In order to study the variety of infectious pancreatic necrosis virus (IPNV) strains involved in outbreaks of infectious pancreatic necrosis (IPN) in Atlantic salmon fish farms, samples were collected from 19 different outbreaks of IPN in the northern part of Norway. The main objective of this study was to examine whether IPNV isolates of different virulence were involved in the outbreaks and could explain the variable IPN protection observed in vaccinated post-smolts in the field. Both the molecular basis of virulence of all field isolates and virulence expressed by mortality after bath challenge of unvaccinated post-smolts with eight of the isolates were studied. Very little variation among the field isolates was detected when the 578-bp variable region encoding the VP2 protein known to be involved in virulence was sequenced. The cumulative mortality after experimental challenge with field isolates genetically characterized as highly virulent was always high (40-56%), while the cumulative mortality of the same strains in vaccinated post-smolts during the field outbreaks varied from 1 to 50%. Although the tested samples came from fish vaccinated with the same vaccine product, the protection against IPN varied. These results demonstrate that differences in virulence of the isolates were not the main reason for the variation in mortality in the field outbreaks. Most of the field isolates were of high virulence, which is shown in experimental challenges to be important for mortality, but clearly other factors that might affect the susceptibility of IPN also play an important role in the outcome of an IPNV infection.
The spotted wolffish Anarhichas minor is a promising new species for cold-water aquaculture. The broad host-range of piscine nodavirus (NV) and infectious pancreatic necrosis virus (IPNV) makes them potentially pathogenic to new fish species in aquaculture. IPNV and NV strains highly pathogenic in farmed Atlantic salmon Salmo salar and halibut Hippoglossus hippoglassus, respectively, in Norway were used for the challenge of spotted wolffish. In general, water-borne infection with IPNV and NV resulted in significant mortality among juveniles <1 g. Cumulative mortality after bath-challenge and cohabitation was 60 to 75% in the smallest juveniles (0.3 g). Intramuscular and intraperitoneal injection of NV was 100% lethal to wolffish of 10 g, and the groups at 12°C died before those at 7°C. No cohabitants of this size died, but NV was still detectable in these individuals after 10 wk. A persistent IPNV infection with low mortality developed in bath-challenged juveniles of 0.7 g, in which IPNV was still detectable 4 mo later. This study comprises a demonstration of experimental viral infections in cultured spotted wolffish, although to date no natural outbreaks of viral diseases have been reported in this species. KEY WORDS: Viral nervous necrosis · VNN · Viral encephalopathy and retinopathy · VER · Nodavirus · Infectious pancreatic necrosis · IPN · IPN virus · Spotted wolffish Resale or republication not permitted without written consent of the publisherDis Aquat Org 59: [101][102][103][104][105][106][107][108] 2004 Nakai 1997), and recently in the USA (Curtis et al. 2001) and Atlantic Canada (Johnson et al. 2002). Like IPN, VNN has caused high mortality in fry of Atlantic halibut (Grotmol et al. 1995), and in Atlantic cod (Starkey et al 2001, Johnson et al. 2002. Recently a persistent infection of NV was reported in halibut juveniles (Johansen et al 2002).The broad host-range of NV and IPNV makes these viruses potentially pathogenic to new fish species in aquaculture. The aim of the present study was to investigate the susceptibility of spotted wolffish juveniles to NV and IPNV strains highly pathogenic to farmed Atlantic salmon and halibut in Norway, and proactively study the possibility of VNN or IPN outbreak in this species. Some factors known to affect the outcome of these viral infections in fishes, such as host size, water temperature and infection route, were examined in 3 separate challenge experiments. However, the virulence of NV and IPNV was not compared. MATERIALS AND METHODSInitially, production of farmed spotted wolffish was small and limited the number of fish available for experimental studies. The present study was performed over a 4 yr period, using 3 different generations of spotted wolffish progeny (P) produced in 1998, 2000 and 2001. Attempts at viral detection were done on all batches of non-challenged wolffish: in 2 pools of 5 juveniles sampled at the start and the end of the trials, as well as in dead individuals. Due to the low number of control fish in Expt 1, these were only...
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