The Changing Face of ISA

NORWAY - Infectious salmon anaemia (ISA) is one of the most economically-damaging diseases in Norwegian fish farming industry.

It is caused by a marine Orthomyxovirus, the same family that produces the influenza A virus that causes disease in birds and mammals, reports the Norwegian School of Veterinary Science. For his doctorate at the university, Turhan Markussen looked at factors of the ISA virus genes that influence its ability to cause disease in salmon.

Infectious salmon anaemia was first recorded in Norway in 1984 and has subsequently appeared in a series of countries around the north Atlantic. In 2008 it was officially confirmed that the disease had also spread to the southern hemisphere, after several large outbreaks in Chile. During the last 25 years, over 460 outbreaks of ISA have been recorded, all of them in farmed salmon.

Around 1990 there was a serious epidemic in Norway, with some 50 - 80 outbreaks annually. Comprehensive preventative measures were introduced and the number of outbreaks was greatly reduced. However, the disease has proven to be difficult to eradicate, and recent years have again seen an increase in the incidence of the disease.

Not all of the ISA virus variants appear to be virulent and the progression of the disease varies from outbreak to outbreak as both the virus and its salmon host fight a struggle to prevail. To be able to develop a vaccine or a treatment regime, it is decisively important to understand the dynamics of this struggle.

One of the goals of Turhan Markussen's doctorate was to map the factors in the genes of the ISA virus that influence its ability to produce disease. Markussen and his colleagues found that even small variations in the genetic material can be of great significance for virulence. Variations in one of the virus's surface proteins, haemagglutinin esterase (HE), was previously been assumed to be an important factor, and the results of this doctorate support that assumption.

This doctorate also revealed that another of the virus's surface proteins, fusion protein (F), is of decisive importance for virulence. Exhaustive analyses of the gene sequence of a series of ISA viruses, isolated both from disease outbreaks and from fish without classical signs of ISA, found an important marker for the virus's virulence in precisely this protein. This result is similar to that found in virulent influenza A viruses.

Viral genes other than those coding for surface proteins were also shown to be significant for the virus's disease-causing abilities. It was shown for the first time that the ISA virus produces substances that inhibit the production of interferons, which are proteins the host uses in its first line of defence.

The results from this doctorate are an important contribution to the understanding of the interaction between the ISA virus and the salmon, and with that to efforts to develop combative measures against the disease.

Cand. Scient. Turhan Markussen defended his Ph. D. thesis, entitled "Molecular determinants of infectious salmon anaemia virus (ISAV) virulence", at the Norwegian School of Veterinary Science on March 19, 2009.

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