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

2 Chronology of the Outbreak

2.1 Notification of a Suspected Outbreak

Information regarding a suspected outbreak of VHS was received by Marine Scotland ( MS) which acts on behalf of the Scottish Ministers as the competent authority for fish, shellfish and crustacean diseases on the 13 December 2012. The suspicions were based on increased mortalities and a commercially-sourced positive test result for wrasse held for a fish farming company (hereafter referred to as the fish-farm client) at a commercial tank-based facility in south-west Shetland mainland. An Initial Designation Notice ( IDN) was served on the site under the Aquatic Animal Health (Scotland) Regulations 2009.

2.2 Confirmation of the Outbreak

The MS Fish Health Inspectorate ( FHI) inspected the site on 15 December 2012. The facility was stocked with multiple marine species at the time of the inspection although none of these were listed as VHS susceptible under EC Council Directive 2006/88/ EC as amended by European Union ( EU) Commission Implementing Directive 2012/31/ EU ^[11] . The wrasse consisted of approximately 10,000 wild-caught individuals comprising ballan wrasse ( Labrus bergylta), corkwing wrasse ( Symphodus melops), cuckoo wrasse ( Labrus mixtus), goldsinny wrasse ( Ctenolabrus rupestris) and rockcook wrasse ( Centrolabrus exoletus) on a diet of commercial dry pellets and cooked frozen pasteurised crab meat originating from the waters around Shetland. The FHI observed evidence of disease in the wrasse population and used targeted sampling to take tissues from a total of 40 individuals for diagnostic and statutory testing.

Samples for diagnostic testing, from 10 individuals, were subjected to pooled laboratory tests for VHS by virus isolation with enzyme-linked immunosorbent assay ( ELISA) confirmation (section 5.2), for Infectious Pancreatic Necrosis ( IPN) by virus isolation and for Infectious Salmon Anaemia ( ISA) using the quantitative reverse-transcriptase polymerase chain-reaction (qRT-PCR). The two pools comprised five individuals of multiple wrasse species. Both pools tested positive for VHS and negative for both IPN and ISA. Tissues were also subjected to histological examination. Overall the most significant lesions were observed in the heart, where mild to severe myocardial necrosis, endocarditis and subendocardial haemorrhage were noted for seven individuals. In addition, all 10 fish showed marked infiltration of the pancreas, one showed renal interstitial haemorrhage, and two showed haemorrhage and congestion in the spleen. This pathology is not regarded as being definitive of VHS. Histopathology also revealed parasites and granulomatous lesions in different organs for a few of the individuals but these are not regarded as unusual observations for fish originating from the wild environment.

Samples for statutory testing, from 30 individuals, were subjected to individual laboratory tests for both Infectious Haematopoietic Necrosis ( IHN) and VHS by virus isolation with ELISA confirmation and also VHS by qRT-PCR ( section 5.3). All individuals tested negative for IHN. All 30 individuals tested positive for VHS by virus isolation with ELISA confirmation and 27 individuals tested positive for VHS by qRT-PCR. A subsequent genotype-specific qRT-PCR assay ( section 5.3) assigned the isolates to VHSV genotype III.

It was concluded that an outbreak of VHS had occurred. A Confirmed Designation Notice ( CDN) was served on the site on 20 December 2012 under the Aquatic Animal Health (Scotland) Regulations 2009. The United Kingdom ( UK) Department of Environment Food and Rural Affairs then informed the European Commission's Animal Disease Notification System in accordance with European Economic Community Council Directive 82/894/ EEC ^[12] . This was the first site at which the VHS outbreak was detected and is referred to as the 'first-reported' site hereafter. This site is not necessarily the origin of the outbreak.

2.3 Contact Tracing

Contact tracing revealed that the first-reported site had received the wrasse as a single delivery by road on 24 October 2012 from a temporary holding facility located on the west coast of the Scottish Mainland. An IDN was served on the temporary holding facility on 14 December 2012. There had been no movements of stock from the first-reported site since the arrival of the wrasse.

2.4 Inspection of the Temporary Holding Facility

The FHI inspected the temporary holding facility on 15 December 2012. The site was stocked with multiple marine species at the time of inspection including Atlantic cod ( Gadus morhua), haddock ( Melanogrammus aeglefinus) and turbot ( Scophthalmus maximus), which are listed as susceptible species under EC Council Directive 2006/88/ EC. The facility had also introduced European seabass ( Dicentrarchus labrax) from France during September 2012 which are considered as vectors of VHS under conditions described in Annex I of EC Commission Regulation 1251/2008 ^[13] . This population originated from a compartment declared free of VHS and the site of origin is not listed as holding VHS susceptible species; the population therefore met the health certification requirements of the regulation. Almost all wild-caught wrasse captured during 2012 had been moved to other sites with the exception of individuals retained by the temporary holding facility as broodstock.

Tissue samples were taken for statutory testing from 70 turbot, 35 ballan wrasse, 25 European seabass, 15 goldsinny wrasse, three whiting ( Merlangius merlangus) and two Dover sole ( Solea solea). Four of the ballan wrasse were representatives of the wild-caught population captured during 2012 and retained as broodstock. The samples were subject to individual laboratory tests for VHS by virus isolation with ELISA confirmation. All 150 individuals tested negative.

Further testing on 150 fish of each susceptible species was not carried out because it would have had a disproportionate effect on the business given the extensive testing of wrasse stocked on other sites which had passed through the facility (sections 2.5 & 2.6).

2.5 Initial Tests at Other Sites

Contact tracing revealed that wild-caught wrasse passing through the temporary holding facility had been stocked onto 16 commercial Atlantic salmon farms in addition to the first-reported site since the beginning of June 2012. These were located around Shetland mainland and the west coast of the Scottish mainland and included all of the sites receiving wild-caught wrasse passing through the temporary holding facility during 2012. One of the farms had harvested its Atlantic salmon and the wrasse euthanased prior to the known start of the outbreak. The temporary holding facility had also supplied hatchery-reared wrasse to another farm on the east coast of Harris. Initial Designation Notices were served on 16 farms (excluding the harvested farm) between 14-18 December 2012 inclusive.

Tissue samples were taken for statutory testing from 15 wrasse from each of 15 farms and four wrasse from one farm on 18-19 December 2012 inclusive. This sampling strategy was intended to provide further information on the origin of the outbreak. Individuals were tested for VHS by virus isolation with ELISA confirmation. Three farms around Shetland mainland tested VHS positive and CDN served on these on 8 January 2013.

2.6 Further Tests at Other Sites

Between 82 and 155 wrasse were sampled from each of 11 of the 13 farms remaining under IDN between 16 January and 12 February 2013 inclusive. Individuals were tested for VHS by virus isolation with ELISA confirmation. Two farms, both located off Shetland mainland, tested VHS positive. Confirmed Designation Notices were served on these farms on 24 January 2013.

The FHI were unable to take samples from two farms, both on the west coast of the Scottish mainland, because they had depopulated after initial testing. One of the depopulated farms had moved the wrasse to supplement those of a neighbouring farm following storm damage. The wrasse on this recipient farm had also passed through the temporary holding facility and were VHS negative at the initial test. The incoming wrasse were distributed between all the cages on the recipient farm and made up about half of the wrasse on this farm during further testing. The further testing on the recipient farm is, therefore, applicable to the original wrasse populations on both the depopulated and recipient farms. The other depopulated farm had harvested its Atlantic salmon and incinerated the wrasse at its shore-base. Any remaining wrasse transported amongst the Atlantic salmon to the processing plant were euthanased, macerated, ensiled, and incinerated off-site. The VHS status of this farm is, therefore, based on the test results of only 15 individuals from the initial testing, although all wrasse on the farm had subsequently been euthanased and disposed of.

A summary of the test results is presented in Table 1. A total of six VHS positive sites around Shetland, comprising all the sites stocking wrasse in this area, were detected and served with CDN. Initial Designation Notices for all other sites were withdrawn on 28 February 2013.

Table 1. Summary of VHS test results on individual fish on farms

^α codes for VHS positive sites only are given;
^β operator names are coded;
^γ number positive/number individuals tested using virus isolation with ELISA confirmation;
^δ test results are always given for wrasse and for other species only when a VHS positive individual was detected;
^ε wrasse originated from two farms with approximately equal numbers tested from each ( section 2.6).