Koi Herpesvirus Disease: symptoms, treatment, prevention

Koi Herpesvirus (CyHV-3) is a devastating herpesvirus of common carp and koi that causes mortality above 90 % at 18-28 °C and has no cure.

Overview

Koi Herpesvirus Disease is caused by Cyprinid herpesvirus 3 (CyHV-3). It is highly contagious, kills more than 90 % of infected common carp and koi at 18-28 °C, and creates lifelong latent carriers in survivors. It is a notifiable disease in many countries.

Symptoms

Necrosis and pale patches on the gills
Sunken eyes
Excess mucus followed by mucus loss
Skin lesions
Lethargy
Mass mortality within days

Causes

Introduced with infected or latently carrying fish, shared equipment, or contaminated water. Outbreaks are triggered when water temperature enters the permissive range of 18-28 °C.

Diagnosis

Mass mortality in koi or common carp during warm seasons with gill necrosis and sunken eyes is highly suggestive. Confirmation requires PCR testing on gill or kidney tissue by a fish veterinarian or accredited lab; rule out spring viraemia of carp.

Treatment

There is no effective medical treatment. Management focuses on biosecurity, official notification where required, and limiting spread to other ponds.

Quarantine

A pond or tank in active outbreak should be isolated; nets, hoses and clothing must not be shared with other systems. Survivors should be considered lifelong carriers.

Medication

No effective antiviral cure exists.
Heat treatment around 30 °C may suppress an acute outbreak but creates lifelong latent carriers and is controversial.
Depopulate, disinfect the system thoroughly, and report to veterinary authorities where required by law.

Recovery

Even when fish appear to recover, they remain infectious carriers. Restocking after an outbreak should follow strict disinfection and a long fallow period; consider PCR-tested stock only.

Prevention

Buy only KHV-tested koi
Long quarantine at 22-24 °C for new fish
Vaccinate where legally available
Dedicate equipment to each pond
Disinfect nets between uses