Download Living with the IPN virus

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Living with the IPN virus
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.
mes 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
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 IPNvirus. 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 program-
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Healthy yolk sack fry of Rainbow trout
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Information Letter
No. 2 / 2007
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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.
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.
Rainbow trout
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
Accumulated mortality, % ± SE
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 followup 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.
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.
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Aqua Gen
Strain A
Strain B
The results from an IPN challenge test of the Aqua Gen strain of first feeding Atlantic
salmon and two other strains.
Aqua Gen AS • PO Box 1240 • Havnegata 9 • N-7462 Trondheim
Phone. +47 72 45 05 00 • Fax. +47 73 54 62 91 • [email protected] • www.aquagen.no
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