Infectious salmon anemia (ISA) is an emerging disease in farmed Atlantic salmon with important commercial consequences. The pathogenicity of the ISA virus (ISAV; an orthomyxovirus) varies, observed as differences in disease development and clinical signs. A small polymorphic region (PR) in the ISAV genomic segment encoding the hemagglutinin (HA) has been described. An analysis of 33 HA gene sequences from historical and recent ISA outbreaks was performed, added to a selection of previously published HA sequences. A differential deletion model explaining the generation of HA polymorphism is proposed. The European ISAV sequences could be grouped according to deletion patterns in PR. Cell-culture replication and cytopathic effect varied between viruses from different PR groups. A rather complex epidemiology is suggested, as (a) HA sequences representing several PR variants were detected in three samples; (b) identical mutations occurred in different genetic lineages; and (c) large genetic differences were present in closely related viruses.

A reverse transcription polymerase chain reaction (RT-PCR) was used to study the early phase of infectious salmon anaemia virus (ISAV) infection in Atlantic salmon Salmo salar L. The detection threshold for the RT-PCR was estimated to be 0.01 to 0.1 TCID 50 . A protocol that closely mimics the conditions in populations of farmed salmon was used. The major port of ISAV entry was most likely the gills, but oral entry could not be excluded. The gills and heart were RT-PCR positive 5 d post infection and there was a rapid viraemic spread of the virus after entry. Ten or more days post infection, most organs yielded RT-PCR positive samples. The viral load of the fish followed a 2-phase curve with the first maximum at approximately 15 d and a minimum around 25 d. After 25 d, there was a steady increase in viral load until all sampled organs eventually became positive. In an experiment in which the transportation of material from field to diagnostic laboratory was simulated, the transportation of whole fish was found to be optimal for the performance of RT-PCR. KEY WORDS: Infectious salmon anaemia virus · ISAV · RT-PCR · Detection of ISAV Resale or republication not permitted without written consent of the publisherDis Aquat Org 47: [175][176][177][178][179][180][181] 2001 viruses in their hosts. However, there are indications that ISAV can cause long-term infection, based on observations in farms with infected fish, and also in experimental settings (Nylund & Jakobsen 1995). Screenings and surveys of ISAV infection in salmon with a clinically healthy appearance would require a highly sensitive detection method as it can be expected that the viral load in such fish would be low.There have been some studies of the early pathological changes in ISA (Falk et al. 1995, Speilberg et al. 1995. However these studies have been based on experiments in which fish have been injected intraperitoneally with ISAV. The initial phase of a primary ISAV infection in Atlantic salmon resulting from a natural route of infection is not well described. The primary target organs of ISAV and the rate with which the infection spreads in a host are not known. However, in the initial phase of a natural ISAV infection, i.e. soon after the introduction of the virus to a farm, the infectious dose that the individual fish would be exposed to and the viral load in the infected fish would presumably be low, and present diagnostic procedures may not be sufficiently sensitive to detect these low levels.To address the problem of a sensitive diagnostic procedure for ISAV, an RT-PCR procedure was optimised and used to monitor the early spread of ISAV within its host after cohabitant infection. MATERIALS AND METHODSExperimental infections. Atlantic salmon presmolts with an average weight of approximately 40 g were used in the experimental infections. The fish originated from a hatchery where ISA had never been diagnosed. During the experiments, the fish were kept in fresh water at 8 to 10°C in 150 l tanks with a water flow of 1 l min -1 . All s...

Atlantic salmon Salmo salar L. were injected intrapentoneally with infectious salmon anaemia virus (1SAV)-infective tissue homogenate to clarify the tissue distribution of ISAV in a time course study. Fish were sampled at 11 different intervals between 1 and 40 d post-infection (p.i.) and mid-kidney, head kidney, liver, spleen, intestine, gills, muscle and heart were tested for the presence of ISAV by reverse transcriptase polymerase chain reaction (RT-PCR). The results showed that during a disease outbreak, ISAV is present in most organs. It was possible to detect ISAV at all sampling times in at least 1 of the fish examined. However, for the first 8 d p.i. positive RT-PCR results were predominantly found in samples from the head hdney and mid-kidney. Fish giving positive samples after Day 13 p.i. were RT-PCR positive in most organs. These results indicated that between Days 8 to 13 p.i. considerable replication of the virus occurred, combined with wide tissue dissemination.

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