- Koi herpesvirus (KHV), is a highly pathogenic virus causing high mortality in common carp and koi, especially in fry. One of the unique features of herpesvirus infections is their ability to become latent following initial infection. During latent infection, only viral genome persists in the host and remains dormant in the infected cells. It is unknown how KHV latency is established and maintained within the infected cells. DNA methylation has been previously shown to play a role in the latency maintenance of the gammaherpesviruses, Epstein-Barr Virus and Kaposi’s Sarcoma-Associated Herpesvirus. The expression of viral DNA polymerase gene, ORF79, is required for viral genome replication and CpG islands were found to be present in the ORF79 promoter region. In this study, DNA methylation of the ORF79 promoter was investigated to understand the regulation of KHV genome replication during productive and latent infection. Using bisulfite conversion and PCR amplification with methylation-specific primers, DNA methylation within the ORF79 promoter was found to be present in viral genome isolated from productive infection in vitro. Total DNA of white blood cells (WBC) containing a small fraction of KHV DNA was also examined, however, the amount of DNA was too low to assess the status of DNA methylation within ORF79 promoter during latent infection.