ISA Virus Spreads via Red Blood Cells in Blood Vessels

Lucy Towers
11 April 2013, at 1:00am

NORWAY - New research shows how the interaction between Atlantic salmon and the Infectious Salmon Anaemia (ISA) virus leads to the development and spreading of the influenza-like disease ISA in fish. The new findings may be of interest to research on influenza generally.

ISA was detected for the first time in Norway in 1984 and still poses a serious threat to the fish farming industry. ISA is the only disease of farmed Atlantic salmon that is listed by the World Organisation for Animal Health. The disease usually breaks out in one cage and gradually spreads over a period of weeksand months to neighbouring cages. The disease cannot be treated and an outbreak of ISA can therefore result in large losses.

In her doctoral thesis, Maria Aamelfot elucidates some phases of the disease development. She has studied what type of cells which are receptive to the virus and what type of cells that actually becomes infected by the virus. Her findings describe the ability of the virus to infect or damage certain cells, tissues or organs. Ms Aamelfot’s research on this interaction between salmon and the ISA virus provides us with new knowledge about how the disease ISA develops and represents an important contribution towards, for example, disease prevention.

Virus and receptor = key in a lock

Ms Aamelfot has developed a method of detecting which cells and organs the virus can attach itself to and use as a port of entry for infecting the organism. If a virus is to be able to infect a cell, the cell must have the virus’ specific receptor (adhesion structure) on its surface. Different viruses use their own specific receptors and the virus adheres to these, just like a key fits in a lock.

The ISA virus’ receptor was found in the following cells in salmon: in endothelial cells (the inner lining of blood vessels), in red blood cells in blood vessels and in cells covering the outside of the gills. This is the first research to reveal the presence and cell location of this type of receptor on sections of tissue. Ms Aamelfot compared the receptor pattern with cells infected by the virus in diseased salmon and discovered a clear correlation.

The cells’ “varnish” can get scratched

When the virus reproduces itself in endothelial cells, it is secreted directly into the blood and adheres to the red blood cells which then transport the virus around the blood system. Red blood cells appear to be damaged by being covered by the virus and this contributes to the circulatory disturbances that are characteristic for Atlantic salmon suffering from ISA.

Endothelial cells have several important functions when it comes to blood circulation and the immune defence system. The endothelial interface with the blood is covered by a “cloud” of sugar chains which have both a protective function and participate in communication between cells.

This layer of sugar can be compared to the paint on a car, which when scratched, leads to the development of rust. In the same way, the immune defence system in fish can be weakened if the sugar layer is injured.

Ms Aamelfot has also demonstrated how one structure of the sugar layer, a type of sialic acid, is distributed in the fish at the cellular level. This sialic acid probably plays an important role in maintaining good blood circulation and protecting the fish against infection.

Ms Aamelfot conducted her doctoral research at the Norwegian Veterinary Institute, where research fellows and engineers at the Veterinary Institute and at the Norwegian School of Veterinary Science were key collaborators. Maria Aamelfot defended her PhD research on 21 March 2013 at the Norwegian School of Veterinary Science with a thesis entitled “Tropism of infectious salmon anaemia virus and distribution of the 4-O-acetylated sialic acid receptor”.