Koi Herpesvirus (KHV)

by Amy Young
June 15, 2020

What is koi herpesvirus?

Takeaways

Koi herpesvirus (KHV) is a highly contagious viral pathogen of common carp and koi carp varieties.
Mortality rates can be 80-100% in some cases.
Lowering the water temperature to <15°C can reduce mortalities, but fish that recover can become persistently infected and are a major source of concern for the spread of the virus.
There is no current treatment for KHV, but supportive care may reduce mortalities.
Prevention is essential to the health of a population. Fish should be purchased from reputable sources and new animals should be quarantined for at least 6 weeks prior to joining the larger population.

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Koi herpesvirus (KHV), also known as Cyprinid herpesvirus 3 (CyHV3) or carp interstitial nephritis gill necrosis virus, is a highly contagious viral pathogen of common carp and koi carp varieties. Mortality rates are influenced by the virulence of the strain of the virus, the age of the fish (younger fish are more affected than older fish), and the temperature of the water, amongst others.

The incubation period of the virus is 3-21 days (on the shorter end in warmer water (15-25°C), and the virus is stable in the water for up to 7 days. Lowering the water temperature can result in reduced, or even no, mortalities, but the fish can become persistently infected and a major source of concern for the spread of the virus. Persistently infected fish exposed to a combination of stress and warmer water can lead to shedding of virus and clinical presentation.

The disease has a worldwide distribution, with the exception of Australia. It has been a World Organisation for Animal Health (OIE) notifiable disease since 2007, meaning that veterinarians and laboratories that encounter positive cases must report them to the state veterinarian.

What are the clinical signs and gross changes of koi herpesvirus diseases?

Clinical signs of KHV vary and can include piping, difficulty breathing, gathering near oxygen-rich areas (such as waterfalls or aerators), excessive mucous production from gills, swollen gills, gill necrosis/discoloration, ulcerations, reddened skin/fins/tail, pale patches, sloughing of scales, increased/decreased mucous production, “sand paper” texture to the skin, decreased appetite/refusal of food, lethargy, sunken eyes, erratic swimming, and “hanging” with a head down position. Mortality rates are highest 48-72 hours after the first onset of clinical signs.

How is koi herpesvirus diagnosed?

Koi herpesvirus can be diagnosed by a variety of methods. Direct methods of diagnosis detect whole virus or virus DNA and include virus isolation and PCR. Quantitative PCR (q-PCR) is very sensitive and specific, can quantify viral load, has been validated, and is the test accepted by the OIE. Indirect methods detect exposure to the virus by looking for antibodies and include ELISA and many others. Just a few ELISA tests for KHV have been validated. It is often recommended to perform both q-PCR and ELISA testing during quarantine to prevent dissemination of the virus.

How is koi herpesvirus treated?

There are currently no treatments for KHV. Supportive care includes increasing or decreasing the water temperature to levels that are non-permissible for the virus (below 60°F/16°C or above 27°C), increasing the salinity to 0.5-3 parts per thousand (g/L) to help with osmoregulation, increasing aeration, and controlling secondary bacterial, fungal, and parasitic infections. It is important, however, not to add antibacterial, antiparasitic, or antifungal treatments to the water unless indicated. These treatments have no direct benefit against KHV and may adversely affect already compromised gills. It is also important to remember that all surviving fish are considered carriers. In cases where large commercial breeding or wholesale populations are affected, depopulation and disinfection are recommended.

What is the prognosis for koi herpesvirus?

Koi herpesvirus can result in 80-100% mortality under ideal conditions for the virus. It is often more severe in younger fish. Fish that recover from infection may remain infected for life and should be considered carriers of the virus.

How can koi herpesvirus be prevented?

In order to prevent KHV, it is important to purchase fish from known, reputable sources. ELISA blood tests can be utilized to look for previous exposure. ELISA testing can detect antibody reactivity long after the virus itself has cleared, allowing for broad pond/farm surveillance. The UC Davis veterinary hospital clinical laboratory services offer both the ELISA and PCR tests for KHV.

It is also recommended to avoid Japanese style koi shows in which fish are mixed in tanks.

New animals should be quarantined separately from the population for at least 6 weeks at virus-permissible temperatures (60-80°F/16-28°C, with higher temperatures being better). Separate equipment should be used for quarantined fish. Quarantined fish should be closely monitored for abnormal appearance and behavior, such as loss of appetite.

Water and equipment can be disinfected with 200 parts per million (ppm) chlorine bleach for 1 hour or 500 ppm quaternary ammonium compounds for 1 hour. A table of additional effective chemical disinfectants can be found in the Southern Regional Aquaculture Center’s Publication, Biosecurity in Aquaculture.

There is currently no vaccine for KHV available in the U.S.

Please contact the UC Davis veterinary hospital’s Aquatic Animal Health Service, part of the Companion Exotic Animal Medicine and Surgery Service, for more information about fish health and medical services.