The epizootic haematopoietic necrosis virus (EHNV) is an iridovirusin Australia causing severe disease in a range of fish including farmed rainbow trout (Oncorhynchus mykiss) and wild populations of redfin perch (Perca fluviatilis). EHNV is a Ranavirus, one of five genera of viruses within the family Iridoviridae. Ranaviruses are large icosahedral DNA viruses measuring approximately 150 nm in diameter with a large dsDNA linear genome of roughly 150-170 kbp, coding for around 100 gene products. Transmission of ranaviruses is thought to occur by multiple routes, including direct contact, waterborne exposure, and ingestion of infected tissues during predation. Ranaviruses are relatively stable in aquatic environments, persisting several weeks or longer outside a host organism. EHNV enters fish through the body surface or gastrointestinal tract, replication occurs in the nucleus and cytoplasm of the host cell once attachment of viral proteins to host receptors mediates the endocytosis of the virus into the host cell. Synthesis of these viral proteins occurs within hours of viral entry with necrosis occurring as early as a few hours after infection. Clinical signs are nonspecific, in perch sudden death may be the first sign. However, some symptoms noted include darkened skin colour, a distended abdomen, haemorrhages at the base of fins and gills, swollen kidney and spleen. Behavioural signs include fish ceasing to feed and juveniles swimming in a disorientated fashion at surface. Clinical outbreaks are associated with poor water quality,and water temperature between 11°C and 17°C in rainbow trout and above 12°C in redfin perch. Kills occur over a short period (several weeks) in summer, and then the disease may disappear from an area for years. EHNV has the potential to negatively impact native fish populations, currently the transmission and control of EHNV is incompletely understood, due to their nonspecific clinical signs, prolonged survival in the environment and resistance to disinfectants.