INTRODUCTION
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.