What Is Koi Herpes Virus (KHV) Fish Disease?

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Koi Herpes Virus (KHV) is a DNA virus that causes a highly contagious disease. KHV is exclusively in common carp, Ghost Carp, and the ornamental variety of the Koi carp. It causes significant mortality. 

KHV can occur in all freshwater lakes and rivers. 

A basic history of KHV Disease

The first recognized case of KHV occurred in the UK in 1996. In 2002 the virus was confirmed in several Carp fisheries in England.  Since then other cases have been confirmed in almost all countries that culture koi and/or common carp except for Australia 

What Are The Other Names Of KHV?

Koi Herpes Virus is the most common name for the disease but many anglers and fishery owners also call it the “fish herpes”, “carp herpes” or even “fish pox”.

What are the factors causing KHV? How does it spread?

The most common concerns are as follows:

  • Stress
  • Poor Water Quality
  • Fish stocking
  • Water Temperature
  • Overcrowding
  • Contamination (from fertilizers and pesticides)

Koi Herpes Virus Disease (KHV) affects fish of all ages. The disease results in 80–100% mortality rates when the water temperature is between 60°-77°F (16°-25 °C). Spring and Autumn are the most common times in Europe for the disease to spread.

KHV stays in the infected fish for life, therefore fish that recover, are considered carriers of the virus.

The process of fish stocking is the greatest risk of introducing the disease to any fishery. Detecting the virus in healthy fish is very difficult, it can spread undetected. It is possible that wet fishing tackle, nets, boots, or the transfer of infected water transmit the virus. It is crucial that diligent fishery management is undertaken to minimise the risks of transferring KHV into any fishery.  

Fishery owners now typically have a quarantine process prior to restocking Carp. This involves a large tank of 100 gallons or more to store the fish for three weeks before releasing them to the main fishery. This process requires constant temperature monitoring at 70°-78°F (21°-25°C)

Why is KHV a major concern?

KHV outbreaks have caused huge fish stock losses at many fisheries. Therefore, sometimes a total removal of all the fish in the lake is the only option. Despite controls, the virus has spread rapidly and is now widespread in England. KHV has never been found in any other species.

What are the signs of KHV?

The fish death rate may begin very rapidly in infected populations, with deaths starting within 24 – 48 hours after the initial signs. In most cases, fish die within the first two weeks.  A fish with the KHV infection shows spots and patches of red and white gill lesions or bleeding.

It is quite common to find Infected Carp near the water surface or grouped together close to the margins, swimming slowly.

KHV Gills Eyes
Sunken Eyes (Top) and Gill Lesions (Bottom picture)

How do fish get infected with KHV?

The spreading of KHV from the infected fish is often due to handling, fluids from infected fish, and contact with water and mud. The highly infectious virus transmits to fish through the skin and gill tissue. Survivors of KHV outbreaks usually become carriers of the virus and are capable of spreading the disease to all other fish in the lake. Despite many studies, the transmission to hybrid common carp and other cyprinids, or non-cyprinid species to native common carp varieties remains unclear.

Another potential cause is stress. Good fishery management is so crucial in reducing disease outbreaks in commercial and syndicate carp fisheries

Rigid fishery management practices are key. Fish overcrowding and excessive angling pressure can increase stress within the fish population.

Infections increase when stress impacts the immune response of carp. Studies have proven that KHV outbreaks occur more in intensively managed fisheries, with high stocks.

Is There A Cure For KHV?

There is no proven treatment for KHV. Antiviral drugs are not available to treat KHV in fish in a fishery or cultured fish in a pond. Basically, any affected fish will have to use its own defense mechanism to fight the virus itself.

Water temperatures rising or reducing due to weather changes may help to reduce the spread to other fish. However, carrier fish do not die from KHV disease or show signs of infection.

Scientists and laboratories across the world are continuously studying Koi Herpes Virus, searching for an effective KHV treatment.

What can we do as Anglers to help reduce KHV?

It is also possible to transfer the virus from wet fishing tackle, nets, and boots, or by the transfer of infected water. The following chart explains what we should do as anglers to help prevent this dangerous KHV disease from spreading.

Protect Your Fishery 1

 Most good fisheries will have a disinfectant wash tank. Dipping your nets as you enter the fishery is a very important practice. 

Net Dip
Always dip and soak all your nets, cradles, etc for 30 seconds before you start to fish

Always, make sure you lay out all your nets, mats, cradle, slings, etc in the sun to dry off any potential bacteria. Do this BEFORE you start fishing for a minimum of 20 minutes. This practice is generally mandatory when fishing matches.

When arriving home after a day of fishing, wash all your relevant equipment with a hosepipe and dry it again.

Whilst this is somewhat unproven to help control the issue (based on scientific studies). Anglers should at least do whatever we can to protect and reduce the risks.

Since many of us visit different lakes and rivers the risk of transfer is significant.

Can Humans Catch KHV?

No, the herpes virus causing KHV disease in fish is not transmissible to humans. In fact, it will not affect any other species of fish except the common carp, Ghost Carp, and the ornamental variety of the Koi carp


Koi Herpes Virus is a devastating disease of common carp and Ornamental Koi. Infections can be detected but there is no fully effective treatment to remove the virus from the fish. It’s crucial for fishery owners to use the correct management practices such as quarantine and testing to prevent the disease from occurring.

KHV is a major concern and anglers must play their part in controlling the spread.

When fishing, please report any suspect fish infections to the attention of the fishery owner or the fishing club bailiffs.

A free 24-hour reporting service is available. Just call the emergency number at the Environment Agency on 0800 80 70 60 to report any concerns. 

I hope that you found this article useful, please check out my other Fish Care posts here!

Steve Fitzjohn