Infectious Salmon Anemia

Volume 3, Issue 3
April 2001
Updated November 2002

Infectious salmon anemia (ISA) is a viral disease of farmed Atlantic salmon Salmo salar in Norway, New Brunswick, and most recently Maine. The first case of ISA was diagnosed in A. salmon parr at a hatchery on the west coast of Norway in 1984. In this outbreak, approximately 80% of the fish died from ISA over a period of several months.

Atlantic salmon infected with Infectious Salmon Anemia virus (ISAV) and showing clinical disease are typically lethargic, fail to swim upright, swim near the water surface; they may have petechial hemorrhage on their skin and fins. ISA also causes anemia, exophthalmia, ascites, congestion, hepatomegally, splenomegally, congestion in the intestinal mucosa and usually petechia in the visceral fat (See Figures 1, 3 & 4).


In 1995 the Infectious Salmon Anemia virus was isolated from cultured cells confirming the viral etiology of this disease. It appears that the virus belongs to the Orthomyxoviridae virus family. Other viruses in this family include the influenza viruses. Orthomyxoviruses are made up of 8 negative strands of RNA enclosed in a lipid envelop. The surface of the virus contains proteins that attach themselves to a host and inject the virus’ RNA into the host’s cell (See Figure 2).


Once inside the host’s cell, the virus tricks the cell into manufacturing new virus particles using up all the cell’s nutrients, building materials, and eventually causing the cell to self-destruct (called apoptosis). When the cell dies, it ruptures releasing the newly formed virus particles ready to infect neighboring cells and neighboring fish.


Although the disease is caused by a virus, it is not known exactly how the virus is spread through the environment. Salmon in close proximity to one another seem to horizontally transmit the virus amongst cohorts. Salmon also shed the infective virus in their feces. There is no evidence that the virus spreads vertically (from parents to offspring). Wild anadromous trout and salmon have been experimentally infected with the virus and some wild anadromous fish have been found naturally infected with the virus without clinical signs of illness. Is not known whether or not the Infectious Salmon Anemia virus (ISAV) has been present in wild salmonid populations or if it causes clinical disease in wild salmonids. It is not known if other animals, birds, fishes, or insects serve as hosts to the virus.

Currently there are two distinct strains of the virus. The strain infecting fish in Maine has been previously found in Atlantic salmon along the Maritime providences of Canada. This strain is called the New Brunswick ISA strain. Fish in Norway have a distinct Norwegian ISA virus strain.


Figure 4. Internal organs of a salmon experimentally infected with Infectious Salmon Anemia virus. Notice the petechiation (red spottedness) in the pyloric caeca and abdominal fat. Image from M. Opitz University of Maine.

Like the Influenza viruses, there may be several different strains of ISAV. And it may be possible for the virus to change by reassorting its RNA with another strain or altering its genetic code with mistakes in viral replication. This phenomenon in Orthomyxoviruses is called “shift and drift.” It is because of these changes that humans get a flu shot every year—Orthomyxoviruses change.

There are still many unsolved mysteries regarding ISAV. Research on the disease is being conducted in the US, Canada, and Norway. ISAV vaccines are being developed for aquaculture. Wild fish are being screened for the virus. As fish health professionals learn more about the virus, they may be able to find effective prevention and/or treatments.

Petechia are minute, pinpoint, non raised, purplish red spots caused by intradermal or submucal hemorrhage.

Ecchymosis is a hemorrhagic spot larger than petechia.

Special points of interest:

Fish with ISAV cannot transmit the disease to people.

ISA is caused by a virus.

ISAV may infect many species of fish.

For more information read: J. Jarp, E. Karlsen. 1997. ISA risk factors in sea-cultured Atlantic salmon (S. salar). Diseases of Aquatic Organisms 28: 79-86.

Images were made possible in part by a grant from the Maine Outdoor Heritage fund 002-4-7.