Infectious Salmon Anemia Virus

Infectious salmon anemia virus (ISAV) is the causative agent of infectious salmon anemia (ISA), a disease of farmed Atlantic salmon (Salmo salar) recorded for the first time in Norway in 1984. ISAV is the type species of the genus Isavirus in the Orthomyxoviridae. The virus is adapted to cold-water salmonid fish and has an optimum growth at 15 °C. All eight genome segments have been sequenced and they encode at least 10 proteins. The virion contains two surface glycoproteins: a hemagglutinin-esterase (HE) responsible for receptor binding and release; and a fusion protein (F) responsible for fusion of viral and endosomal membranes. The overall nucleotide sequence identity between ISAV isolates is high, and highest sequence variation is found in the HE gene. Nucleotide deletion in HE gene is assumed to be important in determining virulence. Both genome segment reassortment and recombination may occur during viral replication. Endothelial cells are assumed to be the most important target cells, and ISA is characterized by severe anemia and hemorrhages in several organs. Mortality during an outbreak of ISA varies significantly and may exceed 90% in severe cases. Salmonid fish of several species can be subclinically infected. During the past ten years, the disease has emerged in Canada, Scotland, the Faroe Islands, and the USA.