The inflammatory responses of salmon may be influenced by new types of feed

An increased understanding of the cell types of the fish immune system, and how they communicate with each other, is important for the development of better vaccines for salmon farming, and to shed light on the ways that new feed types may influence fish health.

The aim of the doctorate was to study how one of the key mediators of inflammation, tumour necrosis factor a (TNF-a ), influences and controls the earlier phases of inflammation and how it influences the innate and adaptive immune systems. The major regulators of the immune response are the cytokines, small proteins that function as messengers among the cells of the immune defence system. One of these cytokines is TNF-a.

In the course of his studies, Øyvind Haugland cloned, sequenced and characterised TNF-a in salmon.The results show that there are two variations of the gene in salmon. Studies in fish after vaccination and of inflammatory cells grown in cell culture show that these two variations are regulated in different ways. The significance of TNF-a was studied in connection with vaccination of salmon and the introduction of alternative feed sources, which also may influence the salmon inflammatory reaction.

Studies in salmon have shown that vaccination with oil-containing vaccines leads to a rapid and intense, but temporary, expression of TNF-a and other cytokines. Three weeks after vaccination, however, the levels of cytokines were nonetheless insufficient to alter the amount or expression of genes influencing the presentation of vaccine components to other parts of the immune system. The key factor here is almost certainly time, in that the inflammatory response of salmon is much slower than it is in warm-blooded animals.

With the aquaculture industry showing strong growth, future increases in production will demand the use of alternative feeds. The availability of marine feedstuffs on the world market is today limited, and the hunt for alternatives to both fish meal as a protein source, and to fish oil, has been going on for a long time. An important question in this regard is whether alternative feed sources affect fish immune defence systems and health. In his doctorate, Haugland investigated whether components in some of these alternative feed sources can influence the inflammatory reaction of salmon.

An alternative to the fish meal currently used in feed is soya protein. The results of the work showed that an extract from the soya plant, Glycine tomentella, reduces the level of one of the central chemical messengers in inflammation, none other than TNF-a.

Fish oil is a scarce resource that in fish feed is being replaced by rapeseed oil. Haugland's studies have shown that this replacement leads to significant changes in the fatty acid composition of salmon tissues, but that neither the degree of inflammation nor the bacteria-killing abilities appear to be affected.

Cand. med. vet. Øyvind Haugland defended his thesis for the degree of Philosohiae Doctor on July 4, 2008 at the Norwegian School of Veterinary Science, with the title ”Studies of inflammation and immunity in Atlantic salmon – with focus on TNF-a expression”.