What is Koi Herpesvirus (KHV)?
Koi Herpesvirus (KHV) is the causative pathogen of a viral disease that causes fatal damage to fish in the cyprinid family. First reported in Israel and Germany in 1998, it subsequently spread rapidly worldwide. In Japan, it was first confirmed in Lake Kasumigaura, Ibaraki Prefecture in 2003, and is now designated as a "specified disease" by the Fisheries Agency.
For goldfish breeders, particular caution is needed regarding the risk of infection spreading through keeping koi together in the same tank or reusing shared water. KHV infects all cyprinid fish (ornamental koi, carp, and goldfish), though the mortality rate in goldfish is sometimes slightly lower than in koi. However, goldfish are not completely immune and can become carriers that spread infection.
Main Symptoms and How to Identify Them
Fish infected with KHV display the following symptoms.
Early Symptoms (1-3 Days After Infection)
Fish may exhibit "head-standing" behavior, floating listlessly at the surface, or sinking to the bottom and remaining motionless. White necrotic spots appear on the gills, and rapid gill flapping due to respiratory distress can be observed. Excessive mucus secretion with a whitish tinge or hemorrhagic spots may appear on the body surface.
Mid-Stage to Terminal Symptoms (3-7 Days After Infection)
As symptoms progress, the eyes may become sunken or bulge outward (exophthalmia), and the body surface skin sloughs off. Gill tissue becomes necrotic and turns chocolate brown, while affected fish completely refuse to eat. In mass infections, massive die-offs within days are not uncommon.
Relationship with Water Temperature
Water temperature significantly affects KHV manifestation. Symptoms are most likely to appear in the 18-25°C (64-77°F) temperature range, where viral replication is most active. At temperatures below 10°C (50°F) or above 30°C (86°F), symptoms are less likely to appear, but the virus does not disappear—it enters a latent state, requiring continued caution.
Infection Routes and Spread Risk
KHV is highly contagious and spreads through the following routes.
Direct Contact Infection
Direct contact with infected fish is the most common transmission route. Infection can also occur from apparently healthy "asymptomatic carrier fish."
Water-Mediated Infection
Water in which infected fish have swum contains virus particles, and infection can occur simply by sharing the same water. Adding water from ponds or rivers to tanks, or reusing equipment across multiple tanks, increases infection risk.
Indirect Infection via Equipment and Human Contact
Sharing nets, buckets, pipettes, and other equipment across multiple tanks without disinfection can transport virus. Virus can also be introduced to other tanks via hands or clothing that have contacted infected water.
Purchase-Related Risk
When purchasing new fish from shops or markets, carrier fish may be inadvertently included. Since infection cannot be determined by appearance alone, newly purchased fish must always be placed in isolation and quarantine for a minimum of 2 weeks.
Legal Regulations and Reporting Obligations
In Japan, KHV is designated as a specified disease under the Fisheries Resource Protection Law, and notification to the prefectural fisheries authority is mandatory if KHV is confirmed or suspected. This reporting obligation applies to breeders of ornamental fish (goldfish and koi).
Moving, transferring, or selling infected fish without notification is a legal violation. If infection is confirmed, administrative authorities may conduct inspections and issue disposal orders.
Breeders raising large quantities of ornamental goldfish and large-scale farmers with ponds must be particularly careful. If suspicious symptoms appear, do not take independent action—first consult with your nearest fisheries research station or agricultural ministry office.
Diagnostic Methods
Definitive diagnosis of KHV requires testing at specialized facilities. General breeders cannot make a definitive diagnosis by visual inspection alone.
The primary diagnostic method is PCR testing (genetic testing), which detects viral DNA from gill or kidney tissue. This highly sensitive test can also detect asymptomatic carrier fish in latent stages. If fish die, preserve the carcass by refrigeration (not freezing) and submit it to your prefectural fisheries research station for testing.
Treatment and Response Measures
Unfortunately, there is currently no definitive treatment for KHV. The following measures are standard once infection is confirmed.
Response Upon Symptom Onset
Immediately isolate any fish showing symptoms. Since other fish in the same tank or pond may be infected, stop removing water or fish from the facility. While some reports suggest slowing disease progression through temperature management (raising temperature above 30°C/86°F), this is not a scientifically established protocol.
Measures After Infection Confirmation
After notification, follow administrative guidance on euthanasia of infected fish and disinfection of tanks and equipment. KHV can be inactivated with sodium hypochlorite (chlorine-based bleach), which should be used to disinfect equipment and tanks. Sand and substrate are effectively disinfected through sun-drying or heat treatment.
Prevention Strategies and Daily Management
The two most important principles are "do not introduce" and "do not spread."
Quarantine of New Fish (Most Critical)
All newly purchased goldfish and koi must be kept in a quarantine tank completely isolated from the main tank (using dedicated nets and equipment) for at least 2 weeks, ideally 4 weeks. If no symptoms appear during this period, the fish can be moved to the main tank.
Equipment Disinfection
When sharing equipment across multiple tanks or ponds, thoroughly disinfect with sodium hypochlorite solution (200 ppm or higher) or boiling water before and after each use.
Purchasing from Trusted Sources
To reduce infection risk, purchase from trusted breeders or shops that work with KHV-tested individuals. Breeder-direct platforms like br-choku allow direct purchase from breeders with transparent rearing environments, reducing risk from distribution channels.
Considerations for Pond and Outdoor Cultivation
In outdoor ponds, cormorants and wild animals can transmit infection. Installing bird netting and avoiding direct intake of river water are effective preventive measures.
Summary
Koi Herpesvirus (KHV) is a threat that is anything but distant for goldfish and koi enthusiasts. It is essential to recognize KHV as a designated disease with legal reporting obligations, and to minimize risk through daily sanitation practices, quarantine of new fish, and selection of trusted sources. Early consultation with specialists and authorities when abnormalities are detected is critical for appropriate response.
