Introduction
Infectious diseases pose a major threat to aquaculture sustainability. Once introduced, aquatic diseases can propagate rapidly through highly connected aquaculture sites and cause substantial production losses due to increased mortalities. Surveillance and control measures such as contact tracing, culling and fallowing are vital for preventing this spread and mitigating impacts on both the industry and the environment. However, the epidemiological and economic impact of these activities may differ depending on the characteristics of the controlled disease. By improving our understanding of specific disease dynamics within aquaculture networks, control strategies can be tailored to be the most effective and impactful, whilst minimising the economic burden to the industry and the competent authority.
England & Wales are free from infectious hematopoietic necrosis virus (IHNv) and viral haemorrhagic septicaemia virus (VHSv), two serious salmonid diseases listed by the World Organisation for Animal Health (WOAH) and nationally controlled under the Aquatic Animal Health (England & Wales) Regulations (2009). Using a network modelling framework, this study aims to compare the complex interplay of epidemics and control measures between the two diseases and contribute evidence to inform tailored disease preparedness.
Materials and Methods
The modelling framework, AquaNet-Mod, was adapted and applied to IHNv and VHSv outbreaks in salmonid aquaculture in England & Wales (Guilder et al. 2023). AquaNet-Mod simulates disease transmission within the network via river connections and live fish movements, integrating data collected by the competent authority through statutory monitoring.
Here, an extensive literature review and expert elicitation was conducted for parametrising specific disease characteristics in the model, such as life cycle stages and seasonal changes. Subsequently, a comparative analysis was conducted to compare the epidemic dynamics between the two diseases and evaluate the effectiveness of various control measures from an epidemiological and economic perspective. Finally, disease characteristics that drive differences in transmission dynamics and efficiency of control measures were identified using sensitivity analysis.
Results
This study found significant differences in disease dynamics throughout the England & Wales aquaculture network, with IHNv epidemics resulting in larger and more costly epidemics than VHSv. For instance, the average overall cost of a IHNv epidemic was 26% higher than an VHSv epidemic under the current control measures in England & Wales. Control measures were, however, shown to similarly prevent the spread and impact of both diseases, notably with a significant decrease in the size, duration and cost of the epidemics. Subsequently, the importance of specific disease control measures, particularly contact tracing, catchment controls and fallowing period were shown. Key disease characteristics, such as seasonality and virus decay rate in sediment, were identified as primary drivers of these differences in transmission and impact.
Discussion
This study facilitates key comparisons between the dynamics of two serious salmonid diseases, aiding informed decision-making with respect to tailored disease control management. The results of this study confirm the vital role disease control strategies play in preventing larger, more costly epidemics, regardless of the disease. Furthermore, it shows the value of mathematical modelling frameworks in better understanding complex interactions between epidemiological and economic impacts of epidemics and control measures, providing evidence to justify the application of effective balanced control strategies, informing resource allocation and enhancing disease preparedness.
References
Guilder, J., Ryder, D., Taylor, N.G., Alewijnse, S.R., Millard, R.S., Thrush, M.A., Peeler, E.J. and Tidbury, H.J. (2023). The aquaculture disease network model (AquaNet-Mod): A simulation model to evaluate disease spread and controls for the salmonid industry in England and Wales. Epidemics, 44, p.100711.