The Disease

Introduction
Infectious Pancreatic Necrosis (IPN) is a highly contagious systemic birnavirus disease of young fish of Salmonid species held under intensive rearing conditions. The disease most characteristically occurs in rainbow trout (Oncorhynchus mykiss), brook trout (Salvelinus fontinalis), brown trout (Salmo trutta), Atlantic salmon (Salmo salar) and several Pacific salmon species (Oncorhynchus spp.).

It mostly occurs under intensive rearing conditions in Salmonids hatcheries or in Post-Smolt Atlantic salmon in sea-cages.

History
In 1941 this disease affecting brook trout fingerlings was described in Canada. The most striking symptom was catarrhal enteritis. Later, in 1955, the cause of this enteritis was found to be a virus. In Europe it was first noted in 1965 in France and has since spread across Europe, including the British Isles. In Norway it was first described in trout in 1975 and in 1985 it was found in salmon. In Chile it was described for the first time in 1985.

IPN used to be a disease of first-feeding fry in the freshwater phase, in recent years it has become a serious problem in marine salmonid farming. Salmonids are predominantly susceptible, particularly brook trout, brown trout, rainbow trout, Atlantic salmon, Pacific salmon and Coho salmon. Furthermore, since its first description, infectious pancreatic necrosis virus (IPNV) has been found in a wide range of fresh water and marine fish belonging to families other than salmonidae.

Etiology
The IPN virus is a birnavirus. The original serotype isolated from rainbow trout fingerlings is called VR-299. Since then, several serotypes have been identified and, in 1995, two serogroups, A and B, were established, each containing serotypes with different geographic origins. The most frequently found and virulent serotype is the Sp serotype.

The lethality of the virus depends on the strain, the species and age of the fish. Predisposing stress-inducing factors, such as first feeding, high stocking density, fluctuations in water temperature, salinity and handling, are believed to favour the onset of clinical disease. Outbreaks can occur all year round, at water temperatures as low as 4 ° C and as high as 18 °C.

Infectious Pancreatic Necrosis virus is a bi-segmented double-stranded RNA virus of the family Birnaviridae. At least nine serotypes exist. Susceptibility decreases with increasing age, with complete resistance to clinical disease in fry/fingerlings being reached at about 1500 degree-days (value obtained by multiplying the age in days by the average temperature during the lifespan) except for Atlantic salmon Smolts, which develop the disease within weeks following transfer from freshwater to seawater.

Sources of the Virus

  • Virus is shed via faeces, sexual fluids and probably urine
  • Infected transport water, contaminated nets, containers and other equipment
  • Infectious virus can be transported and excreted by fish-eating birds and mammals
  • Horizontally by contact through the water route and by ingestion of infected material
  • Vertically (via fertilized eggs) from spawning carrier broodfish
  • Direct contact with secretions from clinically infected fish
  • Asymptomatic virus carrier fish or infected embryonated ova may introduce the virus into healthy stocks


Disease reprinted courtesy of OIE Diagnostic Manual for Aquatic Animal Diseases, OIE (World Organisation for Animal Health), Paris, France.