How to Treat Koi Herpesvirus

Koi pond

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Koi herpesvirus is a viral disease in koi fish caused by Cyprinid herpesvirus-3. It is a highly contagious and deadly disease. All koi owners should be aware of the clinical signs of this disease and the risks associated with adding new koi into their ponds without first testing them and placing the new fish into quarantine. All it takes is one carrier fish to take down an entire pond.

What is Koi Herpesvirus?

Koi herpesvirus (KHV) is a highly virulent virus in all varieties of koi and common carp (Cyprinus carpio). It is an OIE reportable disease and found throughout the world. The disease is caused by Cyprinid herpesvirus-3, a closely related virus to the carp pox virus (Cyprinid herpesvirus-1) (link to article) and goldfish hematopoietic necrosis virus (Cyprinid herpesvirus-2).

Symptoms of Koi Herpesvirus

  • Mottled or white gills
  • Increased mucus on head and body
  • Sudden death
  • Lethargy
  • Decreased appetite

Koi herpesvirus is active only in certain temperatures, and in the winter when the water is cooler the fish may show no signs of disease. The most common sign of KHV is multiple deaths once your pond water temperature increases to the active threshold for KHV of 60 to 77F (16 to 25C). Fish may be lethargic or have a decreased appetite prior to death, or show no clinical signs prior to death. A full physical exam of all remaining fish and water quality testing is critical in evaluating any pond with sudden deaths. Similar clinical signs can be seen in other viral diseases, including Spring Viremia of Carp and Carp Edema Virus or Sleepy Koi Disease.

Causes of Koi Herpesvirus

Fish contract Koi herpesvirus through physical contact with infected fish, transmission between fish on equipment such as nets, and through water shared with infected fish. Classified as a herpesvirus, this disease has a latent stage that resides within the neural tissue of fish. If a fish were to survive an infection, they are assumed to be a carrier for life. Carriers can survive and not show clinical signs, but shed the virus to naïve fish in the pond, spreading the infection.

Like many diseases in fish, the spread of Koi herpesvirus is temperature dependent. Pond temperatures between 60 to 77F (16 to 25C) are within the range for KHV replication. Some ponds may only see clinical signs when water temperatures are within range, then signs subside when the temperature increases or decreases.

Goldfish and some other carp species have been shown to be carriers of KHV.They will not show any clinical signs of the disease, but can spread it to susceptible species. Testing is available to determine if a carrier status exists in other carp species.

Carriers of Koi herpesvirus are not usually persistent shedders. They are more likely to shed viral particles when they are stressed from poor water quality, overstocking, spawning or other causes.

Diagnostic Techniques

Testing for Koi herpesvirus is available through your aquatic veterinary professional. Tissue or blood samples are collected from the fish and require a veterinarian to submit the samples to a diagnostic laboratory. PCR testing, using gill samples from live fish, or internal tissues from freshly dead fish, will test for active infections in fish producing KHV antigens. ELISA testing, using a blood sample, will test for antibodies to KHV and may be used to detect any carriers.


There is no cure for Koi herpesvirus. However, raising the pond or quarantine tank water temperature to 86 degrees F (30 C) has been shown to inactivate the viral infection and help the koi recover. Good water quality, high oxygen saturation in the water, and treatment of secondary bacterial infections are important in helping the koi survive. Once a fish is infected, if they survive, they are assumed to be a carrier for life. KHV can have a very high mortality rate, between 80 to 100%, so do not expect all your fish to survive.

If an outbreak is confirmed, your veterinarian is required to contact your state veterinarian to report the event. You are not required to depopulate your pond. It is up to you and your veterinarian to decide what to do with any survivors. If you choose to keep your fish, the pond should remain a closed system, with no new fish added in and no fish being moved out.

How to Prevent Koi Herpesvirus

The best method of dealing with Koi herpesvirus is to avoid it altogether. The most important aspect of preventing KHV is proper quarantine and testing. Quarantine duration will be temperature dependent, since colder water temperatures will slow emergence of clinical signs and the fish will need a longer quarantine time. Incubation time for KHV is 30 days at 50 F (10 C) but only 3 days at 75 F (23.8 C).

Some koi dealers will test fish before they leave their facilities, but the type of testing will determine if a fish is actually a carrier or not. All testing is required to be submitted by a veterinarian who will discuss any potential positive results with you prior to bringing your fish home.

Is it Contagious to Humans or Other Fish?

Koi herpesvirus is not contagious to humans and most other fish. Infections have been demonstrated in goldfish and some goldfish x common carp hybrids. These other species can carry the disease and never show any clinical signs, but testing is available to be sure they are not carrying the virus if you plan on mixing them in with koi.

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  1. Hedrick RP, Waltzek TB, McDowell TS. (2006). "Susceptibility of koi carp, common carp, goldfish, and goldfish x common carp hybrids to Cyprinid herpesvirus 2 and Cyprinid herpesvirus 3." Journal of Aquatic Animal Health 18:26–34.

  2. Gilad O, Yun S, Adkison MA, Way K, Willits NH, Bercovier H, Hedrick RP. (2003). "Molecular comparison of isolates of an emerging fish pathogen, koi herpesvirus, and the effect of water temperature on mortality of experimentally infected koi." J Gen Virol 84:2661–2668.

  3. Hedrick RP, Waltzek TB, McDowell TS. (2006). "Susceptibility of koi carp, common carp, goldfish, and goldfish x common carp hybrids to Cyprinid herpesvirus 2 and Cyprinid herpesvirus 3." Journal of Aquatic Animal Health 18:26–34.