Epizootic hematopoietic necrosis (EHN) is a systemic iridoviral disease of fish. The epizootic hematopoietic necrosis virus (EHNV) causes EHN in redfin perch and rainbow trout. This disease is highly fatal in redfin perch; affected farms usually have serious economic losses, and severe declines can occur in wild populations. Rainbow trout are less likely to be infected, and the cumulative mortality rate is usually low. Closely related viruses cause serious outbreaks of epizootic hematopoietic necrosis in catfish and sheatfish. Currently, the transmission of EHN viruses is incompletely understood. Their control is also complicated by their prolonged survival in the environment and resistance to disinfectants.