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

12 Origin of the Outbreak

12.1 Background

Hypotheses regarding the origin of the outbreak are summarised and evaluated using information obtained from the outbreak investigation; only minimal reference is made to the scientific literature. The qualifiers used for the analysis are summarised in Table 5.

Table 5. Summary of qualifiers ^[41]

Qualifier	Interpretation
Negligible	Possible under exceptional circumstances
Low	Slight, but possible in some α circumstances
Moderate	Possible
High	A real possibility β

^α translated into English from French as 'certain' in the source reference but modified for this report as 'some';
^β interpreted for the purposes of this report as being beyond reasonable doubt.

12.2 Outbreak Originated from the Marine Environment of the Sound of Arisaig

Evidence supporting the hypothesis includes:

• the VHSV is genotype III ( sections 2 & 10);
• VHS positive sites around Shetland received wrasse caught in this area ( section 11).

Evidence contrary to the hypothesis includes:

• wild-caught wrasse for elsewhere were also captured from this area by the same fisherman but the outbreak was clustered around Shetland only ( sections 4 & 11);
• tests for VHS on some of the wild-caught wrasse originating from this area, carried out while at the temporary holding site prior to the detection of the outbreak, were negative ( section 11).

The chance that the outbreak originated from the marine environment of the Sound of Arisaig is categorised as 'low'. The reason for assigning a low category (rather than negligible) is that VHSV genotype III can be found in the marine environment and it is not possible to definitely determine that no VHS positive wild fish were present in the marine environment around the Sound of Arisaig prior to the outbreak.

12.3 Outbreak Originated During Transportation to the Temporary Holding Facility

Evidence supporting the hypothesis includes:

• VHS positive sites around Shetland received wild-caught wrasse which had been transported to the temporary holding facility ( section 11).

Evidence contrary to the hypothesis includes:

• wild-caught wrasse destined for Shetland and elsewhere were transported to the temporary holding facility using the same transport tank and pickup but the outbreak clustered around Shetland only ( sections 4 & 11);
• tests for VHS on some of the wild-caught wrasse transported to the temporary holding facility, carried out prior to the detection of the outbreak, were negative ( section 11).

The chance that the outbreak originated during transport to the temporary holding facility is categorised as 'negligible'.

12.4 Outbreak Originated at the Temporary Holding Facility

Evidence supporting the hypothesis includes:

• VHS positive sites around Shetland received wild-caught wrasse which had been held at the temporary holding facility ( sections 2 & 11);
• the temporary holding facility held VHS susceptible species at the time of the outbreak ( section 2).

Evidence contrary to the hypothesis includes:

• wild-caught wrasse destined for Shetland and elsewhere were all held at the temporary holding facility but the outbreak clustered around Shetland only (sections 4 & 11);
• tests for VHS on some of the wild-caught wrasse at the temporary holding facility, carried out prior to the detection of the outbreak, were negative ( section 11);
• tests for VHS on stocks other than-wild caught wrasse at or originating from the facility, carried out subsequent to the detection of the outbreak, were negative ( section 2).

The chance that the outbreak originated at the temporary holding facility is categorised as 'low'. The reason for assigning a low category (rather than negligible) is that each susceptible species at the facility would have required testing at the 150 fish level for increased certainty.

12.5 Outbreak Originated During Transportation from the Temporary Holding Facility

Evidence supporting the hypothesis includes:

• VHS positive sites around Shetland received wild-caught wrasse which had been transported from the temporary holding facility ( sections 2 & 11).

Evidence contrary to the hypothesis includes:

• the transport businesses disinfected transport tanks, trailers and trucks prior to entering the temporary holding site and used disinfected water from the temporary holding site ( section 11);
• transport businesses Tr _a and Tr _b made subsequent deliveries of wild-caught wrasse to VHS negative sites out-with Shetland ( sections 11);
• transport businesses Tr _b and Tr _c made only one delivery to single sites on Shetland but the outbreak involved multiple disparate localities around Shetland ( section 11).

The chance that the outbreak originated during transport from the temporary holding facility is categorised as 'negligible'.

12.6 Outbreak Originated at the First-Reported Site

Evidence supporting the hypothesis includes:

• wild-caught wrasse at the first-reported site were VHS positive ( section 2).

Evidence contrary to the hypothesis includes:

• no species listed as susceptible under EC Council Directive 2006/88/ EC were held at the facility while the wild-caught wrasse were on site ( section 2);
• wrasse were fed a commercial dry pellet and cooked frozen pasteurised crab meat ( section 2);
• tests for VHS, carried out subsequent to the detection of the outbreak, on 630 lumpsucker held in the same building as the VHS positive wrasse were negative ( section 9);
• disinfection of effluent discharges was in-place;
• despite investigation no credible epidemiological link between the first-reported site and other VHS positive sites has been discovered.

The chance that the outbreak originated at the first-reported site is categorised as 'negligible'.

12.7 Outbreak Originated from Atlantic Salmon Stocked on Site

Evidence supporting the hypothesis includes:

• wrasse testing VHS positive during the outbreak investigation were stocked with Atlantic salmon ( section 2).
• Evidence contrary to the hypothesis includes:
• all of 750 Atlantic salmon from VHS positive farms tested VHS negative ( section 9);
• at least two independent batches of Atlantic salmon smolts had been split to stock VHS positive farms on Shetland and VHS negative farms elsewhere;
• all Atlantic salmon smolts originated from a continental zone of a member state declared free of VHS under EC Commission Decision 2009/177/ EC ^[42] as amended by EU Commission Decision 2010/171/ EU ^[43] .

The chance that the outbreak originated from stocked Atlantic salmon is categorised as 'negligible'.

12.8 Outbreak Originated Directly from White Fish Processing Activities

Evidence supporting the hypothesis includes:

• VHS has been reported in wild commercial white fish species in the North-East Atlantic Ocean and North Sea ^{[37] , [44] , [45]} ;
• white fish processing takes place in one of the VHS positive localities.
• Evidence contrary to the hypothesis includes:
• white fish processing does not take place in the other two VHS positive localities;
• the distances of VHS positive sites from white fish processors exceed the estimated transmission distance for VHSV ( Appendix 1).

The chance that the outbreak originated directly from white fish processing activities is categorised as 'negligible'.

12.9 Outbreak Originated from the Marine Environment Around Shetland

Evidence supporting the hypothesis includes:

• the VHSV is genotype III ( sections 2 & 10);
• the outbreak clustered around Shetland only ( section 4);
• six species of free-ranging wild fish around south-west Shetland tested VHS positive ( section 9);
• the first-reported site did not use disinfected seawater and was open to infection. The intake was located adjacent to the facility's outlet which was conducive to attracting wild fish thereby creating a potential route of infection into the facility consistent with the estimated transmission distance for VHSV ( Appendix 1). The additional five VHS positive farms used net-pens which are open to infection.

Evidence contrary to the hypothesis includes:

• uncertainty regarding whether VHSV infections in free-ranging wild fish are the origin or a consequence of the outbreak;
• the involvement of multiple disparate localities around Shetland which, together with the limited estimated transmission distance for VHSV, requires either multiple infection events by wild fish or anthropogenic spread ( section 4, Appendix 1).

The chance that the outbreak originated from the marine environment around Shetland is categorised as 'moderate'. The reason for assigning a moderate category (rather than high) is that the available evidence is not sufficiently unambiguous as to be able to state beyond reasonable doubt that this hypothesis is correct.

12.10 Conclusion

The most likely explanation of the origin of this outbreak is the marine environment around Shetland. The VHSV genotype III is recognised as being associated with the area of the North Atlantic to the Norwegian coast and the North Sea ^[10] and VHSV has previously been reported in free-ranging marine species in the vicinity of Shetland ^[44] , ^[45] . Further investigations focussing on how the risk of VHS affects the use of wrasse and other taxa as biological controls for sea-lice, and how the risks can be ameliorated, is desirable.