Carp Pox: What Is It and How Do I Get Rid of It?

 

If you keep koi or other fish in your pond, you may have noticed strange, waxy growths appearing on their skin or fins, especially during colder months. This condition is commonly known as Carp Pox.

While it can look alarming, the good news is that it’s usually more of a cosmetic issue than a life-threatening disease. In this guide, we’ll explain what Carp Pox is, what causes it, whether it’s harmful, and what you can do about it.

 

What Is Carp Pox?

Carp Pox is a viral disease affecting koi and carp species. It’s caused by Cyprinid herpesvirus-1 (CyHV-1), a member of the herpesvirus family that specifically targets carp.

Unlike bacterial infections or parasitic infestations, Carp Pox is caused by a DNA virus. Once a fish becomes infected, the virus remains in its body for life. Like other herpesviruses, CyHV-1 can enter a dormant (latent) state inside the fish’s tissues.

During this period, the fish may show no visible symptoms at all. However, when the immune system is weakened, most commonly due to a change in weather or stress, the virus can reactivate, leading to the characteristic waxy lesions associated with Carp Pox.

The disease primarily affects:

• Skin

• Fins

• Occasionally the mouth or gill covers

The growths form as a result of abnormal thickening of the outer skin layer. These lesions often appear smooth, translucent, and slightly raised, resembling drops of candle wax. In early stages, they may look flat and pale before becoming more pronounced.

Carp Pox should not be confused with more aggressive viral diseases such as Koi herpesvirus disease (KHV), which can cause high mortality rates. In contrast, CyHV-1 is generally slow-moving and rarely fatal.

It’s also important to understand that Carp Pox is species-specific. It affects carp and koi but does not infect humans, pets, or other wildlife species.

Because the virus remains in the fish for life, outbreaks tend to be cyclical, appearing during colder months and often regressing naturally as temperatures rise and the immune system strengthens.

 

What Does Carp Pox Look Like?

The condition appears as:

• Smooth, waxy, white or pinkish growths

• Raised lesions that look like melted candle wax

• Patches on the skin, fins, or sometimes around the mouth

Unlike more aggressive viral diseases such as Koi herpesvirus disease (KHV), Carp Pox develops slowly and is generally non-fatal.

It most commonly appears when water temperatures fall, especially during autumn and winter. For this reason, Carp Pox is often described as a seasonal condition.

However, that doesn’t mean it only occurs in winter. Some fish may show symptoms one year and never again, while others may develop visible lesions every year as temperatures drop. Each fish responds differently, depending on its immune strength, stress levels, and overall health.

 

Is Carp Pox Harmful?

In most cases, Carp Pox is not harmful.

Here’s what you need to know:

• It is usually cosmeti

• Fish often behave normally (eating and swimming as usual)

• It rarely causes death

However, there are exceptions:

• Severe outbreaks can interfere with vision or breathing if growths form near the eyes or gills

• Large lesions may split and become vulnerable to secondary bacterial infections

• It can spread to other carp in the same pond

Importantly, Carp Pox is species-specific. It does not infect humans or other animals.

 

Is Carp Pox Contagious?

Yes, Carp Pox is contagious, but it spreads differently than many other fish diseases.

Carp Pox is caused by Cyprinid herpesvirus-1 (CyHV-1), which can be transmitted between koi and common carp through:

· Direct fish-to-fish contact

· Shared water

·  Nets, bowls, or equipment that haven’t been disinfected properly

However, it is generally considered mildly contagious compared to more aggressive viral diseases like Koi herpesvirus disease.

 

Important things to understand:

·  Not all exposed fish will develop visible symptoms.

·  Some fish may carry the virus for life without ever showing lesions.

·  Outbreaks are far more likely when fish are stressed, or water temperatures drop.

Because the virus can remain dormant, a pond may contain carriers without obvious signs. Symptoms often appear only when environmental conditions allow the virus to reactivate.

 

Should You Isolate Affected Fish?

In most cases, isolation isn’t necessary if the fish are already in the same pond, as exposure has likely already occurred.

By the time visible lesions appear, other carp in the system have usually been sharing the same water and environment for weeks or months, meaning they have already been exposed to Cyprinid herpesvirus-1 (CyHV-1).

Because Carp Pox is typically mild and stress-related, separating the affected fish can sometimes cause more harm than good. Netting and moving fish increases stress levels, which may actually worsen symptoms or trigger outbreaks in other fish.

 

Isolation is generally only worth considering if:

• A fish has severe lesions that are becoming damaged or infected

• You need to raise water temperature in a controlled quarantine system

• You are protecting a separate, previously unexposed collection

Otherwise, the best course of action is usually to focus on improving overall pond conditions, maintaining excellent water quality, minimizing stress, and supporting immune health, rather than removing the fish.

 

What Causes Carp Pox?

Carp Pox is caused by a herpesvirus that remains in the fish for life once contracted. The virus can lie dormant and reappear under certain conditions.

Common triggers include:

 

1. Cold Water Temperatures

Carp Pox outbreaks are strongly linked to cooler water (typically below 15°C). The fish’s immune system slows down in cold water, allowing the virus to become active.

 

2. Stress

Stress weakens immunity and can trigger outbreaks. Common stressors include:

• Poor water quality

• Overcrowding

• Sudden temperature swings

• Handling or transport

• Inadequate nutrition

 

3. Introduction of Infected Fish

New fish added to a pond may carry the virus without obvious symptoms. Once introduced, the virus can spread to other carp.

 

How To Treat Carp Pox

There is no cure that eliminates the virus from the fish’s body. However, Carp Pox often improves on its own as conditions change.

Here’s what you can do:

 

1. Raise Water Temperature (If Possible)

As water temperatures increase (above 18–20°C), lesions often shrink or disappear entirely.

For indoor systems or heated ponds, gradual warming can significantly reduce visible symptoms.

 

2. Improve Water Quality

Testing your water regularly with a reliable test kit is key to maintaining optimal pond conditions:

• Ammonia: 0 ppm

• Nitrite: 0 ppm

• Stable pH & KH

• Effective filtration and aeration – appropriately sized for your pond and the number of fish it contains.

Clean, stable water plays a vital role in reducing stress and strengthening your fish’s immune system. Pond Salt can even be added to help improve water quality and act as a general tonic to all the fish in the pond.

For a deeper understanding of why routine water testing matters and how to identify and correct poor water quality, take a look at our guide on ‘Understanding Your Pond’s Water Parameters’.

 

3. Reduce Stress

• Avoid unnecessary handling

• Prevent overcrowding

• Feed a high-quality, seasonally appropriate diet – for example, wheatgerm-based food during Winter to aid digestion in cooler water, and a high protein food in Summer (such as our Koi Growth food) when metabolism is higher. Explore our full range of Koi Food to find the right option for each season.

 

4. Do NOT Attempt to Scrape or Cut Lesions

Physically removing Carp Pox lesions is not recommended, as it can injure the fish and increase the risk of infection without actually getting rid of the virus.

Although the growths may look unsightly, the safest approach is to leave them alone and allow them to fade naturally over time.

 

5. Monitor for Secondary Infections

If lesions become red, ulcerated, or cotton-like, consult a fish health professional. Secondary bacterial or fungal infections may require treatment.

 

When To Be Concerned

While Carp Pox is usually mild, seek expert advice if:

• Growths rapidly increase in size

• Fish stop eating

• Ulcers develop

• Multiple fish show severe symptoms

Accurate diagnosis is essential, as several other conditions can closely resemble Carp Pox.

If you’re unsure whether your fish has carp pox, or you’d simply like reassurance, visit our store for a comprehensive fish health check or send us clear photos so we can help provide a more informed assessment.

 

Does Your Fish Have Carp Pox?

Carp pox may look worrying, but in most cases, it’s a manageable and seasonal condition. By focusing on water quality, temperature stability, and stress reduction, you can keep your fish healthy and minimize outbreaks.

If you’re unsure whether your fish have Carp Pox, or you’re dealing with recurring issues, don’t guess. Reach out to our team for expert guidance.

Healthy pond, healthy fish. Start with prevention and stay proactive.

For more information or for advice on your pond, check out the rest of our blogs…

Dive Into the World of Koi: The Complete Guide | All About Goldfish: Your Guide to a Happy, Healthy Pond |The Importance of Pond Aeration: How Oxygenation Enhances Pond Health