Infectious hematopoietic necrosis (IHN) is a disease of salmonid fishes, mainly Oncorhynchus spp., including rainbow trout O. mykiss and Atrantic salmon Salmo salar. IHN causes serious economic losses. It was initially known as endemic in sockeye salmon fry O. nerka and chinook salmon fry O. tschawytscha in West Coast of North America since the 1950s. However, it was spread to Japan, Korea and Taiwan in the 1970s. It was spread to Italy and France in the 1990s. Currently, IHN can be found in many parts of the world, including Russia and South America. Mortality due to IHN in fish with ≤ 0.5 g of body weight can reach 60% to 100%. Mortality caused by IHN will reduces when fish grow bigger. However, the onset of IHN has also increased recently in fish with larger. Here, we describe the molecular epidemiology and virulence changes of IHN virus (IHNV) in Japan, and the detection and identification methods for IHNV. Furthermore, we will discuss the future prospects of IHNV researches.
