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Fewer Discoveries of Pathogenic Viruses that are Common in Farmed Fish

03 April 2013

NORWAY - Results from the 'Annual report on health monitoring of wild anadromous salmonids in Norway' showed few discoveries of pathogenic viruses in wild fish that are common in farmed fish.

The Food Safety Authority started with monitoring programmes for disease in wild salmonids in 2012. The purpose of monitoring is to determine whether the contagious diseases of farmed fish lead to infection in the wild. The Veterinary Institute has been responsible for monitoring wild fish in lakes and  in seawater.

The findings of PRV positive fish mainly agreed with previous work on a similar brood fish material sampled in 2007, 2008 and 2009 (Garseth et al. 2012a) that showed a PRV prevalence of 13.4 per cent in wild brood fish and 24.0 per cent in brood fish released from stock enhancement hatcheries.

In the presented results, the Food safety Authority does not distinguish between truly wild salmon and salmon released from stock enhancement hatcheries. It should be noted that three hatcheries localised in Nord-Trøndelag, Hordaland and Østfold respectively, had 95-100 per cent prevalence of PRV.

The FSA had no indications that wild salmonids will develop HSMI as a consequence of PRV infection (Garseth et al. 2012a). However, as the virus is newly identified, it would be premature to conclude about the effects on wild fish at the present time.

The seven R. salmoninarum positive salmon from Hordaland all originated from the same hatchery. The finding was confirmed as BKD by the NVI both by PCR and pathological investigation, and the Norwegian Food Safety Authority has been informed of the findings. R. salmoninarum has previously been found occasionally in this river system, and the infection pressure may have been amplified by extensive cultivation of smolts in cages in lake Evanger during the last years. However, this is just speculations.

The finding of 2 PMCV positive salmon is on par with our previous findings from 2007, 2008 and 2009 (Garset et al. 2012b).

Usually, there are a few findings of IPNV each year, but no findings will still be within the normal range. There has been a decline in IPNV cases in the aquaculture industry in 2012, indicating a lower infection pressure. However, we do not know if our previous findings of IPNV in wild brood fish are related to salmon farming. They may also be due to virus isolates that are specific for wild salmon.

In conclusion, it appears that viral infectious agents that are highly prevalent within the Norwegian aquaculture industry, in particular IPNV and SAV, are found only in low prevalence in wild brood fish. The obvious question, as raised by McVicar in 1997 (McVicar 1997), is whether this is due to a low infection pressure or if wild fish infected by a virulent agent rapidly die and thus avoid to be sampled.

Sequencing of viral agents found in wild fish may show if they are similar to virus found in farmed fish or if they are specific for wild populations, and thus contribute to answer this question.

TheFishSite News Desk



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