Integration of hydrodynamics into a statistical model on the spread of pancreas disease (PD) in salmon farming

Abstract

Pancreas disease (PD) is an emerging disease in salmon farming caused by the salmonid alphavirus (SAV). SAV is evidently spread horizontally between neighbouring salmon farms, but whether such transmission occurs by passive drift in the water current or via fomites is not known. We tested whether hydrodynamic modelling contributes to explain the spread of PD, in which case SAV is likely to spread by passive drift. We present a simple logistic regression model that accounts for the effect of PD in the neighbourhood on the probability of acquiring PD in cohorts of farmed salmonids from an area on the west coast of Norway between 2005 and 2008. For a given cohort, we calculated infection pressure (IP) based on Euclidean distance, seaway distance or estimated water contact to sites with PD, and compared the amount of variance explained in the regression model by the different variants of IP. Water contact between a discharging farm site and a receiving site was calculated by simulating particle discharge using a hydrodynamic model. IP estimated by water contact was the best predictor of PD cases and controls in the model, which performed significantly better than IP estimated by seaway distance or Euclidean distance. Since the spread of PD in the study area was best explained by modelled water velocity, we conclude that PD is likely to be spread by passive drift of SAV in the water current.

Disease interaction and pathogens exchange between wild and farmed fish populations with special reference to Norway

“…Gustafson et al (2007) showed that tidal currents affected the spread of ISA in the Quoddy region of Atlantic USA and Canada. Viljugrein et al (2009) concluded that the spread of PD in an area on the west coast of Norway was significantly better predicted by using hydrodynamic modelling to estimate water contact between farm sites, than by simply using distance between farm sites.…”

“…In parallel, increasing efforts are put into hydrodynamic modelling along the Norwegian coast. Hydrodynamic models used to estimate reciprocal relative water contact between local salmon farm sites, have also been integrated with statistical models on disease spread (Viljugrein et al, 2009). The integration of…”

Host density thresholds and disease control for fisheries and aquaculture

“…Although microparasites tend to be more short-lived, they can also be transported considerable distances by seawater movement, for example, spread between salmon farms of infectious salmon anemia (Gustafson et al 2007) and pancreas disease (Viljugrein et al 2009) outbreaks follow the directions of currents predicted using hydrodynamic modeling. Infection pressure, however, likely declines rapidly with distance from source population.…”

“…Microprasites are primarily viruses, bacteria, and protists, which typically replicate within an individual host, spread by host-to-host contact, and do not have an obligate free-living stage in the life cycle (May & Anderson 1991), although aquatic microparasites may also be transported by water movement (Gustafson et al 2007, Viljugrein et al 2009). The dynamics of a microparasitic disease can be modeled by dividing the host population into classes according to their infection status, such as susceptible, infected, or recovered (SIR) (May & Anderson 1991, Grenfell & Dobson 1995.…”

Disease management mitigates risk of pathogen transmission from maricultured salmonids

“…Experimental work has shown that SAV can survive for ~6 d at 10°C in sterile saltwater with organic loading (Graham et al 2007). This suggests that the virus is sufficiently viable in the aquatic environment to be useful for modelling, as assumed by Viljugrein et al (2009). Phylogenetic data of SAV subtype IV are consistent with a history of repeated transmission between dab and salmon.…”