Network of ship movements and its potential in spreading pathogenic infectious salmon anemia virus among Norwegian fish farms

In Norway, infectious salmon anemia (ISA) is a notifiable and economically important disease. Accurately understanding between-farm transmission remains essential for ISA control and prevention. Using a network approach, our objective was to assess whether ship movements could potentially contribute to the transmission of pathogenic variants of ISA virus (ISAV-HPRΔ) between farms. We described yearly static ship contact (reconstructed according to delay between visits: Δ = 1, 8 and 15 days) and company affiliation networks. We assessed the relevance of salmon production areas as subdivisions of these networks. Finally, we identified ship movements that could have resulted in ISAV-HPRΔ transmission between confirmed ISA-HPRΔ cases and explored whether either network was associated with the spatiotemporal distribution of these cases using a permutation test. Connectivity was high in ship contact networks, with the largest strongly connected component encompassing ≥ 72 % of farms. Farms' affiliations and locations in the same or differing production areas influenced their likelihood of being connected, however increasing Δ enabled the connection of distant regions. Both networks were associated with the distribution of ISA cases. Ship movements and sharing of equipment or personnel are therefore potential viral transmission pathways. While the ship contact network was well structured by production areas, inadequate disinfection of ships could lead to longer ISAV survival times, thus resulting in long-distance ISAV-HPRΔ transmission events throughout the country. This study highlighted the need to further investigate the role of ships and fomites in fish disease spread; the use of genetic data could provide additional insights.