices in mitigating IHN epidemics has not been evaluated

 .  We used an epidemiological simulation model to evaluate a hypothetical,  waterborne spread of IHN virus among a susceptible population of farmed salmon in BC, after initial introduction into a single  farm site.  The purpose  of  the modelling was to evaluate existing viral management practices and  to inform  decision-making of  regulatory agencies about  spatial planning of future marine sites.

 Material and methods

The simulation model  (Romero et al. 2021)  is spatially-explicit  and  allows modelling of 3 components of waterborne spread of IHNv in marine sites: within net-pens (a SEIR model as used for COVID-19 in humans), among net-pens (based on infection prevalence and a user-specified probability of transmission), and spread to distant sites, the probability of which decreases with increasing seaway distance.  For distances  of  >15km, there is almost zero probability of  waterborne  IHNv spread  between sites  (Foreman et al. 2015).  Data used to inform the model were production and location details of all  84 active  farm sites in 2019, distributed across 10 MZs in BC. Expert opinion was used for number of net-pens per site, number of fish per net-pen , baseline daily fish mortality at the net-pen level, and mitigation measures for IHN.  For each of the 10 MZs, different scenarios were explored to evaluate the combined effectiveness of disease surveillance and detection, depopulation procedures and vaccine use in reducing IHNv transmission.  For every scenario , a single net-pen was selected as the infection source (“index”) for the simulated IHNv incursion. This index pen was randomly sampled from the  farm site with the lowest average seaway distance to other sites in the same MZ.

Results

The simulated waterborne IHN transmission is highly influenced by the mitigation measures implemented and the spatial distribution of farms within a MZ. IHN spreads mostly to farms closer to the “index”, as lower seaway distances between farms corresponds to a higher probability of waterborne transmission (Figure 1). IHN spread varies between different MZs, as a consequence of the spatial distribution of farm sites within the MZ (Figure 2); the between-farm spread of IHN increases with the level of spatial clustering of farm sites within the MZ.

Discussion

 The  IHNv  model  demonstrated the ability to  describe spatio -temporal patterns of IHN epidemics and assess the effectiveness of different mitigation strategies. This  can be adapted to other aquatic-farmed  species  and  pathogen  spread in seawater by adjust ing  the open-source modelling framework (https://github.com/upei-aqua/DTU-DADS-Aqua). Particle tracking data from numerical circulation models (e.g., FVCOM) can be incorporat ed in the model to  provide a stronger basis for assessing hydroconnectivity between sites.

References

Foreman MG et al. (2015). Modelling Infectious Hematopoietic Necrosis Virus dispersion from marine salmon farms in the Discovery Islands, British Columbia, Canada. PLoS ONE 10(6): e0130951.

Garver, K., and Wade, J. (2017). Characterization of Infectious Hematopoietic Necrosis Virus (IHNV). DFO Can. Sci. Advis. Sec. Res. Doc. 2017/073. vi + 32 p.

Romero, J.F. et al. (2021). DTU-DADS-Aqua: a simulation framework for modelling waterborne spread of highly infectious pathogens in marine aquaculture. Transbound. Emerg. Dis.(in press).

Saksida, S.M. (2006). Infectious haematopoietic necrosis epidemic (2001 to 2003) in farmed Atlantic salmon in British Columbia. Dis. Aquat. Org, 72(3), 213–223.

St-Hilaire, S. (2002). Epidemiological investigation of infectious hematopoietic necrosis virus in salt water net-pen reared Atlantic salmon in BC, Canada. Aquaculture, 212(1), 49–67.

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