EHN

Aetiology 

Epizootic haematopoietic necrosis (EHN) is a viral disease of perch and rainbow trout. The viruses belong to the genus Ranavirus in the family Iridoviridae. The disease occurs in perch and rainbow trout, but experimentally it is shown that several species are susceptible to EHNV. EHNV is closely related to viruses that are responsible for diseases in toads and frogs. EHNV has until now only been detected in Australia, but there is evidence of other closely related viruses in Germany, France and Italy which have resulted in large disease outbreaks in particular catfish. It is only possible to distinguish the different ranaviruses by sequencing.

EHN has never been detected in Europe and the disease is categorized as exotic under EU legislation. 

Symptoms and pathology 

The clinical signs in rainbow trout are non specific and include increased mortality, often associated with poor water quality and general signs of fish not doing well, e.g. fin erosions, darkening and apathetic swimming pattern. There have been outbreaks at water temperatures from 11-20 ºC. In wild perch disease can affect a large proportion of the population with high mortality. There are no pathognomonic lesions. Swollen spleen and liver, increased serosanginous peritoneal fluid and multiple necrotic foci in the liver can be observed.