How to Treat Carp Pox in Koi

Carp pox on a koi

The Spruce / Dr. Jessie Sanders

Carp pox or koi pox is a common herpes virus infection in ornamental koi. It is characterized by thickened epithelial growths along the dorsal ridge, sides, and fins of koi. It is a relatively benign disease, only causing aesthetic changes and does not affect the fish's overall health and longevity. Severe outbreaks can result in secondary skin infections but are very rare.

What is Carp Pox?

Carp pox is the physical manifestation of Cyprinid herpesvirus-1, a common virus in ornamental koi. This virus results in abnormal skin growths throughout your koi's body, concentrated on the dorsal ridge, sides, fins, and mouth. They can be very mild or quite severe, depending on your fish's individual stress level and immune capacity.

Symptoms of Carp Pox

  • "Candle wax" lesions
  • "Frosting" on skin
  • Thickening of fins, especially the cranial edge
  • Growths around/in mouth

Like many other fish diseases, clinical signs of carp pox are temperature dependent. The skin lesions caused by the Carp Pox herpes virus are more commonly seen in cool water temperatures. When the water temperatures are warmer, up in the high 60 to 70 degrees Fahrenheit, a koi's immune system is significantly more active, and their skin turnover also increases. This will minimize or even resolve the carp pox lesions. This does not mean the virus is gone; it just means the clinical signs of the virus are not present. The signs will likely return when the water cools seasonally.

Many online forums recommend surgical removal of the offending plaques, but this is not recommended. Not only are you subjecting your fish to painful surgery that may cause secondary bacterial infections, but also this will not prevent the lesions from returning. Once the virus has infected your fish and produced clinical signs, they will have the infection for life, even though the skin lesions may come and go. Again, this is not a life-threatening condition, and your fish will only undergo changes in its physical appearance.

Causes of Carp Pox

Carp pox is caused by the herpes virus, Cyprinid herpesvirus-1. It is a cousin of Koi Herpes Virus, Cyprinid herpesvirus-3, and Goldfish Hematopoietic Necrosis Virus, Cyprinid herpesvirus-2. Like other herpes viruses, the virus has a latent or carrier state that persists within an individual without showing any external clinical signs. Fish can be infected very young in life and may not show clinical signs until years later. Secondary stress can activate latent infections and cause skin lesions and infect other fish through direct contact or water-borne transmission.

Once a fish has been diagnosed with carp pox in your pond, it is assumed that all the other fish in the pond are potential carriers. In an infected pond, it is rare for more than one or two fish to show clinical signs at once, unless many potential stressors, such as poor water quality, are present.

It has been demonstrated that goldfish, Carassius auratus, can carry Koi Herpes Virus, so there is the potential that they can also carry carp pox. Goldfish will not show the symptoms of KHV when they are carriers, so it possible they can also carry carp pox between infected systems without showing clinical signs.


Like all other herpes viruses, latent virions will bury deep into neural tissue, making them impossible to remove or treat. As for any species, herpes is for life. There is no treatment available at this time. Surgical removal of the plaques is not recommended since they will only grow back.

You may alleviate the symptoms of carp pox by raising your water temperature. This will allow your koi to better combat the disease and may resolve symptoms entirely. This does not mean the virus is gone from your fish, as the lesions will return when the water cools again.

How to Prevent Carp Pox

As with all contagious fish disease, proper quarantine protocols are essential to ensure the health and safety of all your fish. Pond fish are no exception, however, they will require a larger holding tank. Do not take koi dealers at their word that their fish have been properly quarantined. Ask specifically about their protocols, including duration, treatments, mixing of fish from different suppliers, testing and if they've had a consult from an aquatic veterinarian. If they are hesitant to release any of this information, shop somewhere else.

Testing for carp pox is available for fish actively showing symptoms through your aquatic veterinarian. There is no test available for carrier status at this time. Because this is a non-lethal disease and the symptoms are very minor, koi owners should not be overly concerned about carp pox. In discussions with aquatic veterinarians, this is a very common disease and it is rare in some areas to have a pond that is not infected with carp pox.

Is Carp Pox Contagious to Other Fish?

It has been demonstrated that other cyprinid herpes viruses are transmitted by other fish, such as goldfish, but that those fish will not show other clinical signs. Because carp pox is not a lethal disease, no research has been completed at this point if confirmed carrier status exists in other species.

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  1. Calle, Paul P., Tracey McNamara, and Yvonne Kress. "Herpesvirus-associated papillomas in koi carp (Cyprinus carpio)." Journal of zoo and wildlife medicine (1999): 165-169.

  2. Hedrick RP, Waltzek TB, McDowell TS. (2006). "Susceptibility of koi carp, common carp, goldfish, and goldfish x common carp hybrids to Cyprinid herpesvirus 2 and Cyprinid herpesvirus 3." Journal of Aquatic Animal Health 18:26–34.

  3. Hedrick RP, Waltzek TB, McDowell TS. (2006). "Susceptibility of koi carp, common carp, goldfish, and goldfish x common carp hybrids to Cyprinid herpesvirus 2 and Cyprinid herpesvirus 3." Journal of Aquatic Animal Health 18:26–34.