William H Wildgoose MRCVS
Pet fish vet
Koi herpesvirus (KHV)
This virus is highly infectious and causes severe gill disease and high mortality when water temperatures are over 16°C. Originally identified in 1999, it primarily affects carp and is present in many countries around the world. The virus, cyprinid herpesvirus 3, is similar to that which causes carp pox. It spreads through direct contact with infected fish or contaminated water and equipment. The disease may be triggered by overstocking or poor water quality. Clinical signs of the disease include sunken eyes, lethargy, loss of appetite and sudden death. There is no treatment and mortality is often between 70-100%. It is a notifiable disease in the UK and cases must be reported to the relevant authorities (eg Cefas).
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There is extensive necrosis of the gill tissue (whitened areas) with secondary fungal infection and green algae trapped in the damaged tissue. The virus affects many internal organs although the most visible lesions are on the gills.
The gill disease is less severe in this case although excess mucus is present with early stages of tissue necrosis (whitened areas). The fine cartilage tips of the primary gill filaments are visible where the soft tissue has rotted away (arrows).
This leaflet was produced by Cefas and is available to download here. Although written to advise fishery owners, it applies equally to koi keepers.
There is extensive necrosis of the gill tissue (whitened areas) with secondary fungal infection and green algae trapped in the damaged tissue. The virus affects many internal organs although the most visible lesions are on the gills.