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Living with the IPN virus
Nr. 2 / 2007
Our title reflects the fact that the eradication of the IPN-virus from salmon farming is no longer considered to be a viable strategy. This is one of the most hardy of all the viruses, and it has developed cunning strategies to ensure that it will be reproduced in its fish host, and survive in the environment. However, the industry as a whole, and individual fish farmers, can make substantial efforts to reduce its damaging effects.
The actions that fish farmers can take in their facilities, is not the topic of this information letter, and will not be detailed here. First and foremost, it does deal with our efforts to correct sub-optimal breeding related factors and to reduce the spread and propagation of the IPN-virus. The first part of the value chain, that is selective breeding and the subsequent production of eggs for use in commercial production, plays a fundamental role in the industry's efforts to reduce the consequences of IPN. This applies in particular to two factors: firstly the possibility of the control of vertical transmission of the virus, and secondly the production of a more resistant strain of fish through selective breeding.
Vertical transmission of the IPN virus
It has been demonstrated that vertical transmission of the IPN virus can occur in rainbow trout and brook trout, and most experts consider that this infectious route can also apply to Atlantic salmon. What we do not know is the relative importance of this route in comparison with the horizontal route. In both Norway and Scotland there have been official programmes involving the testing of brood fish and subsequent discard of eggs from parents testing positive. This was carried out to reduce the risk of the possibility of vertical transmission. Neither of these programmes have showed obviously positive effects. The Chilean authorities still do require IPNV testing of National brood fish, and also those which produce eggs for import. Norwegian research has, in recent years, contributed to considerable improvement of the methods used in brood fish testing for IPN virus.
In an ongoing project, led by Patogen Analyse AS, and with Aqua Gen AS as a partner, the genetic variants of the virus (high/low virulence) carried by brood fish will be studied. In this project a study of the virus in hatcheries and also wild sea-running fish will be carried out. The ultimate aim of the project is to make improvements in analytical methods and increase knowledge of different genetic variants of the virus. This will enable us to create more meaningful strategies towards combating the damaging effects of the virus, and to make further reductions in the risk of vertical transmission.
Selection for more IPN resistant fish
Aqua Gen first included resistance to IPN in its breeding program for Atlantic salmon in 2001. Our estimates show that 31 % of the total variation in susceptibility to IPN is controlled by genetic factors in the fish, i.e. the heritability is 0.31. During the autumn of 2007 we are to perform the third selection for IPN resistance in the Aqua Gen breeding nucleus. In Information Letter No. 5/2005 we wrote about a documented effect of selection in two groups of fish from the same breeding population: one was selected for high resistance to IPN (HR) and one for low resistance (LR). Average mortality in a challenge test of the progeny (fry) was 29.9 % for HR and 66.6 % for LR (7 replicates). A group of unselected wild salmon had 42.0 % mortality. A follow-up trial that was not mentioned in the Information Letter was performed on smoltified sibs from the same groups of fish and here the mortality was 31.0 % for HR, 79.0 % for LR and 71.0 % for the wild salmon. This shows that our selection for increased IPN resistance can be highly effective.
Genetic markers for IPN resistance
Information Letter No. 5/2005 also mentioned that we had begun a research project together with AKVAFORSK/CIGENE and The Norwegian School of Veterinary Science. The aim of this project was to find the genetic markers that are linked to IPN resistance. Recently a marker (QTL) was detected that can explain 23 % of the phenotypic and 70 % of the genetic variation in IPN susceptibility. This finding is a significant advance that can give us a new tool in our selection for a more IPN resistant fish. Our aim is now to use this new knowledge in order to have the greatest practical effects for our customers.
In Norwegian aquaculture IPN has primarily affected Atlantic salmon. However in recent years there has also been an increasing occurrence of IPN mortalities in Rainbow trout. This has been seen particularly in the early hatchery phase. Aqua Gen performed the first challenge test for rainbow trout in 2006, with results that estimated heritability at 0.29. This now enables us to add selection for higher IPN resistance in rainbow trout to our breeding programme. Subsequently in collaboration with research groups we have initiated studies aimed at generating more knowledge about genetic variants of the IPN virus that causes mortalities in rainbow trout.
Aqua Gen versus other strains
During the autumn of 2006 we incubated eggs from Aqua Gen and two competing Atlantic salmon strains. This was a new project that had the aim of evaluating differences between the strains. An IPN challenge test was performed at the first feeding stage at VESO Vikan. Average mortality in the Aqua Gen strain was 15.2 % (two groups in five replicates), accumulated mortality in strain A was 14.9 % (one group in three replicates) and corresponding figures for strain B was 50.2 % (one group in three replicates). These results are shown in the following graph.
PDF of the Information Letter is found here