Koi Carp Pox Treatment & Prevention Guide (Updated)

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How to Treat Carp Pox Symptoms in Koi & Improve Recovery (2022 Guide)

koi carp pox virus
Koi suffering from the carp pox virus, with symptoms displaying as white waxy swelling of the caudal fin. Source: (2011) AACL Bioflux. 4. (4).

Carp pox, or koi pox, is an inherited gene passed down through ornamental koi and wild carp which causes outbreaks of the herpes virus Cyprinid Herpesvirus 1, or “CyHV-1”. Although contagious between fish, the virus initially lays dormant within carriers of the herpes gene and usually erupts during periods of stress, lower immune activity (hibernation/torpor), or when koi are still young and developing. The virus is omnipresent in fish born with the carrier gene, and will be with them throughout their life in varying degrees of severity.

Luckily, unlike the highly deadly herpes viruses CyHV-2 and CyHV-3, the inherited CyHV-1 herpesvirus is not dangerous and fish usually outgrow their symptoms as their immune system matures. Symptoms of the common CyHV-1 virus are mostly cosmetic in nature, causing slight disfigurement and swelling of fins and scales which usually disappear as the virus is reduced by the immune response. As a koi’s immune system matures with age, less and less virus activity takes place, and this is why some koi keepers state their fish “outgrow” the virus after reaching full adult maturity.

What Does Koi Pox Look Like? Is Pox Dangerous?

pink waxy spot on koi fin from carp pox
Lesions from common carp pox display as white or pink waxy spots on scales and fins. Source: (2011) AACL Bioflux. 4. (4).

The common herpes virus, CyHV-1, often presents as white or pink waxy spots, commonly on the fins or head of the fish, and looks similar to candle wax in appearance (see koi pox pictures). These semi-translucent lumps may look unsightly, but they’re typically harmless and do not need any direct treatment. These waxy spots are the single most common symptom of the CyHV-1 herpes virus, and if you notice them on your fish, you can safely assume you have the common carp pox virus and not the deadly sub-type varieties.

As with any infection, fish may also become stressed during periods of herpes outbreak, so will be less likely to take food, may swim alone, or prefer hiding away under plants or shelters – all of which are normal as koi try to recover.

The two dangerous herpes virus sub-types, CyHV-2 and CyHV-3 (KHV), attack the blood producing internal organs of the fish, including the kidneys, liver, and spleen. The mortality rate of these sub-types is as high as 90%, and displays as red sores and lesions over the body of the fish, creating a rash-like appearance.

There is no cure for any type of carp pox, but luckily the deadly sub-types are very rare, and in almost all cases your koi will display white/pink spots which are almost definitive signs of the non-deadly herpes virus, CyHV-1.

Koi Pox Symptoms:

  • White or pink waxy spots, often on fins and tail
  • Bacterial infections (if spots are broken)
  • Lack of interest in feeding
  • Dull appearance and color pattern
  • Constant hiding/swimming alone (stress)

What Causes Carp Pox in Ornamental Koi? 

how does koi carp pox virus spread
The pox virus will always be present in fish who carry the herpes gene, but it can still be spread to other fish during an outbreak. Public domain.

Carp pox is one of the oldest recorded carp diseases, and occurs from an inherited gene which allows the herpes virus to become active in fish. Many ornamental koi will have the herpes gene at birth, and symptoms will come and go as the fish matures with age. Even though the common virus is very similar to the rare CyHV-2 and CyHV-3 sub-types, a carrier of CyHV-1 carp pox cannot transmit or become a carrier of the more dangerous types.

This is because although each of the herpes viruses are similar, they’re still different viral species and cannot mutate into other forms. The only way for fish to contract one of the deadly herpes virus types is to come into contact with a fish who is already infected with the disease (very rare).

The herpes virus that causes carp pox will go through periods of dormancy and activity throughout the fishes lifetime. Koi with the virus will generally have outbreaks during periods of lower immune activity, often in winter and spring when water temperatures are lower and they’re hibernating. Other common causes of viral outbreaks are below –

Causes of Carp Pox Outbreaks:

  • Lower water temperature/torpor
  • Increased stress factors
  • Underlying illness, parasites, or injury
  • Poor water quality & conditions
  • Lack of proper nutrition
  • Poor “winterization” routine

Sadly, there is no cure for carp pox, so the fish rely on their natural immune system for support and control of the virus. During summer where water temperature is much higher, a fish’s immune system will be in full-swing, capable of destroying new virion particles before they can find a new host cell to spread the disease. In winter, however, when water temperature is lower, a fish’s immune system will be reduced, and it is during these periods that outbreaks of the virus occur more heavily.

Why Can’t Carp Pox Be Cured? What About Medicine?

herpes virus caused carp pox

Unlike bacterium which are fairly complex micro-organisms, viruses are incredibly simple in comparison, and it’s their simplicity that makes them so difficult to eradicate. Even though the processes that occur within human cells are similar to bacteria cells, the molecular machines that drive these functions are still quite distinct. Likewise, a bacteria will have a cell membrane that is chemically distinct from its human counterpart, which means we can target these differences to help destroy the bacteria without destroying healthy cells – such as with antibiotics.

Viruses, on the other hand, are much more simple and do not require such complicated processes to survive and replicate. Whereas a bacteria will have their own distinct metabolism to create proteins for survival, a virus will instead hi-jack a healthy cell which it will use to carry out this function for them.

Viruses will have just enough resources available to find and hi-jack a healthy cell, and from there they start to fully rely on the host cell for their survival and reproduction. This is why it’s so difficult to cure the herpes virus in koi, as once the virus has found healthy cells to infect, it becomes impossible to destroy without also destroying the cells as a whole, which would in turn kill the fish as a result.

However, to replicate, the virus will need to continually release virion particles which work to seek out new cells to hi-jack. During this period the particles are vulnerable, and the immune system can recognize and destroy them before they find a healthy cell. During summer and warmer weather pox outbreaks are less common as the immune system is working at full capacity, making it difficult for the virus to find new cells fast enough to cause problems.

It is essentially kept in balance, and eventually will go into a dormant period until the immune system is reduced, such as during hibernation or periods of heightened stress. Basically, instead of fighting pox with medicine, we need to fight it by ensuring our koi are healthy, strong, and have a good immune response in case of trouble.

Treating Carp Pox Symptoms & Reducing Outbreaks in Koi (5-Step Guide)

Note:  The virus that causes carp pox cannot be cured, but there are methods to help improve symptoms and reduce the chance of outbreaks in future. Below we have listed some steps which can help alleviate symptoms and encourage a stronger immune response.

Aqua-prazi to treat carp pox1) Treat for Parasites Using Aqua Prazi

When?  Late autumn and early spring, or during onset of carp pox symptoms to prevent any possible parasitical infection. 

The herpes virus that causes carp pox likes to “wake up” when a koi’s immune system goes to sleep, and so does another threat which can lead to worsening pox or a reduction in immune activity – we’re talking about parasites! Just like koi pox, parasites often strike during times when koi are more vulnerable to infection, such as during their winter hibernation period. Parasites that infect carp will not only cause stress and injury (open sores), but will also greatly reduce their immune system as koi struggle to work against the problem.

Basically, if you have koi, you should make a habit of treating for parasites as part of your winter and spring fish keeping routine! Treating the pond in late-fall and early spring is a good way to reduce the risk of parasites, and ensure if koi do get a pox outbreak, there is much less chance of it spreading or worsening due to stress.

Our recommended parasite treatment is always Aqua Prazi, which contains the broad spectrum parasite killer praziquantel. This compound works on a huge range of internal and external parasites, being particularly effective against skin and gill flukes which are some of the worse species in combination with carp pox. Due to how contagious and fast spreading parasites can be, it’s recommended to treat the entire pond system, but it can also be used in quarantine if only a single fish is presenting symptoms. When dosed properly, the treatment should have no ill effects on water quality or the ecosystem. A single dose should be added to the pond over a week period, and then repeated after 2-4 weeks to combat any hatching eggs or larvae.

2) Reduce Risk of Bacterial Infections with Melafix

When?  At first signs of carp pox symptoms, preferably within a quarantine environment. Will reduce risk of bacterial infection occurring if lesion is ruptured.

Although it cannot cure the waxy spots that occur during a carp pox outbreak, treating the pond with a natural anti-bacterial treatment will help ensure any damaged lesions do not become infected. Melafix contains naturally occurring anti-bacterial ingredients (botanical tea tree), as well as ingredients to help speed up the recovery process. Designed for wounds, sores, and injuries, it can still be used as a good deterrent method during a pox outbreak to keep harmful pathogens from infecting areas with swelling.

The treatment can be applied directly to the entire pond system, or the fish can be treated individually in a separate quarantine environment. Due to most of the ingredients being natural, and since it contains no chemical bactericides, it should have little negative effects on the ecosystem if dosed correctly. However, the main ingredient is still a bacterium inhibitor, so it should not be used alongside beneficial bacteria supplements or KoiZyme (step 3), as it will reduce their effectiveness. The treatment can be used daily for a week when there is a carp pox outbreak to speed recovery and sharply reduce harmful bacteria levels. Afterwards, we recommend using KoiZyme (below) as a long term method of safe harmful bacterium control.

3) Balance Harmful Bacteria Strains Using KoiZyme

When? After carp pox symptoms have improved and other treatments finished. Helps reduce harmful ulcer causing bacteria in ponds over time.

As mentioned above, harmful bacteria which naturally lives in the pond can cause issues for fish with pox due to the external nature of the symptoms. Some of the most dangerous bacteria strains are the aeromonas and pseudomonas bacterium, which are a leading cause of ulceration and infection in pond fish. Both these bacteria strains live naturally in all bodies of fresh water, and they do not usually cause problems unless their numbers begin to rise too high. An interesting and very effective method of controlling these harmful bacteria strains is by using KoiZyme, which is a mix of both natural enzymes and harmless bacteria strains. The harmless bacteria KoiZyme introduces is able to reproduce much faster than the aeromonas and pseudomonas bacterium, which leads to a sharp reduction in their numbers over time. Although this can’t remove 100% of the harmful strains, it will help ensure that any possible infections or ulcers in future will be much less serve as there is less harmful bacteria in the environment to present problems.

After carp pox is under control and after anti-bacterial treatments are finished, you can supplement the pond with KoiZyne once a day, every 3 days to kick-start the new treatment. Afterwards, weekly dosing should be enough to ensure harmful bacteria cannot rise to dangerous levels.

immune boosting koi foods help prevent illness and disease4) Make Use of Immune Boosting Koi Foods

When? If your koi are young or prone to have frequent outbreaks of carp pox. Can be used standalone throughout the year, or mixed into normal feed.

There is no cure for carp pox, so the first and last line of defense against the virus is your koi’s natural immune system! Ensuring koi are healthy, comfortable, and happy in their environment is key element of fish keeping, and feeding with a high quality fish food is a cornerstone of any koi keeper’s routine. Not only will a high quality feed ensure your koi stay healthy and grow strong, it will also keep their immune system firing so they can combat any pathogens they come in contact with. The herpes virus can only be controlled when the immune system of the fish is faster to act than the virus can spread, and can be given a “boost” with feeds that contain immune boosting ingredients.

Commonly found immune boosters include, Optimun, Aquagen, Torula Yeast, Bee Propolis, Lactoferrin, and many others. These natural and synthetic compounds have all been shown to improve the immune system in koi, and would be a good addition to your feeding routine during carp pox outbreaks. Optimun, found in Total Koi’s Show Koi range, is a powerful performance enhancer designed for heightened immune response, and is one of our favorite feeds for boosting the immune system. It contains a high protein content from aquatic source (anchovey meal 38%), as well as healthy fats, low ash content, and a wide range vitamin and mineral profile. A very solid choice of immune boosting feed to mix into your regular feed or as a replacement feed during periods of carp pox!

5) Consider Heating the Pond or Quarantine Tank

When? Throughout colder months to keep immune system more active, or during an outbreak in quarantine tank to speed up recovery.

Finally, another aspect to consider which is directly related to the frequency of carp pox outbreaks is the temperature of the surrounding water. The herpes virus almost always springs up in the winter when koi are in torpor, as their immune system is all but shut down due to the cold water temperature. Fish pox is much rarer in summer, as a fish will have a much higher metabolism to process energy, and in turn, a much more active immune response to destroy spreading virus particles.

Although you can’t stop winter from coming, you could instead invest in a powerful in-line heater system  or an immersed heater which will help raise water temperatures and increase immune activity over the winter period. In-line heaters work by directly heating surrounding water which passes through a heating chamber via a dedicated pump from your pond. The warmer water is then added back to the pond system, and gradually the overall temperature increases and can be maintained with a thermostat. If you have a smaller pond, around 500~ gallons, both a high rated immersed heater or powerful de-icer is capable of raising temperatures in the surrounding water enough to improve immune activity. For larger ponds, or if you have bitterly cold winters, the only solution would be a powerful water heater, similar to the heaters you see used for warming large swimming pools. Even though most heaters will only be able to raise water temperatures by a few degrees, this is often enough to maintain much higher immune activity and will help keep winter pox outbreaks at bay.

Alternatively, and if you only have a small number of koi (or young fish), they can be brought indoors and added to a large holding tank during winter. With this method a heater may not even be necessary if you can maintain a higher atmospheric temperature, as water will eventually warm naturally in-line with the surrounding conditions. You can also quarantine fish as needed, either when you first get them or if you suspect illness.


20 thoughts on “Koi Carp Pox Treatment & Prevention Guide (Updated)”

  1. Can Prazi and Melafix be used together or is it best to do 1 at a time? A few of our Koi have Carp Pox and we are seeking some sort of treatment.

    • Hi Edward,

      Both Aqua Prazi and Melafix are fairly “inert” treatments, and I don’t think you’d have problems dosing both assuming your water quality is in good order. However, as with any chemicals you add to the pond, they will still place stress on the eco-system (and fish) to some extent, so it’s always beneficial to treat in stages if you’re able.

      If your koi are just showing signs of pox, and have no visible sores/wounds, I personally would treat with Aqua Prazi first, wait 3 days, perform a 10-20% water change, then dose the pond with Melafix. Afterwards, keep a close eye on their behaviour and appearance, and simply try to make them as comfortable as possible as stress is a major contributor to pox outbreaks, especially in young/weak fish.

      I hope your koi feel better soon!

  2. I have a big yellow koi that is sick. It looks like he has cloudy eye but also has white patches on his body. He is swimming in real abnormal ways. The water is about 15 degrees. He was fine and hibernating at the bottom of the pond just a couple days ago. Anyway to diagnose and how to treat whatever is going on?

  3. Hi. 3 of my young koi are showing signs of Koi Pox. I’ve already treated the pond for parasites/flukes per my local koi shop. They also advised me to add salt which I did. Which steps should I continue with? None of the white patches seem to be infected. Also, the Show food mentioned above is sold out on Amazon. Is there another recommended brand or another place to order it? Thank you, Vanessa

    • Hi Vanessa,

      I’m sorry to hear about your koi and their symptoms!

      You’ve done the right thing treating for parasites, as they have the potential to cause major issues with weakened koi, so it’s always better to be on the safe side.

      From your description, it sounds like your fish are fighting the virus well and they’re fairly healthy overall (no broken welts, bleeding etc.). Sadly, there is little more that can be done now apart from making your fish as comfortable as possible. Adding KoiZyme to the water can help reduce the more harmful bacteria strains that can cause further issues during pox outbreaks. Supplementing with a high quality feed (which should always be done, anyway!), such as the food in this article, can give your fish more energy to fight back. Heating the pond, even by a few degrees or creating a “warm pocket” with an immersion heater, can help increase the immune activity and metabolism of your fish, which is usually very low this time of year.

      These are all things you can try, but sometimes, you just need to let the virus run it’s course! If your koi are looking healthy, and there are no secondary problems/infections, just keep them comfortable and they should be able to ride it out.

      Ps- I’ve updated the article so the links to products now show as in-stock, so you’re welcome to have another look!

  4. Hi Chris,
    thanks for the blog post! I think my goldfish was a carrier. A pet sitter overfed him, the nitrates spiked, and when I got home his fins were in tatters. Shortly thereafter he broke out in the white bumps, which have gotten worse. Otherwise he acts healthy. Has anyone removed these? Can I add new fish? He’s alone with an old catfish right now. He has a page on my old blog.

    • Hi Holly,

      I’m very sorry to hear about what happened to your fish. I’ve just checked the images on your blog and yes, that does look like a particularly nasty outbreak. Your fish must have been under high stress for it to progress so quickly and severely. I really do hope your last goldfish recovers!

      When it comes to carp pox, so long as fish are acting somewhat normal, just keeping them as comfortable as possible is often the best long-term solution. The swelling usually disappears in time, but can take several months if the fish is in bad shape as the immune system needs time to catch up. Physically removing lesions is never a good idea (unless recommended and handled by a trained veterinarian), as it would just open the door to potential secondary infections and even more stress.

      Since in your case it sounds like spikes in harmful substances were the primary trigger, a good way forward would be simply doing all you can to keep water quality in good order and the environment stress-free. More regular water changes to reduce nitrogen compounds, a high quality fish feed, and maybe even raising the water temperature a degree or two for a few weeks to help increase immune activity and metabolism.

      In terms of adding fish, and assuming this is in-fact carp pox, only other cyprinids (carps and minnows) can contract the virus. If you’d want to add more goldfish, there would always be a risk of outbreaks and further spread – as well as the new fish themselves being carriers! In most cases, carp pox is not dangerous and you often don’t even notice the symptoms. In-fact, if an adult fish is happy and healthy, I’d be surprised if the virus ever manages to surface to any noticeable degree. In other words, you shouldn’t worry about how you might cure this problem (it’s not possible!), but instead, how you can best manage it so it doesn’t appear in the first place.

      I hope that helps!

      • We have a year old chagoi koi who developed white spots this late spring..so we pulled him from the pond and put him in quarantine tank..we have treated him for parasites, fungus and bacteria..he will get better but not free from spots…we are suspecting he’s got herpes…we are now treating with melafix..but, should we continue to keep him in quarantine or put him back in pond with the rest of the koi?

        • Hi Leanne,

          Assuming he’s healthy and not showing any other symptoms which could represent other illnesses, I think reintroduction into the main system wouldn’t be a problem. As mentioned in other replies here, carp pox isn’t something we can actually cure, so on-going management and good pond care becomes our main method of ‘treatment’. In my experience, unless you wanted to keep the koi in a separate tank/pond while he fully matures (many years), as long as you keep water quality within good parameters, feed the fish a nutritious diet, and generally just make them as happy as possible, carp pox becomes less and less of a problem as they age – often disappearing in adult life, unless a particularly nasty sickness or injury occurs which causes major stress (a BIG pox trigger).

          On the other hand, an alternative here would be to simply keep him within your quarantine tank through just the upcoming winter, and reintroduce him back in spring. Carp pox is actually the most prevalent in colder months, so being in a warmer (or heated) quarantine tank may actually be more comfortable for him and allow enough time for the spots, as well as any other lingering symptoms, to fully clear. If your winters are mild, it would probably be fine either way, but still something to consider if you’re in a fairly cold climate and have the resources to keep in separate for a few months more.

  5. I have a koi with carp pox but is in a tank with two other koi. does that make the other two carriers? I have 9 koi in a different pond so have kept them separated. I don’t want to infect my other koi. should I keep them separated or will my other koi get carp pox also?

    • Hi Joan,

      That would be almost impossible to know for sure, but if all the fish are offspring of the same parent, they likely do have a higher chance of being carriers.

      However, just remember, carp pox is very common in the hobby, but when managed properly, it’s really nothing to worry about. In-fact, I’d wager most koi keepers have fish which are carriers, but they never notice as they keep the koi healthy and in good conditions which stops the virus flaring up. Carp pox is one of those things which you really can’t avoid (or cure), so it’s better to just manage it as best you can. Whether your other koi contract the virus in future, I have no idea and really couldn’t predict that. But if you keep them all happy, healthy, and in good condition through their lives, even if they have it, you may not even notice!

  6. Many thanks for the informative page on carp pox.

    I have a 3500l pond which is an open system. That is to say, it has continuous springwater infeed (roughly 1 water change every 2 days) as well as a fairly large filter. The filter is only recently set up as I’d hoped that the spring water feed would have been sufficient for water clarity and filtration, but it wasn’t.
    I have 9 mirror carp in the 10-14″ range that are feeding and growing well, but 2/3 of them have very minor signs of carp pox. Water quality is excellent, and as mentioned, they are growing rapidly.
    As it’s an open system and treatment would simply be washed away, is there anything I can do to mitigate the CP? I think the original flare up stems from them having been delivered by post in April and the sender not adequately packaging them. They came with some scuffs and scrapes that took a while to heal. They are fine now though.

    Any pointers are much appreciated.

    • Hi Jonathan,

      Carp pox is difficult to manage, and basically impossible to treat, as the symptoms are self-limiting and there is no actual “cure” for the disease. However, you can do things to help relieve symptoms during particular bad periods, and in your case with your open water system, I’d suggest the following to help:

      1) Ensure the feed you’re using is of high quality, with high protein (35-40% – from aquatic sources) and a wide range of vitamins and minerals (vitamin C & E, especially). ‘Immune boosting’ feeds can also be used during these periods, as there is some evidence certain ingredients help aid in bodily function, effectively reducing stress and strain on the fish.

      2) Provide plenty of hiding places, such as fish shelters, to allow your koi to relax. If they’ve been newly introduced, and also have pox, I’d wager they’ll want some ‘down-time’ to recover and get used to their surroundings, and stress only makes carp pox worse.

      Other that that, all you can really do is give them time to recover. If water quality is good, and they’re healthy and eating, the symptoms will eventually pass and you should not expect any on-going issues.

      Wishing you the best luck! I hope your koi recover soon.

  7. Hi I have been given two Koi. One had a bit of a scar the other I thought blind in one eye. Both seem happy and stick together most of the time. Looking at the blind eye it looks now as though there is a gold cloudyness to it. I want to take it to a vet but have no idea how to transport it so it doesn’t get stressed. I would be grateful for any advice

    • Hi Dawn,

      Apologies for the delay in responding!

      I would advise that you see if a vet is able to come out to you so that you don’t have to transport your koi. If that’s not possible, try transporting it in a small aquarium. It should be just fine for a couple of hours. To minimize the risk of shock, use at least 50% of the pond’s water in the aquarium and 50% fresh water, making sure the temperature of the new water is as close as possible to that of the water your koi is normally in.

      Best of luck!

  8. Hi, we have a fish which looks like he has pox, he isn’t a young fish and this is the first time he has shown sign of the waxy lumps. We have had a very warm spring but the weather over the last week has been dramatically cooler. Could this have caused the outbreak? He is still feeding well, he has the lumps on the front edge of one pectoral fin and near his mouth. What would you suggest doing? Thanks in advance

  9. Hi, I have a Koi with Carp Pox. It was getting quite bad, the mass was growing in size and his eye was beginning to become distorted. I thought he had a tumour.
    Took him to the vet and decided to let her operate. She used a special piece of equipment that reduced bleeding as it went.
    My fish came home and the diagnosis was that the mass was proud flesh.
    For a while he appeared to be like a new fish. More vigour. However within a month the sores began to return. My vet had heard that experiments were being done using a cold sore cream for humans called Zovirax. I gave it a go but it was hard to keep my fish still for a minute while the ointment had a chance to sink in.
    I stopped doing this when I noticed his eye was now popping out of his head and it was clouded over. I also believe catching him everyday was very stressful for him.
    I have a series of photo’s I would very much like to show you but don’t seem to be able to get them to attach to this.
    I don’t know what I am asking you for. As you state there is little to be done, but if you have any suggestions they would be appreciated.

    • Hi Libby,

      I am so sorry that you’re dealing with this, and apologies for the delayed response, as well! I will pass your info onto our fish expert, but in the meantime I’ll let you know how you can share images with us.

      We don’t allow pictures to be posted on the site for security reasons, but I’ll link you to a different site we use for images that’s secure and private: Postimage.org — free image hosting / image upload

      If you’d like to upload your pictures there, I can take a look and let you know what I think it could be! After you upload the image(s), you’ll have to also leave a comment here with a link to the image you just uploaded so that I can find it. Then we’ll take a gander and comment back here with a response!

  10. Hi Chris. I have a single koi in a 70 gallon tank with a bunch of goldfish. I have been reading about this koi pox and it sounds and looks similar to what is going on with my fish. It is on the tail and fins as per the pictures but is also in a thin layer on the face. When it is touched it is like a thin film that can easily be wiped off. Is this koi pox or something else.


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