Koi herpesvirus (KHV) in the common carp (Cyprinus carpio): studies on latency, transmission and serology

Koi herpesvirus (KHV) is an emerging viral disease that has been associated with mass mortality in Cyprinus carpio, which includes the common carp and the koi carp. Its similarity to other piscine herpesviruses suggests that latency is likely and may be a major contribution to the spread of the virus through the international trade in koi carp. This work is an investigation into the conditions necessary to induce reactivation of infections which may be persisting latently and the danger previously exposed fish play to naive fish, upon co-habitation. This study also investigates the possibility of antibody production to the virus as a means of detecting previously exposed fish. Common carp were experimentally exposed to KHV and then subjected to 2 temperature scenarios indicative of conditions that may be met in the wild. Carp exposed to the virus for 2 hours at 21 °C, then dropped to l2°C and held there for 6 months, showed very little initial mortality, but when raised to 23°C suffered a re-emergence of clinical signs and a subsequently mortality (53%). These previously exposed clinical fish were able to transmit infectious virus to naive fish that subsequently suffered 100% mortality. Although virus was not isolated during the 6 months at l2°C the temperature scenario used may be similar to that met by carp in a mild winter or early spring, and into summer and may be evidence for latency. Common carp, exposed to KHV and not lowered in temperature (held at a permissive temperature for pathogenesis) showed high initial mortality but no later signs of disease or transmission to naive fish. Antibody to KHV was detected using an Enzyme Linked Immunosorbent Assay (ELISA) and was consistent with exposure. Seroconversion in common carp experimentally exposed to KHV was between 40 - 60% and was more rapid in the fish that were held for longer at a high initial temperature. The applicability of antibodies as a screening tool for KHV exposed fish is discussed.The Centre for Environment Fisheries and Aquaculture Science (CEFAS)- Weymout