Koi Herpesvirus Disease

The aetiological agent is koi herpesvirus (KHV) in the family Herpesviridae (18, 45) although it has also been given the name carp interstitial nephritis and gill necrosis virus (CNGV) (20, 30). Waltzek et al. (42) provided evidence to support the classification of the virus as a herpesvirus, and named it cyprinid herpesvirus 3 (CyHV-3), following the nomenclature of other cyprinid herpesviruses: CyHV-1 (carp pox virus, fish papilloma virus) and CyHV-2 (goldfish haematopoietic necrosis virus). Sequence analysis of part of the genome has shown that KHV is closely related to CyHV-1 and CyHV-2, and distantly related to channel catfish virus (Ictalurid herpesvirus: IcHV-1) and Ranid (frog) herpesvirus (RaHV-1) (42). Recently, Aoki et al. (2) described the complete genome sequence of KHV and identified 156 unique protein-coding genes. They suggest the finding that 15 KHV genes are homologous with genes in IcHV-1 confirms the proposed place of KHV in the family Herpesviridae. Early estimates of the genome size of KHV varied from at least 150 kbp (14) to 277 kbp (20) but the size is now confirmed as 295 kbp (2, 42). Estimates of virion size also vary. Nucleocapsids of negative-stained virus have been measured at 103–112 nm in diameter surrounded by an envelope (18, 20, 40). The nucleocapsids of thin-sectioned virus have been measured at 78–84, 80–110 and 110–120 nm in diameter (5, 6, 18, 28).