Koi herpesvirus (KHV / Cyprinid herpesvirus 3)
A highly contagious and often fatal viral disease affecting koi (Cyprinus carpio) and common carp. Causes gill damage, lethargy, sunken eyes, and mass mortality typically at water temperatures between 16–25°C (60–77°F). A notifiable disease in many countries. There is no cure; management focuses on biosecurity and welfare decisions.
Do first
- Do NOT move fish from an affected tank to other bodies of water or ponds. This spreads KHV.
- Isolate the affected tank/pond from other water systems and equipment immediately.
- Contact your local aquatic veterinarian or animal health authority. KHV is notifiable in many jurisdictions.
- Do not handle water from affected systems and then access other ponds or fish-holding facilities.
- Humanely euthanize moribund fish to limit suffering.
Escalate if
- Multiple koi dying rapidly in spring or autumn (peak temperature range for KHV): contact a veterinarian immediately.
- A notifiable disease authority needs to be informed. Check your local regulations.
- Mass mortality in a public or commercial koi facility: immediate professional involvement required.
Water clues
These readings can push this pattern higher or lower in the triage result.
KHV is less active above 30°C. Raising temperature above this threshold is used diagnostically and as a management tool, but is not a cure.
KHV is less active below 16°C. Cold temperatures suppress outbreaks but the virus persists.
Care protocol
Follow only the steps that fit your species, tank inhabitants, and medication label.
KHV suspicion: what to do
- KHV should be suspected when: multiple koi are dying rapidly, gill tissue appears pale or mottled, eyes appear sunken, and water temperature is 16–25°C.
- Definitive diagnosis requires PCR testing. Send a freshly dead fish (chilled, not frozen) to a veterinary diagnostic laboratory.
- Do not attempt to treat with antiparasitics or antibiotics. These will not affect a viral infection.
- Contact a fish veterinarian immediately. Do not delay for a definitive test result before biosecurity measures.
- KHV survivors become lifelong carriers and can shed virus during temperature stress.
- Do not sell or move fish that have survived a KHV outbreak without disclosure and veterinary guidance.
Biosecurity and management
- Quarantine all new koi for a minimum of 30 days before introducing to an existing pond.
- Disinfect all equipment, nets, and clothing before and after visiting different fish-holding facilities.
- Consider raising water temperature above 30°C for 21 days in an affected system. This suppresses active viral replication (management tool only).
- Euthanize humanely: clove oil solution (eugenol) is widely used for fish. Consult a veterinarian for dosing.
- After an outbreak, disinfect the pond before restocking. Use bleach or potassium permanganate depending on setup.
- Temperature manipulation does not cure KHV. It suppresses replication and may allow fish to survive the acute phase.
- KHV-positive koi should not be resold. This is ethically problematic and may be illegal in some jurisdictions.
Source notes
References and context notes used for this triage entry.