How To Treat Koi Sleepy Disease (Carp Edema Virus)

We are 100% reader supported. We may earn commission at no extra cost to you if you buy through a link on this page. Read our disclosure.

Share this page!

too many koi in a pond increase chances of koi sleeping sickness
Originating in Japan, sleepy sickness has now spread into Europe through trade, koi shows, and non-carp carriers. Public domain.

Koi sleeping sickness, sleepy disease, or scientifically, the carp edema virus, is an infectious disease that can ultimately result in a slow and unpleasant death if left untreated, particularly in juvenile fish. Its two most common names stem from some of the symptoms of the disease: the development of swollen skin lesions (called edemas), and extreme lethargy or “sleepiness.” Carp edema virus (CEV) was originally discovered in Japan in the 1970’s and has since spread globally because of the international trading of koi, with outbreaks occurring in North America, India, and Europe. Electron microscopy examination has found the disease to be a pox virus that directly affects DNA.

Because the disease tends to cause a large number of secondary infections, it can be very difficult to actually recognize and diagnose CEV, contributing to the very high 80% mortality rate found in juvenile koi.

juvenile koi usually die from sleeping sickness sleepy disease
Adult koi usually survive the illness, but the mortality rate is as high as 80% in juveniles. Public domain.

Its spread has been linked to a multitude of reasons – intensive fish culture, international koi shows, animal vectors (or carriers) and other carp that carry the disease but aren’t affected by it, and regional domestic and international trading. Another means of disease spread is within pond water itself; natural water movement causes infected gill and skin filaments to break off, which then come in contact with other koi.

Water temperature and severe stress (such as that caused by transporting fish to a new environment) are two of the most commonly accepted triggers for the onset of the disease. The virus seems to be carried most prominently in the gills of koi, and is more common in koi that have been transported from earthen ponds to concrete tanks or vice versa (a very significant and stressful environment change). So far, only carp species are known to be impacted by the disease, though other fish may act as carriers as mentioned earlier.

What Are Symptoms of Sleepy Koi Disease?

An orange and white koi with swollen, damaged gills from carp edema virus sleepy disease
Sunken eyes, dull colouration, and swollen/red gills can be symptoms. Photo: Deborah B. Pouder, University of Florida

As already discussed, the most telltale symptoms of koi sleeping sickness are skin lesions, damaged or discolored gills, and extreme lethargy (‘sleepiness’). Other symptoms include sunken eyes, loss of appetite, scale discoloration or even complete necrosis, swollen gills, clubbing of gill filaments, and hyperplasia. Studies have found that CEV replication is both fastest and most common in gills (predominantly the epithelial tissue), which explains why the gills of koi suffering from the disease are often discolored and have impaired functioning after only a couple of days of contracting carp edema virus, and why certain parasites, in particular flukes, can worsen the disease.

flukes help spread and worsen fish illnesses
Parasites, such as flukes, can cause or worsen the spread of the illness. The Scottish Government – Gyrodactylus Salaris Working Group (GSWG)(Archive), OGL v1.0OGL v1.0, via Wikimedia Commons

In particular, fish with sleepy koi disease will, as might be expected, appear to simply sleep for extended periods. Juvenile koi will tend to float just beneath the water’s surface, while adult koi are more apt to sit at the bottom of the pond. They may also swim in what appears to be an aimless manner for a short period before becoming inactive and non-responsive again.

In advanced cases, fish will also appear very thin and sickly as they will refuse to eat, which in turn further weakens them and inhibits their ability to try to fend off the disease. In many cases, especially with young koi, there is sadly not much you can do accept make them as comfortable as possible in their environment.

How to Prevent & Treat Koi Sleeping Sickness

1) Raise Water Temps (or bring indoors)

Rubbermaid Commercial 150-Gallons Stock Tank, Structural Foam, Heavy Duty, Resilient to Weather &...
  • MULTI-PURPOSE: 150-gal stock tank can withstand a variety of uses. Use cases range from farm life to a creative solution for daily life...
  • DURABLE: Structural foam resists weathering and crackingand cleaning.
  • STRONG CONSTRUCTION: Stepped sidewalls resist bowing and provide added stability.

Most CEV outbreaks have been reported in waters that are somewhere between 15 and 23 degrees Celsius (or 59 to 74° Fahrenheit). Temperatures above or below this range appear to stop further incubation of koi sleepy virus. If you find your fish suffering from sleeping sickness, try altering the pond’s temperature over the course of a few days (so as to not shock your fish), and monitor for any improvements. Ideally, you should raise rather than lower the water temperature, as koi can become more stressed or ill in cooler temperatures. If you have a very large pond, or young fish, you can also consider bringing the fish indoors inside a quarantine tank which would be easier to heat and allows more convenient administration of treatment.

2) Regular Salt Baths

CrystalClear ParaSalt Outdoor Pond & Water Garden Rock Salt Crystals, Anti Fish Parasite, Beneficial...
  • Pond Salt Builds and Maintains a Healthy Slime Coat -- Formulated to increase koi and goldfish electrolytic intake to build a strong slime...
  • Improves Poor Parasite Conditions - ParaSalt will help ward off the most common protozoan parasites found in ponds. Helps prevent common...
  • Increases Electrolytic Intake and Reduces Fish Stress -- formulated to help build a protective slime coat, aid in wound repair, keep fish...

Many koi breeders employ salt baths as a means of both treating and preventing outbreaks. The recommended strategy is a concentration is 0.5%, with a maximum concentration of 3%, and maintaining the concentration for several days (usually four). While studies have found that salt baths do not fully eliminate the virus, they do vastly reduce mortality due to improved overall physiological health of koi during and after salt baths, thus boosting their ability to battle the virus.

3) Treat Bacterial Infections

Kanaplex, 5 g(0.18 oz)
  • For use in freshwater or marine aquaria.
  • Easily absorbed through both skin and gills; ingestion not required.
  • Does not adversely affect the filter bed and is easily removed with carbon.

Koi sleeping sickness is typically caused by stress, and the presence of injuries or certain harmful bacteria can really put your fish at risk. Wounds, injury, and ulcers in particular can allow any CEV that is present to infiltrate your fish and take hold. If you suspect injury, utilizing antibiotics such as KanaPlex will help keep your koi healthy, thus improving their resistance to serious conditions like koi sleepy sickness. Antibiotics can be administered either via injection (by a professional) or into food or supplements, or put directly into the water where it will be absorbed by the skin and gills. It should be noted that antibiotics have not been found to be useful in treating CEV, but rather are more of a preventative aid. If you’re making use of antibiotics, binding them with koi food is the safest method of administration, as this means 100% of medicine is absorbed  and it cannot cause problems with the surrounding eco-system.

If you’re using Kanaplex, which can be mixed with food (recommended), 1 scoop is added to 1 scoop of binding agent Focus, and then this mixture is added to a table spoon of feed (regular pellets) and a few drops of water for softer feeding. This can be carried out once a day, and then after a week of daily feeding you can evaluate the results, and if there is no improvement, you can repeat the dosing for an additional week.

4) Treat For Common Parasites 

Aqua Meds Aqua Prazi 50 grams
  • Koi Pond Treatment for Fluke parasites

Parasites that are present in the water, such as flukes, can cause skin injuries that may then become infected with diseases or viruses like carp edema virus. Thus, removing any present parasites will improve the overall health of the pond and help prevent the contraction of koi sleeping sickness. Praziquantel (Aqua Prazi) and formalin/malachite green (Proform C) will kill most external parasites and some internal ones as well. The best times to treat for parasites are before winter and early spring, as this is when koi are most likely to contract them due to their  slow running immune system in colder weather. However, if you suspect parasites or notice unusual behaviour (rubbing/flashing), you can treat for parasites just to be on the safe side at almost any time of the year.

5) High Quality Nutrition

IKU KOI KICHI Color Enhancer Koi Fish Food, 40-Pound
  • Iku Koi Kichi premium koi foods contain high quality ingredients that nourish your fish throughout the seasons
  • These wholesome foods come in three varieties including Color Enhancer, Wheat Germ and Staple Food
  • Iku Koi Kichi comes in a range of sizes from 40lb bags to 2lb canister featuring convenient handle and shaker top

Providing your koi with a high quality diet will ensure optimum overall health and decrease the likelihood of them falling prey to various viruses and ailments, such as CEV. As covered in a previous article, koi should ideally be fed a diet of 35 to 40% protein, less than 10% carbohydrates, 5-10% lipids, and can be supplemented with various fruits and veggies to provide other essential vitamins and minerals as well as fiber. Making use of a  high quality pellet feed is essential to healthy carp, and incorporating one with color-enhancer “astaxanthin” (pictured) has actually been found to help strengthen the immune system of fish against various bacteria strains, especially those which may cause ulcers and infection.

6) Good Pond Maintenance

OASE 032232 Pondovac 4 Pond Vacuum Cleaner
  • Max; Suction Depth: 7 Ft
  • Suction Hose Length: 16 Ft
  • Discharge Hose Length: 8 Ft, Max; Flow Rate: 1300 Gph

To prevent the establishment or spread of bacteria, viruses, and disease, you should test your water quality regularly for pH and dissolved oxygen levels, as well as the presence of nitrates, nitrites, and ammonia, and water temperature. Keep your water moving with aerators, well-filtered, and make sure that you clean out your filters regularly. In addition, try to drain your pond and replace the water at least once a year (though doing it in both the fall and the spring is best), as well as making use of weekly smaller water changes. Make sure that you’re clearing away any dead fish and vegetation, and not overfeeding your fish as any excess food will break down in the pond and can contribute to reduced water quality that will provide an ideal habitat for parasites and potentially harmful bacteria to thrive.

7) Don’t Overstock (And Always Quarantine!)

Having too many fish can cause a host of issues that all weaken the defenses of your fish and make them more susceptible to koi sleeping sickness. Having too many fish will result in an excess of excrement and nutrients, increased stress from lack of space, and greater ease of spreading disease and infections, all of which are vectors for carp edema virus. If your pond is already heavily stocked, and you suspect a fish has illness, they should be removed from the main system and quarantined in a separate holding tank to prevent further spread. Another good time to quarantine is when adding new fish to your stock, especially those bred internationally in very different conditions. Keeping new arrivals separate for a  a month or two will allow you to monitor them for signs of illness, disease, or parasites before it can infect your main pond system.

Chris G
About the author

Chris G

Pond consultant and long-time hobbyist who enjoys writing in his spare time and sharing knowledge with other passionate pond owners. Experienced with pond installation, fish stocking, water quality testing, algae control and the troubleshooting of day-to-day pond related problems.

Read more about Pond Informer.

2 thoughts on “How To Treat Koi Sleepy Disease (Carp Edema Virus)”

  1. Hi
    Think you have a typo in the headline – you might want to change it from enema to oedema.
    Thanks for the rest of the info – really useful


Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.