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Health guide, not a diagnosis · Serious

Koi herpesvirus (KHV)

Also called khv, koi herpes, KHVD, CyHV-3, Cyprinid herpesvirus 3

Rules and reporting. KHV is a WOAH-listed disease. England and Wales: a listed (notifiable) disease; suspicion in any farmed or wild fish must be reported immediately to the Cefas Fish Health Inspectorate (01305 206700, fhi@cefas.gov.uk), and not reporting is breaking the law. USA: KHV is considered endemic; USDA APHIS asks accredited vets and labs to report positive cases to the Area Veterinarian in Charge and the state veterinarian, but there is no federal mandatory depopulation. Rules differ by country; owners elsewhere should ask their national veterinary authority.

Facts checked against sources on 16 Oct 2026. See sources

Signs

  • Gills mottled red and white (dead patches), pale or bleeding gills
  • Sunken eyes
  • Too much or too little mucus, rough or pale skin patches, bleeding at fin bases
  • Lethargy, gathering at the surface, inlets or shallows
  • Many koi or common carp dying fast (often 80-100%)

Cause

Koi herpesvirus, Cyvirus cyprinidallo3 (formerly Cyprinid herpesvirus 3, CyHV-3; family Alloherpesviridae). Causes disease in koi and common carp (Cyprinus carpio). Disease occurs at about 16-25 °C (GOV.UK: 16-29 °C). Survivors stay carriers for life. Listed by WOAH since 2007.

What to do

There is no treatment or antiviral. Isolate, contact a vet and (where required) the authorities; do not move fish. Survivors are lifelong carriers, so vets strongly recommend or advise considering depopulation, then cleaning and disinfecting everything the fish touched. Raising water to 30 °C only slightly improves survival and leaves survivors as carriers.

If a fish is badly ill, talk to an aquatic vet.

Prevention

Quarantine new koi in a separate system with their own equipment for at least 30 days at 24 °C (75 °F), then test-mix a few new and resident fish for at least 2 more weeks. Buy only fish of known origin; repeat quarantine after koi shows.

Sources

7 facts checked with sources

Species often affected

0 species