INDUCTION OF INFECTIOUS PANCREATIC NECROSIS (IPN) IN COVERTLY INFECTED ATLANTIC SALMON, SALMO-SALAR L., POST SMOLTS BY STRESS EXPOSURE, BY INJECTION OF IPN VIRUS (IPNV) AND BY COHABITATION

Atlantic salmon post-smelts were given an intraperitoneal (ip) injecti on of tissue homogenate of Atlantic salmon fry from an outbreak of inf ectious pancreatic necrosis (IPN), and cohabitants were given an ip in jection of Earle's balanced salt solution (EBSS). Parallel treatment g roups were exposed to recurrent episodes of environmental stress by wa ter drainage twice a week. Fish injected with EBSS and non-injected fi sh were exposed to water drainage. The control fish were left untreate d. Mortality due to IPN started 3 weeks after challenge in noninjected and EBSS-injected fish that had been exposed to water drainage. This showed that the fish used in the experiment were covertly infected wit h IPN virus (IPNV) prior to challenge, although no virus was detected in the fish sampled before the experiment. In fish that received an in jection of IPNV, mortality started 5-6 days after challenge, regardles s of the presence or absence of stress exposure. The EBSS-injected coh abitants started to die after an additional 5-6 days, also regardless of the presence or absence of stress exposure. The final cumulative mo rtality in the IPNV-injected fish was significantly lower than in the EBSS-injected cohabitants, thus suggesting that the secondary immune r esponse after injection of IPNV provided more protection than the resp onse after a waterborne infection. No disease outbreak was observed in the control fish.