Skip to content

Carp pox (CyHV-1)

By the koi.wiki editorial team · Updated

Key facts: Carp pox is a skin disease of koi and common carp caused by Cyprinid herpesvirus 1 (CyHV-1), producing smooth, waxy or candle-wax-like growths on the skin and fins, most often seen at cold water temperatures[1][2].
Carp pox, also called fish pox or epithelioma papillosum, is caused by Cyprinid herpesvirus 1 (CyHV-1), a DNA virus in the family Alloherpesviridae[3][4]. It is distinct from koi herpesvirus (KHV), which is caused by the related but separate Cyprinid herpesvirus 3 (CyHV-3)[4][2]. Carp pox is globally distributed in koi and common carp populations[1][2].The disease is best known for soft, pink to white, translucent wart-like growths on the skin, sometimes described as resembling candle-wax drippings[1][2]. A commercial koi health resource, Kodama Koi Farm, similarly describes the condition as producing waxy, white lesions on the skin and fins[5]. In most mature koi the disease is non-lethal and self-limiting, though it may be associated with high mortality in fry under two months old[2].

Cause and classification

Carp pox is caused by Cyprinid herpesvirus 1 (CyHV-1), also referred to as carp herpesvirus, koi pox, herpesviral epidermal proliferation in carp (HEPC), or herpesvirus septicemia in carp (HSC)[2]. It belongs to the genus Cyvirus within the family Alloherpesviridae and order Herpesvirales[3]. CyHV-1 was isolated and classified as a herpesvirus in 1985, although carp skin symptoms consistent with the disease had been noted for centuries before that[3]. The virus has not been successfully cultured in the laboratory, likely due to host and tissue specificity typical of herpesviruses[3].Whole genome sequencing shows the CyHV-1 genome is approximately 291,144 base pairs, though this can vary due to tandem repeats, and it contains about 143 open reading frames (ORFs)[3]. One of its gene families, JUNB, is an oncogene not seen in other Cyvirus species and may play a role in the tumor-like growths associated with the disease[3]. CyHV-1 is closely related to CyHV-2 (goldfish herpesvirus) and CyHV-3 (koi herpesvirus, KHV), and more distantly related to channel catfish virus and frog herpesvirus[4].

Symptoms

The hallmark sign of carp pox is soft, pink, translucent, wart-like skin growths that can often be removed simply by rubbing a finger over them, unlike many other tumors or parasitic growths[1]. The University of Florida IFAS Extension describes these as smooth, raised growths resembling candle-wax drippings on the skin and fins of older fish[2]. Kodama Koi Farm, a koi dealer, lists the visible symptom as waxy, white lesions on the skin and fins[5].In the acute systemic phase of CyHV-1 infection, which is lethal, affected common carp have shown atypical swimming behavior, eyes protruding from their sockets, darkened skin, and hemorrhaging on the body and operculum, with tissue death in the liver, kidney, and intestines in fish nearing death[3]. Survivors of this acute phase typically enter a recurring, non-lethal proliferative epidermal phase, which is the stage responsible for the visible wart-like skin growths[3].

Transmission and temperature effects

Like other fish herpesviruses, CyHV-1 can persist for long periods in a latent stage within apparently healthy carrier fish, with no active virus production, before being reactivated by stress and producing disease or shedding virus to infect other fish[1]. Survivors of carp pox outbreaks are considered lifelong carriers capable of spreading the virus to new fish[1].Water temperature strongly affects the disease. Growths typically appear when the immune systems of infected fish are slowed by cold temperatures, and the optimal temperature range cited for the virus is below 68 °F (20 °C)[1][2]. Temperatures above about 72 °F (22 °C) cause signs of carp pox to disappear, and water above 68 °F (20 °C) helps reduce the skin and fin growths in older fish, though this does not eliminate the virus from the fish[1][2]. Research on virus production in relation to temperature has demonstrated that temperature influences the virus's pathogenicity and replication, which may have implications for wild common carp populations as water temperatures rise[3].

Management and treatment

There are no drug or chemical treatments available that can clear herpesvirus infections, including carp pox, from carrier fish[1]. The Southern Regional Aquaculture Center states that carp pox disease is so widespread that avoidance is nearly impossible, and management instead focuses on reducing the likelihood and severity of outbreaks[1].The recommended approach is to ensure koi and common carp have adequate nutrition, good water quality, and low parasite loads during winter when temperatures are low, and to avoid very low temperatures and rapid temperature swings where possible[1]. Because the disease resolves with warmer water, the general management strategy is to wait for spring or to gradually warm affected fish into the mid-70s to mid-80s °F range[1]. It is rare for fish to die from carp pox, but large growths can sometimes leave permanent scars on valuable show koi, so warming show fish while growths are still in early stages may help limit scarring[1]. Physically removing growths by rubbing is discouraged, since this damages the skin and makes it more prone to secondary bacterial infection, and the growth is likely to return regardless[1].

Distinguishing carp pox from other koi diseases

Carp pox is caused by a different herpesvirus than koi herpesvirus disease (KHVD): carp pox is caused by CyHV-1 while KHV is caused by CyHV-3, and the two conditions have very different severity and management implications, as described on the page about koi herpesvirus (KHV)[2][4]. Unlike KHV, which can kill 70 to 95 percent of newly exposed fish populations and is treated as a disease to be avoided through strict biosecurity, carp pox is managed on the assumption that the virus is already present, since avoidance is considered essentially impossible[1].Carp pox is not a reportable or notifiable disease in the United States, in contrast to some other cyprinid viral diseases; readers can review notifiable disease lists on the page about notifiable koi diseases[2]. The soft, fragile, rubbery texture of carp pox growths helps distinguish the condition from other tumors and some parasite infections that can look similar, a distinction also relevant to the broader topic of sunburn, tumors and injuries[1]. For comparison with another viral disease of koi, see spring viremia of carp (SVC), which is caused by an unrelated RNA virus rather than a herpesvirus[2].

Significance for wild and farmed carp populations

CyHV-1 is found worldwide and infects both koi and wild or farmed common carp, as covered on the page comparing koi vs common carp and wild carp[3][1]. The US Geological Survey notes that outbreaks of cyprinid herpesviruses, including CyHV-1 and CyHV-3, are among several possible contributing factors to declining common carp populations observed in the Upper Mississippi River system[6].Because disease severity in CyHV-1 is temperature dependent, researchers have raised concerns that warming global water temperatures could alter patterns of virus replication and pathogenicity in wild carp populations, given the virus's combination of temperature sensitivity and potentially high mortality during acute infection[3].

Common questions

Is carp pox the same as koi herpesvirus (KHV)?
No. Carp pox is caused by Cyprinid herpesvirus 1 (CyHV-1), while KHV is caused by the distinct but related Cyprinid herpesvirus 3 (CyHV-3); the two differ greatly in severity, with carp pox rarely fatal in older fish while KHV can kill 70–95 percent of newly exposed populations[2][4][1].
What do carp pox growths look like?
They are soft, pink to white, translucent, wart-like or waxy growths on the skin and fins, sometimes described as resembling candle-wax drippings, and are fragile enough to be rubbed off with a finger[1][2].
Can carp pox be treated?
There is no drug treatment that clears the virus from carrier fish; management instead focuses on good nutrition, water quality, and avoiding very low or rapidly changing winter temperatures, and growths typically resolve as water warms above about 68–72 °F (20–22 °C)[1][2].
Is carp pox fatal to koi?
In mature fish it is typically non-lethal and self-limiting, though it may cause high mortality in fry younger than two months old[2]. It is also noted that it is very rare for fish to die from carp pox, though large growths can leave scars on valuable show koi[1].
Is carp pox a reportable disease?
No, carp pox is not a reportable or notifiable disease in the United States[2].

References

  1. ^ SRAC Publication No. 4710: Herpesviruses in Fish. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4710%20Herpesviruses%20in%20Fis…. Retrieved November 2026.
  2. ^ UF/IFAS EDIS VM113: Koi Herpesvirus Disease (KHVD). University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/VM113. Retrieved November 2026.
  3. ^ Cyprinid herpesvirus 1 (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Cyprinid_herpesvirus_1. Retrieved November 2026.
  4. ^ 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.
  5. ^ HELP, Is My Koi Sick?!? Diagnose Symptoms & Fish Diseases - FREE Health Checklist. Kodama Koi Farm (koi dealer). kodamakoifarm.com/koi-sick-health-diagnosis-symptoms-diseases/. Retrieved November 2026.
  6. ^ Common Carp (Cyprinus carpio) - Species Profile. US Geological Survey. nas.er.usgs.gov/queries/FactSheet.aspx?speciesID=4. Retrieved November 2026.