Scottish Marine and Freshwater Science Volume 4 Number 3: Epidemiology and Control of an Outbreak of Viral Haemorrhagic Septicaemia in Wrasse Around Shetland Commencing 2012
Report on an outbreak of viral haemorrhagic septicaemia in multiple stocked species of wrasse on six sea-water sites around Shetland Mainland commencing December 2012.
Appendix 1
Modelling the Dispersion of VHSV
N.K.G. Salama
To determine the potential transmission distances of VHSV from infected fish a coupled simplified transport-epidemiology model is used [31] , [46] . The model accounts for the biomass of infectious individuals at a site, the biomass of a potentially exposed site, the mean transmissibility rate of a pathogen between infected and susceptible individuals, the mean expression probability of an infected individual, the mean recovery probability of infected individual, the mean shedding rate of an infected individual, the minimum infective dose of a pathogen required to cause an infection to occur in a susceptible individual, the disease agent decay rate, the diffusion coefficient of the disease agent in the water, the tidal current amplitude, residual current speeds and tidal period. The model calculates the time until the pathogen abundance in the environment decreases below the minimum infectious dose. The model then estimates the average distance that the peak amount of shed pathogen particles can travel in this calculated time.
It is difficult to obtain the actual parameters that may occur for this current strain of VHSV and its interaction with wrasse species, as the strain characteristics are yet to be documented. Furthermore, research is limited as wrasse species had not been described as being susceptible to marine VHSV. As such, many of the parameters have to be approximated for other strains of VHSV with non-wrasse susceptible marine species, and also characteristics of other marine pathogens such as Infectious salmon anemia virus ( ISAV) infecting salmonid hosts and the related Rhabdoviridae species Infectious hematopoietic necrosis virus ( IHNV). The parameter values implemented in the model are described in Table 6 on the following page.
Table 6. Parameter values used for the VHSV transport-epidemiology model
| Parameter | Value | Note on Source Value |
|---|---|---|
| Transmission | 0.015 d -1 | as for ISAV in salmon [47] |
| Expression | 0.14 d -1 | as for ISAV in salmon [47] |
| Recovery | 0.04 d -1 | as for ISAV in salmon [47] |
| Shedding | 10 3 pfu mL -1 | as for IHNV in salmon and within range of VHSV IVb in muskellunge [48] , [49] |
| Min. infective dose | 10 2 pfu mL -1 | as for VHSV in Pacific herring [50] |
| Decay rate | 0.06 h -1 | within range for North American strains in seawater [51] , [52] |
| Diffusion coefficient | 10 4 cm 2 s -1 | within range for Scottish waters and previously used in model development [53] |
| Residual current speed | 4cm s -1 | lowest recorded range observed between October 2004-2005 offshore eastern Shetland. For simplification, although unlikely, advection occurs in all directions. [54] |
| Tidal period | 12.42 h | footnote [55] |
| Tidal current amplitude | 0.255 m s -1 | published approximation for Shetland [55] |
The host number of infectious individuals is obtained by the biomass of wrasse recorded by MS FHI for each confirmed site. As the prevalence had yet to be determined, simulations were undertaken for each site with prevalence assigned within the range between 10-100%, increasing in 10% increments. The exposed site was taken to be the nearest site by Euclidian distance of sites held in the MS viewfarms GIS database. The biomass of this site was obtained from the records held in the MS FHI Aquadat database.
Although the model cannot be validated, using parameter values specific for ISAV and applying the physical characteristics described in Table 6 and using farm biomass values characteristic for the Scottish industry, it is possible to produce transmission distances in a similar scale to those designated for disease management areas for the control of ISA [31] . This indicates that the model is able to capture features which have proved useful in restricting the spread of ISA [56] .
Implementing the parameters in to the model expression [46] and undertaking multiple simulations for varying prevalence, and biomass at each positive farm site and incorporating the biomass of salmon on the nearest neighbour it is possible to estimate that a transmission radius occurring within the range of around 400 m for farms with possible high prevalence values. However, this distance may be an underestimation due to the parameter values being taken from related host and pathogen species being an underestimation, therefore it is likely that a radius of 500 m will encapsulate the transmission distances of VHSV. Likewise, a more cautious estimate could have been obtained, should this VHSV strain be more conserved than ISAV or other VHSV strains, however, it is likely that the offshore residual current speed are a significant overestimation for the inshore areas where salmon farming occurs in this region. Previous work has demonstrated that for a range of pathogen species, transmission distances diminish with reduced residual current speeds [31] .
Although the distances may appear low in comparison to the radius established for disease management areas for ISA control, it must be noted that salmon biomass on farms is between 10 - 20 times greater than the biomass of wrasse held on the positive sites identified in Shetland. Although, it is not a linear relationship between biomass and potential transmission distances, the general pattern is that the lower the biomass held on the pathogen positive farm, the lower the transmission distances of shed pathogen [ 31].
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