A stochastic model for infectious salmon anemia (ISA) in Atlantic salmon farming

Abstract

Infectious salmon anemia (ISA) is one of the main infectious diseases in Atlantic salmon farming with major economical implications. Despite the strong regulatory interventions, the ISA epidemic is not under control, worldwide. We study the data covering salmon farming in Norway from 2002 to 2005 and propose a stochastic space-time model for the transmission of the virus. We model seaway transmission between farm sites, transmission through shared management and infrastructure, biomass effects and other potential pathways within the farming industry. We find that biomass has an effect on infectiousness, the local contact network and seaway distance of 5 km represent similar risks, but a large component of risk originates from other sources, among which are possibly infected salmon smolt and boat traffic.

Epidemiological investigation into the re-emergence and control of an outbreak of infectious salmon anaemia in the Shetland Islands, Scotland

“…Jarp & Karlsen (1997) identified an enhanced risk of infection (odds ratio 8) if infected farms were within 5 km of the farm assessed, the odds increasing as the distance decreased. Scheel et al (2007) and Lyngstad et al (2008) have also identified proximity as a risk factor.These modes of transport are effective over different distances and result in a variety of patterns of spread. In the present study, the significance of different forms of spread is assessed in the light of the outbreak pattern and data on contact distributions implied by these different routes.…”

A strategic model for epidemic control in aquaculture

“…This comparatively local spread for ISA is in agreement with Jarp and Karlsen (1997), who found that approximately 50 % of ISA cases in Norwegian sea sites were within 10 km of other ISA positive sites. Scheel et al (2007) suggest that the risk for ISA of being in the same local contact network is equal to that due to seaway distance at a distance of 5 km. We use these data to parameterise the transmission kernel.…”

“…From Scheel et al (2007), we assume an effective farm size at the distance where local contact is equivalent to seaway distance: d 0 = 5 km. We then re-express equation (14) into the following form, where φ represents a proportional drop in infectivity due to the distance kernel over the…”

“…However other potentially infectious links between sites are probable. Wellboat movements have been implicated as a potential disease transmission risk (Murray et al, 2002); and for ISA, one recent study indicated risk from sources aside from the local contact network and seaway to be a large component of risk of infection in the Norwegian salmon industry (Scheel et al, 2007). Though an important route of pathogen introduction, for salmon, site-to-site movements outside of stocking and and harvest are relatively infrequent (Green et al, 2009).…”

“…A stochastic modelling approach is more appropriate for modelling such risk of disease incursion, as has been carried out (in the field of aquaculture) for risk of inter-river transmission of Gyrodactylus salaris (Paisley et al, 1999;Høgåsen and Brun, 2003), and inter-site spread of infectious salmon anaemia (ISA; Scheel et al, 2007), and more generally by Thrush and Peeler (2006). These approaches are complemented by risk evaluation studies of low-frequency, high-impact disease transmission events, e.g.…”