| Infectious salmon anemia virus (ISAV) is an orthomyxovirus recently assigned to the genus Isavirus. It is currently the most important viral pathogen threatening commercial salmonid aquaculture in the Northern Hemisphere. The virus causes infectious salmon anemia (ISA) a highly fatal disease in marine farmed Atlantic salmon (AS). The disease is characterized epidemiologically by variable mortalities, grossly by exopthalmia, pale gills and ascites, and microscopically by congestion and hemorrhagic necrosis of internal organs. Difficulties have been reported in reproducing the lesions associated with the infection experimentally and in some cases infected AS are asymptomatic. Also the virus seems to persist in recovered or asymptomatic fish, a phenomenon unusual for other orthomyxoviruses. In this study, in situ hybridization (ISH) was used to monitor the virus gene expression in vitro and in vivo, while histology was used to correlate lesions with presence of viral mRNA.; ISH conditions were established for the detection of mRNA transcripts of ISAV RNA segments 6--8 in infected TO, SHK-1 and CHSE-214 cell lines. Hybridization signals were observed first in the nucleus and then in the cytoplasm of infected cells, which is consistent with the nuclear transcription and replication of other orthomyxoviruses. Transcription signals were intense and widespread for all three mRNAs in TO cells and in few cells in SHK-1 cells from one day post infection (dpi). In CHSE-214 cells, maximum hybridization signals were seen in the nucleus at 2, 4 and 5 dpi with segments 8, 7 and 6 riboprobes respectively. The earlier transcription of segment 8 and later transcription of segment 7 might suggest that there is a difference in the time requirement and amount of proteins encoded by these segments. ISH with segments 7 and 8 riboprobes, and histology were used to study the relationship between the presence of lesions and viral mRNA in AS liver, spleen, kidney, heart, gills and pyloric caeca collected during clinical phases of the infection. Three groups of AS were infected with three ISAV isolates, two belonging to North American (RPC and CH7) and one to European (NSC) HA genotypes. Lesions consistent with ISA were observed in 100% of spleen, 95% of livers and pyloric caeca, 82% of hearts, 60% of gills and 27% of kidneys. (Abstract shortened by UMI.)... |
