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Nr. 1 / 2008

Different research results imply that breeding for IPN resistance produces an increase in the general resistance to several infectious diseases in addition to IPN.  Aqua Gen has recorded these results in a field test where salmon from strongly IPN resistant families had a lower mortality at the outbreak of both gill infection and Pancreas Disease than families with weak IPN resistance.

Pancreas disease (PD) has become a great challenge for the European salmon industry, and the viral disease, in addition to historic problems in Scotland and Ireland now seems to have an increasing prevalence along the Norwegian coast. As in most other infectious diseases which have been investigated, it seems that there are also genetic variations in susceptibility to PD.

In Ireland there are clear indications that farmed fish with differing genetic origins have a differing susceptibility to PD. This has been confirmed through field experiments and infection experiments (McLoughlin et al., 2006). In Ireland studies were also carried out to find genetic variation in PD susceptibility in spontaneous field outbreaks and it was concluded that it is possible to carry out genetic selection to increase the salmon's resistance to PD (Norris, 2007).

Since 2005 Aqua Gen has studied the genetic conditions surrounding the salmon's susceptibility to PD in order to, if possible, adopt PD resistance as a new breeding goal. Due to difficulties in obtaining good laboratory challenge test results, field trial work was the method chosen.

Results from field tests

In 2005/06 Aqua Gen and Marine Harvest started a field test by placing 400 salmon families at a location in Hordaland, Norway where PD infection challenge had been experienced in previous production cycles. The families were divided into four groups based on genetic values for IPN resistence. The fish were stocked as first-year smolt in the spring of 2006. In May/June 2007 PD was diagnosed in the test fish, and there was a clear correlation for families with high genetic values for IPN to have the lowest mortality (Figure 1). The family group with high IPN genetic values had 40 % lower mortality compared to the family group with low IPN genetic values.

Prior to the PD outbreak there was a mortality event where proliferative gill infection was diagnosed in the test fish. Similarly to the correlation during the PD outbreak, it was shown that the highest genetic values for IPN had the lowest mortality. IPN was not diagnosed in this field test.

Figure 1. Mortality during a natural PD outbreak in a field test in 2007. The 400 families of test fish were divided into four categories depending on genetic values for IPN resistance. There was a clear correlation to lowest mortality in the IPN strong families. The most dominant cause of death before the PD outbreak was gill infection (epitheliocystis).

Better non-specific immune defence

Resistance to IPN is well established in Aqua Gen's genetic program. Since 1997 infection tests have been carried out on families in Aqua Gen's breeding population. The results of these tests have formed the basis for selection of families with high IPN resistance. These tests are run on start feeding ready fry, which are very susceptible to IPN. In such young fish the specific immune response is not highly developed. To fight the virus the fish must therefore mobilize its non-specific immune defences.

The non-specific immune response is the body's first line of defense against micro-organisms, and it is active against a great number of pathogens, not just a specific disease. Since the IPN-test is run on small fish, it is probable that the non-specific immune response is what is important for survival, and that increased IPN resistence also carries with it a selection for a generally better non-specific immune defence. The results from the trials so far indicate that fish which have high resistance to IPN are also less susceptible to other viral diseases, and perhaps also for diseases caused by intracellular bacteria.

Future PD Projects

There are still not any experimental infection tests for PD which produce a noticeable mortality, therefore more field tests of spontaneous PD outbreaks must be carried out in order to study genetic conditions. Aqua Gen now has plans to repeat a field test in 2008/09. An important goal will be to validate preliminary findings, namely that the correlation between PD and IPN resistance is so strong that PD " goes along for the ride" when breeding is carried out for increased IPN resistance. We are also planning a project to identify genetic markers for PD resistance, which will make possible marker-assisted selection. For IPN we have now identified such a marker, which explains most of the variability in genetic variation for resistance against IPN.

References

McLoughlin, MF; Graham, DA; Norris, A; Matthews, D; Foyle, L; Rowley, HM; Jewhurst, H; MacPhee, J; Todd, D. Virological, serological and histopathological evaluation of fish strain susceptibility to experimental infection with salmonid alphaviruses. Dis. Aquat. Org. 72, no. 2, pp. 125-133. 2006.

Norris, A. Heritability of mortality in response to a natural pancreas disease challenge in Atlantic salmon post-smolts on a West of Ireland sea site. Aquaculture 272, S296 p. 2007.

PDF of the Information Letter is found  here