Carp Pox on a koi fish

Cyprinid Herpesvirus 1, more commonly known as Carp Pox, is one of the oldest recognized viral afflictions for koi. Carp Pox is easily recognizable by the pencil eraser-sized white, grey, blue or pinkish spots that look like molten candle wax. They usually manifest themselves around the head, shoulders and fins, but can sometimes cover the whole body.

What is Carp Pox?

Carp Pox is one strand of the herpes virus that affects koi. This strain results in abnormal skin cell growth (similar to warts).

The virus causing the spots is related to KHV (koi herpes virus), but is a separate virus that has neither the virulence nor the same rate of mortality. Thankfully, Carp Pox will not develop into one of the more serious types of Cyprinid Herpesvirus.

The good news is that if your koi is showing signs of Carp Pox, however unsightly it may seem, it is ordinarily not a life-threatening disease.

When Does it Happen?

Carp Pox is most often observed in colder fall and spring water temperatures and so is mostly understood to be a cold water affliction. In colder water temperatures, a koi’s white blood cell count is naturally lower and without the the warmer water that will help to boost the immune capabilities and stave off the virus, it is more likely to occur.

It isn’t just cold water that will cause an outbreak of Carp Pox, though. Any type of stressor that lowers your koi’s immune response will allow the virus to replicate and an episode to occur.

Is it Contagious?

As a virus, the answer is yes. You can mitigate the likelihood of it spreading, though, by keeping the water quality high and properly oxygenated, as well as not overcrowding your pond or tank.


There is currently no antiviral remedy that exists to combat this virus. As the virus uses normal cellular functions to replicate, the means of stopping it would require destroying the infected cell—which would more than likely kill the host (your Living Jewel).

As Carp Pox is mostly non-lethal, the advised course of action is to simply wait for water temperatures to rise. As the immune system’s natural processes start to kick into higher gear, it will start to fight off the virus and the spots will “disappear.”

Even though the spots might not be visible, however, it doesn’t mean the virus is gone. Once the temperatures fall (or a stressor is introduced and the immune system cannot handle the destruction of replicated viral cells), the spots will likely reappear.

However, over time, as your koi’s immune system becomes better at identifying the Carp Pox virus, the response to combatting it will gradually become quicker and the outbreak less severe.


The usual measures of clean water, low fish load and good parameters (ammonia, dissolved oxygen, pH) will help to reduce the stress that can cause the virus.

Some enthusiasts introduce Chloramine T into the environment as a precautionary measure to lower the incidence of opportunistic pathogens that might use the Carp Pox as a means to gain a foothold in your otherwise healthy koi.

Next Day Koi takes the utmost precautions to ensure that all of our koi fish for sale are as healthy as possible. From extended quarantine periods for incoming stock and using some of the industry’s top sources, we continue to keep our koi’s quality and your koi’s safety in the top of our minds.

You can check out our comprehensive list of koi illnesses and diseases, and how they can be treated here.

2 responses

  1. J. A. Brown :

    This article was a huge relief to me to realize what my epidemic was caused from.
    This Fall I recently went thru a round of Koi Pox that resulted in the loss of 100% of my Koi collection some which I have kept for 20 plus years. A horrible experience that leaves me puzzled about what to do with my 1800 gallon set-up. I am thinking of switching to pond lilies and vieltail comet goldfish. How long does it take for the pond to be safe for new fish? Are Goldfish susceptible to Koi Pox. Do I need to sterilize the pond. How do I safely restock?????? Replies appreciated.

  2. Carp pox is typically not lethal to fish. If you lost all of your fish, there may have been other factors that contributed to their deaths. Take a look through our blog for much more information on water quality as well as other disease and parasite issues.

    Good luck.

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