Vets probe ISA virus issue
Posted by Mainstream Canada, Tue, 2011-10-25
Professional veterinarians from all around North America are discussing the recent reports that ISA had been discovered in two B.C. sockeye salmon.
The American Veterinary Medical Association regularly publishes the "AquaVetMed" newsletter for its members and recently sent out the following information in an email news update. It is reproduced here as it appeared in the update to AVMA members:
British Columbia, Canada - The following information was provided by Paul Kitching (British Columbia Provincial Veterinarian, Ministry of Agriculture) and Gary Marty (British Columbia Provincial Fish Pathologist) regarding the recent Infectious Salmon Anemia PCR test results from wild sockeye salmon in British Columbia (Canada), and some recent press releases and news stories.
1. Results from a laboratory in Prince Edward Island, Canada, were positive for Infectious Salmon Anaemia Virus (ISAV) nucleic acid in two wild sockeye salmon smolts out of 48 tested. The fish were not showing any clinical signs consistent with ISA (the diseases caused by ISAV). Therefore, these are positive PCR test results only; they do not confirm that the fish had ISA.
2. The tests indicated a European strain of ISAV, and European strains of ISAV have also been identified in farmed Atlantic salmon in Chile.
3. The test used, polymerase chain reaction (PCR) is extremely sensitive, and because contamination of samples within the laboratory can occur, the PCR result by itself is not conclusive (i.e., we need confirmatory tests to determine whether the original PCR test results are false positives or true positives). The OIE (World Animal Health Organization) always requires additional evidence such as laboratory isolation and growth of the virus, or clear evidence of clinical disease in animals (in this case fish) from which the positive sample is taken.
4. Therefore, the Canadian Food Inspection Agency (CFIA) will probably not report the initial PCR results to OIE until at least one of these two other criteria (virus culture or evidence of disease) are fulfilled.
5. From 2003 - 2010, the British Columbia Ministry of Agriculture operated a scientifically designed Fish Health Auditing and Surveillance Program that tested farmed salmon in BC. This program examined freshly dead fish on the farms for evidence of disease, including ISA. During this time, 4,726 fish were sampled and tested for ISAV. Using a highly specific and sensitive PCR test, all samples were negative for ISAV--no virus. The program is now operated by the Canadian Federal government, and results from farm fish continue to be negative--no virus.
6. All published evidence suggests that Pacific salmon are resistant to ISA virus infection; controlled laboratory studies have not been done with sockeye salmon and ISAV, but Pacific salmon species tend to respond similarly to fish diseases.
7. The follow-up investigation being conducted by CFIA might take longer than 6 weeks.
8. Every year Chile produces more coho salmon (originally derived from wild BC stocks) than the total annual farm fish production in British Columbia (which is >90% Atlantic salmon). When Chile had an outbreak of highly pathogenic ISA in 2008, in some cases coho salmon were reared in pens adjacent to infected Atlantic salmon. Thousands of Chilean coho salmon were tested for ISAV over the past 3 years, and all samples were negative for ISAV. This provides strong evidence that coho salmon are highly resistant to ISAV, and a reasonable hypothesis is that sockeye salmon are similarly resistant.
9. The PCR tests were done in a laboratory that is a designated OIE reference laboratory. However, OIE has no responsibility for the quality of the work, and makes no financial contribution to the running of the laboratory. OIE has no laboratories of its own, so the test was not "carried out by OIE". A positive PCR by itself is not sufficient, by the OIE's own definition, to say that either the virus or disease is present in BC. The laboratory in PEI, even though it is an OIE reference laboratory, has responsibility to report a positive ISA result to the Canadian OIE representative: the Canadian Chief Veterinary Officer, Brian Evans. Dr. Evans would then report a positive case to OIE. At this stage we do not have a positive case for OIE reporting purposes.
[Moderators comment: While published data (Rolland & Winton, 2003. Relative resistance of Pacific salmon to infectious salmon anaemia virus. J.Fish.Dis., 26(9):511-520 - http://tinyurl.com/3s7jflz) suggests that ISA-infected Pacific salmon species do not show clinical signs, and that 2003-2010 BC ISA surveillance data (www.commissioncohen.ca/DownloadExhibit.php?ExhibitID=1562) did not detect the presence of ISA in Pacific salmon, it remains to be seen if the current diagnostics are false positives or, if they are confirmed as true positives, whether sockeye or other Pacific salmon species can harbor and shed ISA virus. ADS-Mod.]
