SUSCEPTIBILITY OF ATLANTIC HALIBUT HIPPOGLOSSUS-HIPPOGLOSSUS TO INFECTIOUS PANCREATIC NECROSIS VIRUS

Infectious pancreatic necrosis virus (IPNV), serotype N1, isolated from Atlantic halibut Hippoglossus hippoglossus, was used to bath-challenge (2 x 10(5.0) TCID 50 ml-1 for 1 h) Atlantic halibut fry of different sizes at 12 and 15-degrees-C in 3 trials. In all trials, the fry challenged at 15-degrees-C experienced significantly higher cumulative mortality compared to the unchallenged fry. The smallest fry (mean weight 0.1 g) also experienced significantly higher mortality compared to the controls when infected at 12-degrees-C, whilst the medium-sized fry (mean weight 1.0 g) did not display any mortality when infected at this temperature. The largest fry (mean weight 3.5 g) were only challenged at 15-degrees-C, resulting in 30% cumulative mortality. All fry infected at 15-degrees-C and the small fry infected at 12-degrees-C remained IPNV positive during the entire experimental period. In contrast, the medium-sized fry infected at 12-degrees-C seemed to be able to clear the infection after 3 wk. Moribund and diseased fry showed clinical signs such as distended stomach and uncoordinated swimming. Pathological findings included necrosis of the liver, kidney and intestine, but the pancreatic tissue was unaffected. Immunohistochemistry revealed strong positive reactions to IPNV in the livers of challenged individuals.