Modelling the spread of infectious salmon anaemia among salmon farms based on seaway distances between farms and genetic relationships between infectious salmon anaemia virus isolates

Abstract

Infectious salmon anaemia (ISA) is an important infectious disease in Atlantic salmon farming causing recurrent epidemic outbreaks worldwide. The focus of this paper is on tracing the spread of ISA among Norwegian salmon farms. To trace transmission pathways for the ISA virus (ISAV), we use phylogenetic relationships between virus isolates in combination with space-time data on disease occurrences. The rate of ISA infection of salmon farms is modelled stochastically, where seaway distances between farms and genetic distances between ISAV isolates from infected farms play prominent roles. The model was fitted to data covering all cohorts of farmed salmon and the history of all farms with ISA between 2003 and summer 2009. Both seaway and genetic distances were significantly associated with the rate of ISA infection. The fitted model predicts that the risk of infection from a neighbourhood infectious farm decreases with increasing seaway distance between the two farms. Furthermore, for a given infected farm with a given ISAV genotype, the source of infection is significantly more likely to be ISAV of a small genetic distance than of moderate or large genetic distances. Nearly half of the farms with ISA in the investigated period are predicted to have been infected by an infectious farm in their neighbourhood, whereas the remaining half of the infected farms had unknown sources. For many of the neighbourhood infected farms, it was possible to point out one or a few infectious farms as the most probable sources of infection. This makes it possible to map probable infection pathways.

Figure 1.

Most probable infection pathways in the cluster of ISA-infected farms in Troms County after 2007. Each infected farm is denoted by a pie chart that represents probabilities of infection attributed to neighbourhood farms (red colour) or to other sources (blue colour), (a) when genetic information was accounted for and (b) when genetic information was ignored. The arrows indicate the most probable infection pathway for each infected farm, and the colours represent the probability of each specific pathway.

Figure 2.

Salmon farms in Norway with ISA during 2004 to August 2009. Each infected farm is denoted by a pie chart that represents probabilities of infection attributed to neighbourhood farms (red colour) or to other sources (blue colour), (a) when genetic information was accounted for and (b) when genetic information was ignored. Note that locations of farms are displaced in areas with dense clusters of infected farms to avoid overlapping symbols. The histograms show the probability distributions of being infected by neighbourhood farms.