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Bacterial Kidney Disease (BKD), caused by the bacterium Renibacterium salmoninarum, is a systemic, chronic infection of salmonids that has been reported throughout the world wherever salmonids are cultured or occur naturally (with a few exceptions such as Ireland), writes Kevin Denham, Fish Health Inspectorate for the Centre for Environment, Fisheries and Aquaculture.

Introduction

It can affect fishing both the freshwater and seawater environment. All age groups of fish may become infected, but losses may be dependent on environmental conditions or predisposing factors such as water quality.

BKD was first recorded as an observation in wild Atlantic salmon from the Rivers Dee and Spey in Scotland in 1930. It was made a notifiable disease in Great Britain in 1978 following a serious outbreak of the disease in farmed rainbow trout. Control measures comprised the prohibition of movements of live fish from affected sites, and contact testing of suppliers and recipients. There was no requirement to cull out affected stocks. The control measures differentially affected restocking farms and farms producing juvenile fish for on-growing as cull and disinfection was the only option for the continuation of business on these sites. Under what amounted to a passive surveillance programme, confirmation of BKD in fish farms was rare.

BKD controls continued under the single market measure: EC Directive 91/67/EC categorised BKD as a List III disease, and more latterly through, what were known as additional guarantees under EC Decision 2004/453. This required an active targeted surveillance programme on fish farms, and control and eradication measures on infected sites.

However the validity and expedience of controlling BKD on a statutory basis were becoming of increasing concern to both regulatory authorities and stakeholder groups, as has the likelihood of eradicating the disease in GB. A great deal of Government funding has been invested in BKD research in areas such as improved diagnostic testing, epidemiology of the disease, and the interactions between wild and farmed fish. The transposition of legislative controls from the additional guarantees decision 2004/453/EC to controls under 2006/88/EC offered an opportunity to review the statutory control of BKD in GB.

A UK Group on BKD was established to consider various options for the future control of BKD in GB. The aim of the Group was to develop a domestic control strategy for BKD in Great Britain, acceptable to the regulatory authorities and to stakeholder groups, to control the further spread of the disease through a risk based assessment approach. Following consideration of a number of proposals and options by both the regulatory authorities and stakeholder groups, a policy for the future control of BKD was agreed by both Defra and Scottish Government in February 2011.

Key policy Features

  • The UK will not pursue an extension to national control measures for BKD in Great Britain through Article 43 of Council Directive 2006/88/EC. A commitment was given to both English and Scottish Ministers that circumstances meant we seek an alternative domestic approach to controlling BKD in GB and thus drop our Article 43 measures.
  • BKD will be subject to domestic control measures in Great Britain.
  • Clinical BKD will remain a notifiable disease in GB – This is an essential non-negotiable requirement in order to facilitate the establishment of approved farms (compartments) for BKD and so allow export trade to other Member States/third countries that require BKD attestation.
  • Clinical outbreaks of BKD will be controlled through the application of initial designations (IDs) and confirmed designations (CDs) under Aquatic Animal Health Regulations. Both Marine Scotland Science and Cefas will maintain current maps of all confirmed designations for BKD on their websites.
  • Statutory controls will only be applied when clinical BKD is confirmed. The presence of R.salmoninarum in itself will not be sufficient to initiate statutory actions.
  • There will be no routine targeted surveillance on fish farms for BKD.
  • Surveillance will be limited to active surveillance (during fish farm inspections for other purposes) and passive surveillance.

Definition of clinical BKD

In order to confirm the presence of clinical BKD, evidence from each of the following stages must be noted during the course of any investigation:

  • Evidence of morbidity and/or mortality in the affected population and
  • Presence of external clinical signs, which may comprise some or all of the following: pale gills, exophthalmia, abdominal distension, skin blisters filled with clear or turbid fluid, darkening of the skin, petechial haemorrhaging particularly around the vent and / or internal signs which may comprise some or all of the following: ascites and/or fluid in the pericardial cavity, haemorrhages on the walls of the body cavity and viscera, ibrinous layer on one or more of the internal organs, granulomatous lesions in the kidney, spleen or liver and
  • Evidence for the presence of R.salmoninarum in one or more laboratory tests. The tests used will be ELISA and PCR.

Control Strategy

1. On Confirmation of Clinical BKD
  • Statutory controls (a designation) will be placed on the affected farms
  • All movements of live fish for stocking other farms/waters will be prohibited during the clinical disease outbreak (this is already prohibited under the AAH Regulations).
  • Movements of fish for processing for human consumption will be permitted (only those not showing clinical disease).
  • Movements of fish to another fish farming site subject to Designation for BKD will be permitted subject to the requirements of welfare in transport legislation (only clinically healthy animals may be transported).
  • The Fish Health Inspectorates will work with the affected fish farm in the development of a management plan to control the disease outbreak (e.g. removal of affected fish, de-stocking of affected populations, and implementation of biosecurity measures).
  • Two official inspections at least 28 days apart will be required to confirm that the clinical disease outbreak is over.
2. Post Clinical Outbreak Controls

Movements of live fish for stocking fish farms and other waters will be permitted subject to the following caveats:

  • Movements of live fish from fish farms subject to BKD controls will not be permitted to unaffected freshwater farms (including wild fish restocking hatcheries) holding Atlantic salmon.
  • Movements of live fish from fish farms subject to BKD controls will not be permitted to on-line (unenclosed) waters (whether they are farms or fisheries), or other on-line locations (including river waters) in catchments where Atlantic salmon are farmed, (including wild fish restocking hatcheries).
  • Movements of live fish which are permitted from a fish farm subject to BKD controls to another fish farm site not subject to controls for BKD will result in an Initial Designation Notice being served upon the receiving farm(s). Following a 28 day period, the receiving site will be inspected by the competent authority. Where no evidence of clinical disease is observed the IDN will be revoked.
  • Movements of susceptible species from affected freshwater sites to marine sites for on-growing will be subject to a risk assessment to determine the likelihood of spread to other populations.
  • Movements of fish for processing for human consumption will be permitted. Movements of fish between farms subject to BKD controls will be permitted.

[Note: For the purpose of this article the term catchment refers to an individual river catchment rather than a supercatchment. Where farms and fisheries are located in different river catchments but within the same supercatchment, they will be treated as separate catchments unless the distance between sites within a supercatchment is less than 5km upstream or downstream. In such a case they would be considered to be an increased risk and not permitted.]

Contact Tracing

  • Contact tracing from positive cases will be based upon the inspection of fish stocks held on sites directly supplying, or having been supplied by any infected site within the six months prior to the date of the confirmation of BKD.
  • Where clinical BKD is confirmed, all fish farm sites located within the same river catchment or within one tidal excursion at sea will be inspected for the presence of clinical BKD.
  • Only where moribund or sick fish are observed will fish be taken for examination.
  • Where fish have gross pathological changes consistent with BKD observed at post-mortem samples will be taken for histopathological examination, and testing by ELISA and PCR methods
  • Confirmation of BKD will only occur where gross pathological changes consistent with BKD have been observed, histopathological changes consistent with BKD have been observed and the PCR or ELISA sample provides a positive result.

Removal of Movement Controls from Farms Subject to bKD Designation

There are two options for the removal of statutory controls on BKD affected farms:

Option 1: Cull and disinfection

Farms may elect to depopulate, disinfect and fallow the site following a clinical disease outbreak. The Designation will be removed from the farm following the successful completion of this process.

Option 2: Testing of fish stocks

The Designation will also be lifted following a negative test of a sample comprising 150 fish, individually tested by PCR, from the affected farm. This test will be carried out following the resolution of clinical BKD. Where this test is positive, any subsequent retest will not be undertaken for six months, unless other actions to remove the infection within the affected population are undertaken.

Approved Farms

  • The Official Services (MSS and Cefas) will support farms that seek approved farm (compartment) status for BKD through the provision of advice on requirements (equivalent to those stipulated in Annex V Part II of Council Directive 2006/88/EC).
  • The Official Services will provide an appropriate sampling and diagnostic testing service to industry in order to facilitate the establishment of approved farms. The control strategy will be subject to review after 12 months from the date of implementation, i.e. Feburary 2012.
October 2011

the Fish Site Editor

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