Nr. 1 / 2013
Since the introduction of IPN resistant QTL-eggs in 2009, there has been seen a reduction in the number of recorded IPN
diagnoses for salmon by 50 % during the period 2009-2012. Information from both farming companies and national statistics also shows that there has been a considerable reduction in mortality for both 2011 and 2012 generations of salmon in Norwegian fish farming. In the same period the number of QTL-eggs delivered to Norwegian fish farmers has increased greatly. Field investigations to document performance that began at the time of introduction of QTL-eggs underpin our conclusion that both the number and severity of outbreaks has been reduced.
Field investigations with IPN resistant QTL-fish
The first QTL-fish that had their origin in egg intakes from 2009/2010 egg generation were transferred to sea as S0 smolts in the autumn of 2010, and as S1 smolts in spring of 2011. From this egg generation fish were also transferred to sea that had not been selected with the assistance of the IPN gene marker. A total of 30.6 million fish distributed across 44 locations were followed up so that the relative performance of QTL-fish and non QTL-fish could be compared.
The results of the field investigation showed that both S0 and S1 classes of QTL had a lower mortality (1.1 % and 4.6 % compared to 6.4 % and 12.7 %) and fewer IPN diagnoses (0 and 3 compared to 1 and 7) than S0 and S1 of non QTL-fish up to 90 days after sea transfer (figure 1).
IPN resistance tested with a highly virulent field isolate
It has been questioned as to whether the IPN virus would adapt in response to the use of IPN resistant QTL-eggs. As a part of AquaGen following-up on performance of QTL-eggs in the field, highly virulent IPN strains involved in outbreaks have been isolated and analysed for genetic changes (mutations). Part of this follow-up involved a new challenge test that was carried out using a highly virulent IPN virus field isolate by VESO Vikan in the spring of 2013.
A total of three genotypes were tested, qq (homozygote for the trait IPN sensitive), qQ (heterozygote for the trait IPN resistant) and QQ (homozygote for the trait IPN resistant).
Accumulated mortality when the challenge terminated was 59.2 % for qq, 8.0 % for qQ and 2.6 % for QQ. This result gives a relative percent survival (RPS) of 86.5 % for qQ and 95.6 % for QQ, measured against qq as a control (figure 2). This agrees well with the expected degree of protection for this product (AquaGen declares a RPS of 82 % for QTL-innOva IPN).
The results from these analyses, four years after introduction to the market, don’t give any indication that the IPN virus has “found a way round” the defence mechanism posed by the QTL-innOva IPN.
Reduction in IPN diagnoses on a national level
As the proportion of QTL-eggs used in Norwegian aquaculture has increased, the economic importance of the effect of IPN disease has been greatly reduced. National statistics from the Norwegian Veterinary Institute for fish health in 2012 showed that the number of outbreaks for the country had reduced by 50 % from 221 diagnoses in 2009 to 110 diagnoses in 2012 (figure 3). We believe that this trend will be maintained and show further reductions in 2013-2014 by virtue of the fact that 70 to 80 % of all salmon eggs laid down will be of QTL origin.
A large reduction in the prevalence of IPN will also make a positive contribution towards the overall health status and resultant fish welfare of farmed fish. IPN outbreaks in freshwater often result in consequential effects in the form of variable or poor smolt quality. Furthermore it has been recently documented that fish populations that have previously experienced an IPN outbreak have a resulting higher risk for developing PD later in the production cycle.
A fish is a homozygote when it has two identical variants of a gene on a chromosome pair. This can be described as either qq or QQ.
A fish is a heterozygote when it has two different variants of the gene (qQ or Qq).
Genes can be dominant (denoted by a large letter, Q) or recessive (denoted by a small letter, q). Dominant genes will be expressed in the fish phenotype as long as there is at least one on the chromosomes in a pair (qQ, Qq or QQ). Recessive genes must be present on both chromosomes of a pair (qq) to be expressed in the fish.