Infectious Pancreatic Necrosis (IPN): A Disease Guide

Infectious pancreatic necrosis virus causes high mortality in salmonid fry and lifelong carriers in survivors. There is no treatment; control relies on SPF stock and biosecurity.

Overview

Infectious pancreatic necrosis (IPN) is a contagious viral disease of farmed salmonids. The causative agent is infectious pancreatic necrosis virus (IPNV), a double-stranded RNA virus with a bisegmented genome belonging to the genus Aquabirnavirus, family Birnaviridae. The virions are non-enveloped icosahedral particles about 60-70 nm in diameter. IPNV is notably stable in the environment, tolerating a wide pH range and remaining viable after heating, which makes it persistent and difficult to remove.

Affected species

IPNV mainly affects young salmonids such as rainbow trout (Oncorhynchus mykiss), Atlantic salmon (Salmo salar), and brook trout (Salvelinus fontinalis), as well as post-smolts. Fry are most susceptible, especially around yolk-sac absorption and first feeding. The virus or IPNV-like viruses have been isolated worldwide from at least 32 families of saltwater and freshwater fish, many of which can act as carriers without obvious disease.

Clinical signs

• Characteristic whirling or corkscrew swimming, with fish rotating about their long axis
• Darkening of the skin and exophthalmia (bulging eyes)
• Pale gills and a distended, swollen abdomen
• Anorexia, lethargy, and inability to maintain a horizontal position
• Trailing white mucoid faecal casts
• Necrosis of the pancreatic acinar tissue (the primary target organ)

Transmission and the carrier state

IPNV spreads both horizontally, through water and faeces and urine, and vertically, through reproductive fluids to offspring. A defining feature of the disease is that fish surviving an outbreak typically become lifelong carriers, shedding infectious virus in their faeces and sexual products for the rest of their lives. These asymptomatic carriers are a major challenge for control because they silently maintain and spread the virus.

Diagnosis

Diagnosis is confirmed by virus isolation in cell culture and by RT-PCR, supported by histopathology showing characteristic pancreatic acinar necrosis. Because survivors are often subclinical carriers, molecular screening is important for detecting infection in apparently healthy stock and broodstock.

Treatment, control and prevention

There is no treatment, and the only way to eliminate the virus from carrier fish is to destroy them. Management therefore focuses on excluding the virus and reducing both horizontal and vertical transmission.

• Source eggs and stock from specific-pathogen-free (SPF), tested broodstock
• Disinfect egg surfaces and incoming water and maintain strict hatchery biosecurity
• Screen and remove carrier fish and avoid mixing tested and untested stock
• Use IPN-resistant strains selected by breeding programs (including QTL-based selection) where available
• Use vaccination as part of an integrated control program where appropriate