Koi herpesvirus or KHV is proposed as a third cyprinid herpesvirus (CyHV-3) in the family Herpesviridae. Others resist the classification as a herpesvirus and prefer the designation CNGV.  Most knowledgeable scientists concur that it is a herpesvirus.

KHV was diagnosed in koi and common carp (Cyprinus carpio) beginning in Israel in  May of 1998 and later from 1996-archived tissue of fish from  Europe. It has quickly spread around the entire world and has been reported in the U.S. since 1998. Other cyprinids such as common goldfish and crucian carp seem to be immune to the disease but can become infected with the virus and may become carriers.  As with other herpesvirus infections, KHV is believed to remain in surviving fish for life, thus exposed and recovered fish should be considered dangerous potential carriers of  the virus. There are anecdotal reports of naïve koi being added to a pond containing KHV survivors and resulted in new outbreaks of the disease.  

KHV disease may cause 80–100% mortality in affected populations, and fish seem most susceptible at water temperatures of about  20° to 26° C (about 68° to 79°F), although the disease is reported to have occurred from 17° to 28° C.  KHV effects koi & carp of all ages. Infections are transmitted via virus in the water, in fecal material, in sediments and from fish to fish.  Death may occur very rapidly in affected populations, starting within a very few days after the onset of clinical signs. KHV infection usually results in severe gill damage and erosion.  Secondary bacterial and parasitic infections may also be present. Affected fish often remain near the surface, swim lethargically, and may exhibit respiratory distress and/or uncoordinated swimming.

It is impossible to diagnose KHV based on clinical signs alone as these vary and are non-specific.  A lab capable of testing for the KHV viral DNA, its proteins or the virus itself (viral isolation), is currently the only way to gain a definitive diagnosis.

Too little is known about KHV and much research needs to be done before science can protect our koi collections from this deadly disease.