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Simulation offers insight into emerging-disease spread on salmonid farms

A new computer simulation approach may help better understand how emerging diseases could spread across salmonid farms in England and Wales and the best measures to limit their impact.

The Aquaculture Disease Network Model (AquaNet-Mod) was designed by scientists at the Centre for Environment, Fisheries and Aquaculture Science (Cefas). It uses data drawn from numerous sources, including information on aquaculture sites, river catchments, disease dynamics and controls.

Viral outbreak analyzed

To test its performance, the team modeled an outbreak of viral hemorrhagic septicemia, a World Organisation for Animal Health (WOAH)-listed disease which predominantly affects rainbow trout. It showed that 96% of sites farming susceptible fish could be infected over a 10-year period following the arrival of the virus that causes the disease, if no controls are put in place.

However, running the model also suggested that controls on the movement of live fish could significantly reduce the impact of the disease, while other control measures such as detection and contact tracing could help further.

Risk assessments which consider local circumstances are important, the researchers stressed, as while the work underlines the need for high-risk fish movements to be controlled, restricting the activity of uninfected sites within a wide infected catchment could be economically prohibitive.

Another key finding of the work was that a small subset of highly connected fish-farming sites play a disproportionate role in how disease spreads. Increased surveillance of these sites may help in curbing transmission, they said.

Broader applications possible

“AquaNet-Mod…provides a flexible tool that can be adapted based on different diseases and up-to-date live fish-movement data that can be used to inform policy on the efficacy of disease-control measures,” they wrote in the journal Epidemics.

Real outbreak data is needed to validate the findings, they noted. They also conceded that the model doesn’t consider possible real-world developments during a disease outbreak, such as pressure on diagnostic services potentially delaying control measures.

The framework used by the scientists could be adapted to simulate disease transmission in different aquaculture settings, they suggested, noting carp as a possible example. The model could also be adapted for other WOAH-listed salmonid diseases such as infectious hematopoietic necrosis virus.

Read the full research report in the journal Epidemics.