Koi herpesvirus (KHV)
Key facts: Koi herpesvirus (KHV), also known as cyprinid herpesvirus 3 (CyHV-3), is a highly contagious double-stranded DNA virus of the family Alloherpesviridae that causes severe disease and mortality in koi and common carp (Cyprinus carpio)[1][2].
Koi herpesvirus disease (KHVD) is one of the most significant viral diseases affecting koi and common carp worldwide, with cases confirmed in almost all countries that culture koi, with the exception of Australia[1]. The first recognized case occurred in the United Kingdom in 1996 according to the UF/IFAS EDIS fact sheet[1], while the UK government states KHV was first recorded in Israel in 1998 following large-scale mortalities of farmed carp[3]. A secondary source, Wikipedia, also states the first case was reported in 1998 and confirmed in 1999[6].The virus is classified as Cyvirus cyprinidallo3, previously known as Cyprinid herpesvirus 3 (CyHV-3), within the family Alloherpesviridae, and is closely related to carp pox virus (CyHV-1) and goldfish hematopoietic necrosis virus (CyHV-2)[1][2]. KHV is a notifiable (listed) disease under World Organisation for Animal Health (WOAH) standards and under UK regulations, and outbreaks must be reported to relevant authorities[4][3].What is KHV?
KHV is a double-stranded DNA virus belonging to the family Alloherpesviridae, with a genome of approximately 295 kbp[2]. Research by Waltzek and colleagues confirmed it is a herpesvirus based on morphology and genetics, closely related to carp pox virus (CyHV-1) and goldfish hematopoietic necrosis virus (CyHV-2)[1]. Virus nucleocapsids measure 100–110 nanometers in diameter and are surrounded by an envelope, with enveloped virions ranging from 170 to 230 nanometers depending on infected cell type[2].KHV disease has been diagnosed in koi and common carp[1]. Other cyprinids such as goldfish (Carassius auratus) and grass carp are typically unaffected by disease, although KHV DNA has been detected in their tissues using PCR after exposure[1]. Goldfish can become infected and spread the disease to koi without showing disease signs themselves[1]. Hybrid goldfish x common carp crosses showed moderate resistance to mortality in experimental infection[1].Signs and symptoms
According to the UK government, signs of KHV disease can include patches of dead (necrotic) tissue in the gills, often with secondary fungal or bacterial infection, sunken eyes, over- or under-production of mucus, rough dry patches of skin, sloughing of mucus, bleeding on the skin and base of fins, and patchy pale coloration or reddening of the skin[3]. A 2026 GOV.UK outbreak notice lists similar signs: necrotic (white or brown) patches on the gills, rough patches on the skin and sloughing mucus, and sunken eyes[7].Affected fish often become lethargic and gather at water inlets or in shallow water during an outbreak[3]. A secondary source also describes gill mottling, red and white patches on gills, bleeding gills, pale patches, blisters, and behavioral changes such as disorientation, hyperactivity, and isolation from the shoal[6].Mortality and temperature dependence
KHVD affects fish of various ages, often resulting in 80–100% mortality in susceptible and naïve populations when water temperatures are between 60°F and 77°F (16°C and 25°C)[1]. The WOAH Aquatic Manual states morbidity can reach 100%, with mortality of 70–100%, though some common carp strains have shown differential resistance, with the most resistant strains experiencing cumulative mortality of approximately 40%[2]. The UK government notes the disease occurs in water temperatures between 16°C and 29°C, and that stress events can trigger an outbreak[3]. Clinical signs may become apparent 3–21 days after naïve fish are introduced to a pond containing infected fish[2].Fish that recover from a KHV outbreak should be considered lifelong carriers of the virus and should not be exposed to naïve fish[1]. The Southern Regional Aquaculture Center notes that when first introduced into new fish populations, KHV may kill 70 to 95 percent of fish[5]. According to the koi dealer Kodama Koi Farm, for an outbreak to occur the water temperature must be within the optimum range and the koi need to be under stress[8].Transmission and carrier status
A study by Costes and colleagues found that the skin covering the fins and body of carp is the major portal of entry for KHV[2]. The virus spreads systemically to internal organs, with high levels of KHV DNA detected in kidney, spleen, liver, and gut tissue[2]. According to the UK government, the virus is released via feces, urine, gills, and skin, and can spread through movement of infected fish, contaminated water, and contaminated equipment such as husbandry, angling, and transport equipment; infected fish do not need to be visibly sick to spread the virus[3].No species of vector has been demonstrated to transmit KHV to susceptible species, though KHV DNA has been detected in plankton (particularly Rotifera), swan mussels, freshwater shrimp, and in migratory wild ducks in areas where fish and ducks coexist[2]. Survivors of infection may become persistently infected and retain the virus for long periods without clinical signs; a Japanese survey of carp in Lake Biwa found 54% of older carp seropositive and 31% PCR positive after prior outbreaks, suggesting periodic reactivation of latent virus[2].Diagnosis and PCR testing
Real-time PCR is the test recommended by WOAH for surveillance in apparently healthy populations to declare freedom from KHV infection[2]. The most commonly used quantitative assay is the Gilad TaqMan real-time PCR assay, though published real-time PCR methods may not detect novel strains of cyprinid herpesvirus closely related to KHV[2]. When testing clinically affected fish, it is recommended to sample gill, kidney, and spleen tissue, where virus levels are highest during overt infection; fins are also a suitable, easily collected organ for PCR detection[2].The Southern Regional Aquaculture Center notes that cell culture tests for herpesviruses will almost always be negative even when fish carry the virus, making PCR tests for viral DNA a better detection approach, although PCR may still fail to detect very low-level carriers[5]. Antibody-based blood testing is also available through specialized laboratories for KHV[5]. According to the koi dealer Kodama Koi Farm, PCR testing is used as part of screening protocols before koi are sold[9].Prevention and management
There is no treatment for KHV, according to the UK government, and prevention is considered the best approach to disease control, including awareness of signs, regular stock health checks, good husbandry and biosecurity practices, and avoiding fish of unknown origin[3]. The Southern Regional Aquaculture Center likewise states there are no drug or chemical treatments that can clear the virus from carrier fish, though outbreak losses can sometimes be reduced by manipulating water temperature; any fish treated this way must still be considered virus carriers with the potential to infect others[5]. There are currently no vaccines for fish herpesviruses available in the United States, though a KHV vaccine is available in some other countries[5].Koi breeders, dealers, exhibitors, and veterinarians should be aware of international export health regulations regarding movement of KHV-vaccinated fish, as import requirements in some countries specify that vaccinated fish will not be permitted entry[1]. Readers concerned about introducing disease to an established collection may consult guidance on quarantine practice and on recognizing signs of illness.Notifiable status and regulation
Both KHV and spring viremia of carp (SVC) are listed as notifiable fish diseases by WOAH[1]. Within the United States, KHV is considered widespread, and USDA APHIS asks accredited veterinarians and laboratories to report cases to the area veterinarian-in-charge of the state where an outbreak occurs, though not every outbreak is notified to WOAH[1]. In England and Wales, KHV is a 'Listed' or notifiable disease that must legally be reported immediately to the Fish Health Inspectorate (FHI), part of Cefas, under the Aquatic Animal Health (England and Wales) Regulations 2009; it is an offence to fail to report suspicion of KHV[3].KHV outbreaks have been subject to statutory controls in the UK since 2007, and the UK maintains a surveillance program for the disease[7]. When an outbreak is confirmed, the FHI applies disease controls including restrictions on moving aquatic animals, eggs, and gametes into, out of, or within a designated area, and may require culling of fish and disinfection of premises[7]. More broadly, readers can consult the page on notifiable koi diseases and on importing and exporting koi for related regulatory context.KHV compared with other cyprinid viral diseases
It is important to differentiate KHV from other viruses affecting carp and koi, including spring viremia of carp (SVC) and carp pox (CyHV-1), as these diseases have significantly different management and regulatory implications[1]. SVC is caused by a single-stranded RNA virus (Sprivivirus cyprinus) and affects a broader range of species including grass carp, bighead carp, silver carp, Crucian carp, and goldfish; see the page on spring viremia of carp (SVC)[1]. Carp pox (CyHV-1) typically causes smooth raised skin growths in older fish and is not a reportable disease in the US; it is described further at carp pox (CyHV-1)[1].| Feature | Koi herpesvirus | Spring viremia of carp | Carp pox |
|---|---|---|---|
| Viral agent | DNA herpesvirus (Cyvirus cyprinidallo3) | RNA rhabdovirus (Sprivivirus cyprinus) | DNA herpesvirus (Cyvirus cyprinidallo1) |
| Optimal water temperature | 64–81°F (18–27°C) | 41–64°F (5–18°C) | Below 68°F (20°C) |
| Species affected | Common carp, koi; goldfish may carry but not show disease | Common carp, koi, goldfish, grass carp, bighead carp, silver carp, Crucian carp | Common carp, koi |
Zoonotic risk to humans
There is no zoonotic concern with KHV; the herpesvirus responsible for the disease in fish does not cause disease in humans[1]. The UK government similarly states that KHV does not affect humans and that there is no risk to public health[3][7].Common questions
What temperature range triggers KHV outbreaks?
Can KHV spread to humans?
Is KHV a notifiable disease?
How is KHV diagnosed?
Related koi topics
References
- ^ UF/IFAS EDIS VM113: Koi Herpesvirus Disease (KHVD). University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/VM113. Retrieved November 2026.
- ^ Manual of Diagnostic Tests for Aquatic Animals, ch. 2.3.6 Koi herpesvirus disease (WOAH). World Organisation for Animal Health. woah.org/fileadmin/Home/eng/Health_standards/aahm/current/2.3.06_KH…. Retrieved November 2026.
- ^ Koi Herpesvirus (KHV) - GOV.UK. UK Government. gov.uk/government/publications/koi-herpesvirus-khv/koi-herpesvirus-…. Retrieved November 2026.
- ^ Koi herpesvirus disease - WOAH - World Organisation for Animal Health. World Organisation for Animal Health. woah.org/en/disease/koi-herpesvirus-disease/. Retrieved November 2026.
- ^ SRAC Publication No. 4710: Herpesviruses in Fish. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4710%20Herpesviruses%20in%20Fis…. Retrieved November 2026.
- ^ Koi herpesvirus (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Koi_herpesvirus. Retrieved November 2026.
- ^ Outbreaks of Koi herpesvirus (KHV) disease in 2026 - GOV.UK. UK Government. gov.uk/government/news/outbreaks-of-koi-herpesvirus-khv-disease-in-…. Retrieved November 2026.
- ^ Safely Managing Japan KHV Outbreak with New Processes and Technology at the Farm. Kodama Koi Farm (koi dealer). kodamakoifarm.com/managing-japan-khv-outbreak/. Retrieved November 2026.
- ^ Koi Herpes Virus (KHV): In-depth Exploration of the Deadly Virus to Prevent & Manage. Kodama Koi Farm (koi dealer). kodamakoifarm.com/koi-herpes-virus-khv-disease-guide/. Retrieved November 2026.