Archived - Epidemiological Evaluation of Surveillance Activities in British Columbia Farmed Salmon
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The complete text of this report is available upon request.
This report details the quantitative epidemiological evaluation of the existing surveillance programs in British Columbia farmed salmon for the following viruses: infectious salmon anemia virus (ISAV), both pathogenic (ISAV HPRΔ) and non-pathogenic (ISAV HPR0) strains; infectious pancreatic necrosis virus (IPNV); and infectious hematopoietic necrosis virus (IHNV). Stochastic modelling was used to conduct the evaluation. The substantiation of the health status of these agents in both farmed and wild fish in British Columbia is considered a National Aquatic Animal Health Program (NAAHP) priority for the purposes of both domestic disease control and international trade. The findings derived from this evaluation provide scientific background for surveillance requirements in the salmon farming industry. This assessment is limited to the BC farmed salmon sector that rear Atlantic and Pacific salmon in both marine and freshwater environments.
The Canadian Food Inspection Agency (CFIA) has carried out this evaluation with the collaboration of several groups, including the BC Salmon Farmers Association (BCSFA), Mainstream Canada, Creative Salmon, Grieg Seafood, Marine Harvest Canada, the BC Ministry of Agriculture (BCMA), the Fisheries and Oceans Canada (DFO) Aquaculture Management Division in British Columbia, the BC Centre for Aquatic Health Sciences (BC CAHS), and DFO's National Aquatic Animal Health Laboratory System (NAAHLS). Data used in this evaluation were shared with the CFIA; confidentiality agreements have been put in place between the CFIA and the BCSFA and its members, the BCMA and DFO.
As the competent authority for aquatic animal health in Canada, the CFIA is responsible for the substantiation of Canada's health status for the diseases listed as reportable and notifiable under the Health of Animals Act and Regulations. As part of this responsibility, existing governmentFootnote 1 and industry surveillance programs were evaluated for the period of 2006 to 2011 inclusive. During this period, over 550 farm audits were carried out by the government, which resulted in over 9500 molecular diagnostic testsFootnote 2 being completed for the diseases in the study (3183 tests for ISAV HPRΔ and ISAV HPR0, 3183 for IHNV, and 3183 for IPNV). During the same period, the aquaculture industry completed 5132 diagnostic tests for ISAV HPRΔ, 2395 for ISAV HPR0, 31,086 tests for IHNV, and 2737 tests for IPNV using a combination of molecular methods, tissue culture, and histopathology.
The evaluation revealed that both government and industry programs are effective contributors to the attestation of the health of farmed salmon in British Columbia for ISAV HPRΔ, IPNV, and IHNV. Achieving disease freedom targets for ISAV HPR0 (non-pathogenic) will require additional active surveillance, and sample collections are planned in the spring of 2014.
A regional approach to surveillance activities in British Columbia is required as a component of Canada's national surveillance system. Surveillance sampling for ISAV, IHNV, and IPNV in wild salmon stocks has already been implemented as part of this regional approach. In 2012 and 2013, over 8000 samples were collected from wild salmon, and molecular diagnostic testing has been completed on all of these samples, with negative results. A comprehensive evaluation of the health status of BC finfish in all industry sectors is required and will be initiated in 2014.
The objective of this quantitative epidemiological evaluation was to assess the efficacy of the existing surveillance activities for ISAV HPRΔ, ISAV HPR0, IPNV, and IHNV. The evaluation estimated the probability that BC farmed salmon were free from each of the pathogens, believed to be exotic to the province, at the end of the study period. The report was then used to identify additional requirements for surveillance in this industry sector for the agents being studied.
The following programs were included in the assessment:
- The government Fish Health Audit and Surveillance Program, which includes ongoing testing for IHNV, IPNV, and ISAV as well as numerous other pathogens. The program covers verification of industry compliance with licence conditions, including biosecurity measures and record keeping, to confirm that mortality collection and classification are occurring as required. The audit also confirms that recent mortality events have been investigated by industry fish health staff. This program was delivered initially under the authority of the BCMA and has subsequently been transferred to the authority of DFOFootnote 3.
- Industry fish health monitoring programs are in place for multiple purposes, including but not limited to the ongoing monitoring of the health of farmed fish, the prevention of disease introduction by fish movements, and compliance with government requirements. Syndromic surveillance is ongoing on BC salmon farms for the early detection of disease. Syndromic surveillance is a form of surveillance that looks for changes in signs and syndromes in individual animals and populations, and investigation and testing follow when changes are detected.
Approach and Methodology
In this evaluation, a scenario tree model was used to assess the sensitivity of the surveillance activities. This methodology makes it possible to quantitatively analyze complex surveillance activities and to combine evidence of freedom from multiple sources. Three surveillance components were examined: i) the testing data from the government audit; ii) the testing data from the industry's pre-movement and routine mortality sampling; and iii) an estimation of the efficacy of syndromic surveillance that models the probability that an infected population will show clinical signs and that observation, investigation, detection, and reporting will occur.
The model results were then used to calculate the probability of disease freedom for ISAV HPRΔ, ISAV HPR0, and IPNV. For any pathogen, estimating the probability of disease freedom requires knowledge of the probability of pathogen introduction and the sensitivity of the combined surveillance activities. Confidence in disease freedom increases over time with the accumulation of negative results, provided that the ongoing surveillance activities occur at a level sufficient to detect pathogen introduction from all potential sources.
Results and Recommendations
Pathogenic Infectious Salmon Anemia Virus and Infectious Pancreatic Necrosis Virus
Syndromic surveillance is considered a reliable means for the early detection of ISAV HPRΔ and IPNV in BC farmed salmon, primarily because of effective ongoing monitoring for abnormalities on farms through routine mortality collection and classification. Clinical disease is expected to manifest in Atlantic salmon if either of these agents are present in British Columbia.
Due to ongoing surveillance for ISAV HPRΔ (pathogenic) and IPNV, the mean probability of disease freedom as of December 2011 was greater than 99% for both agents. The existing ongoing surveillance activities in BC farmed salmon are sufficient to support claims of disease freedom for both of these agents, assuming that these activities are continued at the current level. Additional surveillance beyond what is currently in place is not required to substantiate the absence of ISAV HPRΔ and IPNV in BC farmed salmon. This finding is dependent on the continued enforcement of import and domestic movement controls to prevent pathogen introduction.
Non-Pathogenic Infectious Salmon Anemia Virus
Syndromic surveillance is not suitable for the detection of ISAV HPR0 (non-pathogenic), because the agent is not known to produce clinical signs in infected hosts. The focus for ISAV HPR0 remains the prevention of its introduction through strong controls on both imports into Canada and movements within Canada; only animals shown to be free from ISAV HPR0 should be released into British Columbia.
Although surveillance for this agent is ongoing in the salmon farming industry and the government audit of salmon farms, further testing must be conducted to achieve 95% confidence in freedom from this agent. The mean probability of disease freedom as of December 2011 was 84%. Implementation of active surveillance to confirm the status of ISAV HPR0 in British Columbia will begin in the spring of 2014.
Infectious Hematopoietic Necrosis Virus
During the study period, no outbreaks of IHNV were reported in the salmon farming industry. Incursions of IHNV did occur in 2012, and the existing surveillance activities proved to be effective for early detection prior to spread between farms. Additional surveillance beyond what is currently in place is not necessary to address the requirement for the early detection of clinical cases of the disease. These conclusions do not take into consideration the impact of vaccination on syndromic and active surveillance. It is expected that vaccinated populations will be less likely to manifest disease and will be resistant to infection.
Other Priority Diseases under the National Aquatic Animal Health Program
A collaborative survey under the NAAHP is proposed to validate the health status of BC farmed salmon for salmonid alphavirus (SAV). This disease was accepted for listing by the General Assembly of the World Organisation for Animal Health (OIE) in 2013, and surveillance for this agent in susceptible populations is required to attest to the health status of farmed salmon populations, for the purposes of safe international trade and domestic disease control. The syndromic surveillance carried out by the industry is expected to detect clinical cases of this disease if it is present in British Columbia. The audit of farmed salmon populations for this agent has been initiated under the DFO audit program, and in 2013, 604 Atlantic salmon and 89 Pacific salmon were tested, with negative results. A short-term active surveillance program will be conducted to validate the health status of this agent in BC farmed salmon.
Surveillance sampling, to address the gaps in the existing data and to validate the results of the evaluation, is expected to be initiated in the spring of 2014. The CFIA has prepared a draft surveillance plan for the period of 2014 to 2016 that describes the sampling regimen and identifies the roles, responsibilities, and proposed delivery options for the surveillance activities.
Determining the health status of BC finfish for all reportable, notifiable, and OIE-listed agents requires a comprehensive assessment of the region (the province of British Columbia and the territorial sea) on a disease-by-disease basis, as well as the assessment of additional industry sectors (e.g., trout industry and wild marine finfish harvesters). This assessment will be initiated in 2014 and is expected to be completed by 2016; it will be used to determine any additional surveillance requirements in British Columbia for reportable and emerging diseases.
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The complete text of this report is available upon request.
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