ABSTRACT: This paper examines the association between (1) specific antibodies against infectious pancreatic necrosis virus (IPNV), (2) plasma chloride levels, (3) infection with erythrocytic inclusion body syndrome (EIBS), and the risk of clinical infectious pancreatic necrosis (IPN) and cumulative mortality in sea-cultured Atlantic salmon post-smolt. The field study initially included 29 smolt groups raised in 10 freshwater hatcheries. The groups were transferred to seawater in spring 1993 and monitored from just prior to seawater transfer until 1 October the same year. Before seawater transfer, specific antibodies against IPNV were found in 62.1% of the groups. Anti-IPNV antibodies were most frequently detected in fish which had suffered from clinical IPN during the fry period. However, only antibody-positive groups in which no earlier outbreaks of IPN had been recorded during the freshwater stage appeared to be protected against IPN outbreaks after transfer to the sea. The cumulative mortalities in seawater for groups which were positive or negative for IPNV-antibodies prior to seawater transfer were 3.9 and 6.6%, respectively. No relationship was found between the hypo-osmoregulatory capacity or infection with EIBS and the risk of clinical IPN after seawater transfer. The group-level prevalence of EIBS was 48.3% before seawater transfer, decreasing to 42.1 and 28.9% for the first and second samplings after seawater transfer, respectively. The protective effect of acquired specific humoral immunity shown in the present study may be important for the control of IPN in farmed salmonids.

KEY WORDS: Epidemiology · Infectious pancreatic necrosis · Atlantic salmon · Immunity · Smoltification · EIBS

Published in DAO Vol. 27, No. 2 (1996) on November 14
ISSN: 0177-5103. Copyright © Inter-Research, Oldendorf/Luhe, 1996