Infectious pancreatic necrosis ( IPN ) is a highly contagious viral disease of young fish of salmonid species held under intensive rearing conditions ( 14, 32, 33 ) . 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. ) . Susceptibility generally decreases with age, with resistance to clinical disease in salmonid fish usually being reached at about 1500 degree-days ( value obtained by multiplying the age in days by the average temperature in degrees centigrade during the lifespan )( 11 ) , except for Atlantic salmon smolts, which can be affected after transfer from fresh water to seawater ( 29 ) . The first sign of an outbreak in salmonid fry is frequently a sudden and usually progressive increase in daily mortality, particularly in the faster growing individuals. Clinical signs include darkening pigmentation, a pronounced distended abdomen and a corkscrewing/spiral swimming motion. Cumulative mortalities may vary from less than 10% to more than 90% depending on the combination of several factors, such as virus strain ( 17 ) and quantity ( 24 ) , host and environment ( 10 ) . For further details see reviews by Hill ( 14 ) , Reno ( 27 ) and Wolf ( 32 ) .
The disease is transmitted both horizontally via the water route and vertically via the egg ( 3, 4, 12 ) . Surface disinfection of eggs is not entirely effective in preventing vertical transmission ( 6 ) .
The disease has a wide geographical distribution, occurring in most major salmonid-farming countries of North and South America, Europe and Asia. Oceania is free of the disease.
IPN virus ( IPNV ) , or viruses showing serological relatedness to IPNV, have been reported to cause diseases in some farmed marine fish species, such as yellowtail (Seriola quinqueradiata)( 21 ) , turbot (Scophthalmus maximus)( 7, 20, 22 ) , dab (Limanda limanda) (25), halibut (Hippoglossus hippoglossus) (20, 28) and Atlantic cod (Gadus morhua). Subclinical covert infections have been detected in a wide range of estuarine and freshwater fish species, such as loach (Misgurnus anguillicaudatus)( 8 ) , pike (Esox lucius)( 2 ) and numerous other species in the families Anguillidae, Atherinidae, Bothidae, Carangidae, Cotostomidae, Cichlidae, Clupeidae, Cobitidae, Coregonidae, Cyprinidae, Esocidae, Moronidae, Paralichthydae, Percidae, Poecilidae, Sciaenidae, Soleidae and Thymallidae (27) .
The causative agent, IPNV, is a bi-segmented double-stranded RNA virus belonging to the family Birnaviridae ( see review by Dobos & Roberts [ 10 ]) . Isolates display wide antigenic diversity ( 11, 16, 19, 23 ) and fall into two serogroups that do not cross-react in serum neutralisation tests ( 3, 25, 31 ) , with the majority of strains belonging to serogroup A, which comprises at least nine serotypes ( 16 ) . Isolates also show marked differences in degrees of virulence ( 14, 15, 17 ) .
Control methods currently rely on the implementation of control policies and of hygiene practices in salmonid husbandry, through the avoidance of the introduction of fertilised eggs originating from IPNV-carrier broodstock, and the use of a protected water supply ( e.g. spring or borehole pond ) where the ingress of fish, particularly possible virus carriers, is prevented. In outbreaks, a reduction in the population density ( 'thinning out' ) may help to reduce the overall mortality. No treatment or entirely effective vaccine is available at present.