Abstract
Infectious salmon anaemia (ISA) is an important, systemic viral disease of farmed Atlantic salmon, Salmo salar L. Endothelial cells are the main target cells for highly virulent HPR-deleted ISA virus (ISAV) types. Here we examine the pathogenesis of non-virulent ISAV HPR0 infections, presenting evidence of an epithelial tropism for this virus type, including actual infection and replication in the epithelial cells. Whereas all HPR0 RT-qPCR positive gills prepared for cryosection tested positive by immunohistochemistry (IHC) and immunofluorescent labelling, only 21% of HPR0 RT-qPCR positive formalin-fixed paraffin-embedded gills were IHC positive, suggesting different methodological sensitivities. Only specific epithelial cell staining was observed and no staining was observed in endothelial cells of positive gills. Furthermore, using an ISAV segment 7 RT-PCR assay, we demonstrated splicing of HPR0, suggesting initial activation of the replication machinery in the epithelial gill cells. Immunological responses were investigated by the expression of interferon-related genes (e.g. Mx and γIP) and by ELISA for presence of anti-ISAV antibodies on samples taken sequentially over several months during an episode of transient HPR0 infection. All fish revealed a variable, but increased expression of the immunological markers in comparison to normal healthy fish. Taken together, we conclude that HPR0 causes a localized epithelial infection of Atlantic salmon.
Highlights
Infectious salmon anaemia virus (ISAV) is the causative agent of a systemic and lethal disease (ISA) in farmed Atlantic salmon, Salmo salar L
We demonstrate that ISAV putatively non-virulent variants of ISAV (HPR0) causes a localized and superficial epithelial infection in mucosal surfaces of the gills and skin of Atlantic salmon
We present valuable data from healthy Atlantic salmon from five independent HPR0 positive farms located in the Faroe Islands providing unique information on HPR0 infections in the general environment
Summary
Infectious salmon anaemia virus (ISAV) is the causative agent of a systemic and lethal disease (ISA) in farmed Atlantic salmon, Salmo salar L. The virus belongs to the genus Isavirus of the family Orthomyxoviridae. Clinical signs suggest circulatory failure with severe anaemia, ascites, congestion and enlargement of the liver and spleen (reviewed in [1]). Endothelial cells are the main target cells for virulent virus replication [2], epithelial cells may be infected in early stages, especially by low virulent isolates [3, 4]. Immersion infection experiments suggest variations in replication dynamics in gill [5] and other mucosal surfaces [4] between ISAV.
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