LABORATORY DIAGNOSIS OF INFECTIOUS SALMON ANEMIA (ISA): EXPERIENCE GAINED FROM THE OUTBREAKS ON THE FAROE ISLANDS 2000-2003

The first outbreak of ISA on the Faroe Islands was diagnosed in March 2000. Despite intensive surveillance, control and eradication of ISA, the disease has since spread to most of the Faroe Islands affecting about half of the 23 aquaculture farms. Sampling and laboratory diagnosis of ISA is performed according to the EU Commission Decision draft on sampling and diagnostic procedures for ISA. Inspection, clinical and gross-pathological examination and tissue sampling is performed by the veterinarians on the islands. Laboratory examination is done in collaboration between the Veterinary Department at the Faroe Islands and the Danish Veterinary Institute (DVI). Fish tissue samples were investigated by several different methods, including various RT-PCR procedures on kidney material, immunoflourescence staining (IF) of kidney imprints and cell cultures inoculated with organ homogenates, immunohistochemical staining (IHC) of formalin fixed tissue sections a.o. In a few cases haemadsorption to infected cell cultures was evaluated. Cell cultivation is of prime importance as it produces isolates for further study. The SHK-1 cells have proven difficult to use for isolation of ISAV, as the cells tend to become less efficient with higher passage numbers. The problems with the SHK-1 cells have also caused some difficulties in making IF staining for ISAV in these cells. IF on kidney imprints have a high potential in the rapid and low cost diagnosis of ISA from clinical outbreaks. The on-site preparation of the imprints, however, is a critical step. The RT-PCR method has proven very reliable in ISA diagnosis. A number of primers targeting the ISAV genomic segment 2, 5 and 8 were examined. The primers targeting the segment 8 (Mjaaland et al. 1997) were the most efficient in detecting virus positive samples. RT-PCR on supernatants from infected cell cultures has shown that although the SHK-1 cells do not give a clear cytopathic effect they are at least in some cases able to replicate the virus. In addition to the methods recommended by the Commission Decision an IHC method developed by one of the authors has been used. This seems to be a very promising technique, because there is almost a total correspondence between samples positive by RT-PCR and samples positive by this method. In conclusion proper clinical examination on site is still the most efficient method for tentative and primary diagnosis of ISA in Atlantic salmon. IF on imprints and IHC are reliable ways of verifying clinical ISA diagnosis. RT-PCR could be used as a way of screening fish stocks without clinical symptoms of ISA. Cell cultivation must be used in order to facilitate further studies of the viral agent. Methods for reliable surveillance of freedom of ISA should be established in order to prevent further spread of the disease. In order to harmonise diagnostic procedures for ISA recurrent international inter-laboratory proficiency tests will be organised. References: Mjaaland, S., Rimstad, E., Falk, K. & Dannevig B.H. (1997). Genomic characterisation of the virus causing infectious salmon anemia in Atlantic salmon (Salmo salar L): an orthomyxo-like virus in a teleost.