Carp Pox in Koi and Carp (Cyprinid Herpesvirus 1)

Carp pox is a wax-like skin growth on koi and common carp caused by cyprinid herpesvirus 1. It appears in cool water, usually fades as water warms, and the virus stays latent.

Carp pox is a benign skin disease of common carp and koi (Cyprinus carpio) caused by cyprinid herpesvirus 1 (CyHV-1). It produces smooth, milky, candle-wax-like growths that develop in cool water and usually regress as the water warms; adult fish rarely die, but the virus stays in the fish and lesions can return in later seasons.

Cause

CyHV-1 is a herpesvirus of the family Alloherpesviridae; its current species name is Cyvirus cyprinidallo1. Carp pox (also called epithelioma papillosum) is one of the oldest recognised fish diseases and has been reported worldwide, including most European countries, China, Japan, the USA, Russia, Israel, Korea and Malaysia. The virus has not been grown in cell culture. Like other herpesviruses it becomes latent: carrier fish show no signs until a stressor reactivates it.

CyHV-1 infects all scaled and colour varieties of common carp, including koi. The MSD Veterinary Manual treats common carp and koi as the only susceptible species, although CyHV-1 DNA has since been detected in skin tumours of wild barbel (Barbus barbus), carp pox has been reported in golden ide (Leuciscus idus), and a related but separate herpesvirus caused similar lesions in roach and asp. Goldfish carry a different virus, cyprinid herpesvirus 2, described in a separate article.

What the lesions look like

The growths are areas of epidermal hyperplasia (overgrowth of the outer skin layer). They are milky-white to grey, sometimes pigmented, raised about 1–3 mm above the skin and often compared to molten or melted candle wax. They appear on the head, fins and body, may be single or multiple, smooth or lumpy, and can spread until they cover large areas of the body before regressing. In one documented outbreak in a backyard koi pond, white proliferations covered up to 30% of the body of 15 adult koi.

Seasonality and temperature

Lesions typically develop in winter and spring at low water temperatures and regress in summer. A 2021 UK study describes carp pox in older fish as a recurring proliferative skin disease linked to cooler periods, typically 9–16 °C. Because the infection becomes latent rather than cleared, lesions can reappear the following year. Researchers also note that CyHV-1-induced skin overgrowth can appear transiently in response to water temperature.

Prognosis in koi and carp

Adult fish: usually no other signs; lesions are self-limiting and mortality is very rare. In uncomplicated cases the growths are eventually replaced by normal skin.
Juvenile fish: an acute systemic phase can cause high mortality, with loss of appetite, intermittent immobility, eye protrusion (exophthalmia), darkened skin and haemorrhages on the gill cover and abdomen. Survivors may develop skin growths later.
Koi: the main impact is cosmetic and economic, because lesions lower the appearance and market value of the fish. Secondary infections can complicate lesions.
Tumours: CyHV-1 DNA has been found in cutaneous squamous cell carcinoma of koi, but a cause-and-effect link has not been established.

Diagnosis

A presumptive diagnosis is usually made from the typical appearance. Skin overgrowth can have several causes, so histopathology alone cannot confirm CyHV-1 infection. A veterinary laboratory can examine a skin biopsy (epidermal hyperplasia with intranuclear inclusions, herpesvirus-like particles on electron microscopy) and run a PCR assay targeting the CyHV-1 thymidine kinase gene. That assay does not cross-react with CyHV-2 or CyHV-3, and it detected viral DNA in both affected and normal-looking skin of infected koi.

Management and prevention

There is no specific antiviral treatment. Veterinary sources describe control mainly as keeping infected fish out of a collection:

Quarantine new koi and remove fish that show carp pox during quarantine, before they join the resident population.
Surgical removal of lesions is reported by the MSD Veterinary Manual as not rewarding; any such procedure is a decision for an aquatic veterinarian.
WikiVet lists warmer water, liming of ponds and moving fish to ponds with plenty of clear, well-oxygenated water as measures that may speed recovery. Temperature changes stress fish and can trigger other diseases, so plan them with an aquatic veterinarian.
Disinfect nets, buckets and other equipment between ponds, and do not move fishing gear between water bodies without cleaning it.

How carp pox differs from KHV

Koi herpesvirus (CyHV-3) is a related herpesvirus with a very different course; it is covered in the Koi Herpesvirus (KHV) article. In short, carp pox causes raised wax-like skin growths in cool water and rarely kills adult fish. KHV disease occurs in warmer water (gov.uk gives 16–29 °C), damages the gills and skin with necrosis, sunken eyes and abnormal mucus, and can kill 80–100% of a population. KHV is a WOAH-listed disease and is notifiable in the UK; carp pox is not on the WOAH list of fish diseases. In the UK, spring temperatures can overlap the permissive ranges of CyHV-1, CyHV-2, CyHV-3 and carp edema virus, so laboratory testing is needed when fish are sick or dying.