Infectious pancreatic necrosis virus infection in Atlantic salmon Salmo salar post-smolts affects the outcome of secondary infections with infectious salmon anaemia virus or Vibrio salmonicida.

Infectious pancreatic necrosis (IPN) virus (IPNV) infection in Atlantic salmon Salmo salar L. post-smolts and its influence on the outcome of secondary infections with infectious salmon anaemia (ISA) virus (ISAV) or Vibrio salmonicida were studied. The infections with ISAV or V salmonicida were performed both in a period of acute IPN and in the following IPNV carrier stage, 3 and 6 to 8 wk after experimental IPNV challenge, respectively. An IPNV carrier condition at low virus titre did not influence the mortality rates after secondary infections. Neither the ISAV infection nor the V. salmonicida infection in experimentally induced IPNV carriers resulted in mortalities different from those observed after challenge of IPNV-free fish. At higher IPNV titres in Atlantic salmon with acute IPN, the outcome of secondary infections was quite different from that observed in IPNV-free fish and in IPNV carriers. In 2 different experiments significantly more fish died when fish with acute IPN were infected with V salmonicida than when fish were infected with V salmonicida alone. Mortality also started earlier in the double-infected group than in the group challenged with V. salmonicida alone, 3 to 4 and 8 d after V salmonicida infection, respectively. Similar results were observed independent of whether mortalities due to IPN alone were registered in the experiments. When Atlantic salmon with acute IPN were infected with ISAV, significantly fewer fish died than when fish were infected with ISAV alone. The ongoing IPNV infection seemed to provide some protection against development of ISA.