Introduction

Infectious pancreatic necrosis virus (IPNV) was the first virus to be isolated from teleosts. This work led to the development of fish virology research. IPNV is classed as a birnavirus and is an unenveloped icosohedral virus approximately 60 nm in size, containing a double-stranded, bisegmented RNA genome. The virus is stable and can survive in fresh, brackish and sea water. Various serotypes exist and a number of families of fish have been recorded as being infected including: Anguillidae; Clupeidae; Gadidae; Percidae; Pleuronectidae; and Salmonidae, as well as invertebrates, i.e. Ostreidae.

Early reports of infectious pancreatic necrosis virus (IPNV) were from epizootics in cultured brook trout, Salvelinus fontinalis, but subsequent investigations showed that it was responsible for disease in a number of other salmonid genera, including Oncorhynchus and Salmon. Within salmonid farming in Scotland and Norway, mortality associated with IPNV occurs during the freshwater first feeding stages and latterly has been a major factor in mortality within seawater culture. In sea water, IPN occurs within a few weeks following the transfer of smolts, usually in June following a spring transfer.

Gross Signs

The disease is characterised by behavioural changes including anorexia and an agonal corkscrew swimming motion interspersed with ataxia. Non-specific external signs include hyper-pigmentation, emaciation, exophthalmia and petechial haemorrhage on the ventral surfaces. Internal gross lesions include: visceral petechia; pale liver; and spleen, reduced fat body and an empty gut containing a yellow exudate. Similar signs are seen within diseased smolts following seawater transfer.

Histopathology

In fresh and sea water, the principal lesions are focal or coagulative inflammatory infiltrate with moderate to marked necrosis of the pancreatic acinar tissue with acute enteritis. Exocrine cells become necrotic with basophilic inclusions recognised as products of cell breakdown and pyknotic nuclei. Pathology extends into the gut mucosa and necrotic areas are replaced with fibrous/fatty tissue and cell sloughing in combination with increased mucous resulting in an acute, catarrhal exudate. In moribund, fish necrosis of the renal tubules, haematopoietic tissue and liver has been recorded with mild to moderate focal necrosis, slight congestion and scattered pyknotic cells. Survivors of outbreaks can exhibit fibrous replacement of the exocrine pancreas. Hyaline degeneration of skeletal muscle has also been reported.

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Natural infection of rainbow trout with Infectious Pancreatic Necrosis virus.  Fish show shrunken body and darker colouration

IPN affected pancreas from Atlantic salmon, x20, H&E stain

IPN affected pancreas from Atlantic salmon, x40, H&E stain

IPN affected caeca from Atlantic salmon, x20, H&E stain

IPNV localisation by IHC from Atlantic salmon, x20