CyHV3

CyHV-3, or koi herpesvirus, is a large double-stranded DNA virus in the genus Cyprinivirus of the family Alloherpesviridae. It is the etiological agent of koi herpesvirus disease (KHVD), which primarily affects koi and other species of carp (notably Cyprinus carpio). First identified in the late 1990s, CyHV-3 has since caused widespread outbreaks in ornamental koi populations and in some commercial carp operations across multiple continents. The virion is enveloped and icosahedral, and the genome is approximately 295 kilobases in length, encoding around 150 to 160 predicted genes. The virus replicates in the nuclei of infected cells and disseminates through infected tissues and secretions.

Host range and susceptibility are greatest for koi and juvenile carp, though other cyprinids can be affected under certain conditions. Transmission is primarily horizontal, via water, direct fish-to-fish contact, and contaminated equipment or surfaces. The virus can persist in the environment and be shed into the water column, promoting rapid spread in crowded or poorly managed facilities. Outbreaks are associated with moderate water temperatures, and clinical signs typically emerge over several days to weeks.

Clinical presentation includes lethargy, loss of appetite, abnormal swimming, gill necrosis, skin and fin lesions, and high mortality in naïve populations. Survivors may become carriers. Diagnosis is based on laboratory confirmation, most commonly polymerase chain reaction (PCR) detection of CyHV-3 DNA from gill swabs, kidney, or fin tissues; virus isolation and histopathology can support confirmation.

There is no universally licensed antiviral treatment. Control relies on rapid diagnosis, removal and culling of infected stock, strict biosecurity, disinfection of equipment and facilities, and quarantine to prevent movement of potentially infected fish. Vaccines have been developed and used in some regions, but they are not universally licensed and typically offer partial protection. Ongoing research focuses on improved diagnostics, vaccines, and biosecurity strategies to mitigate KHVD outbreaks.