Infectious haematopoietic necrosis (IHN)
Infectious haematopoietic necrosis is an acute viral disease of trout and salmon fry in North America and Japan. It is caused by a bullet-shaped virus and can be transmitted from fish to fish and from parent to progeny through seminal fluids or infected eggs. The disease is generally seen in fry and fingerlings, but this depends also on the host species. For example, in chinook salmon and steelhead and rainbow trout, mortality may occur from the sac fry stage to the yearling stage. Except for rainbow trout in certain areas, older fish rarely die from IHN.
Dark coloration, weakness, abdominal swelling and pale gills are some of the external signs of the disease. The internal signs are very similar to those caused by other viral infections. In infected sockeye salmon fry, the kidney becomes translucent and speckled with pigment cells. Diagnostic confirmation requires isolation and identification of the virus by neutralization tests with anti-IHNV serum IHN virus can reliably be detected only during the spawning season in carrier fish. Sometimes the IHN infection may be combined with that of IPN and so checks should be made for the presence of other viruses.
The primary mode of transmission is through infected eggs, but other means of transmission such as raw feedstuffs have also been recorded. Sockeye and Chinook salmon and rainbow and steelhead trout appear to be the most susceptible hosts. Coho salmon and other trout species are more resistant. Differing responses have been observed and are ascribed to interactions between strains of the virus, the amount of virus present and the species, strain and age of the host. The incubation period of IHN is temperature-dependent and ranges from 5.5 days at 21°C to about 16 days at 3°C.
No drugs or chemicals are known that will control IHN outbreaks. As in other virus diseases, prevention is the only means of control. The introduction of infected eggs and fish should be avoided. As carrier status for IHN can be reliably detected only at the spawning time and during epizootics, repeated inspections employing thorough virological samplings are necessary