Koi Herpes Virus

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January 2003 by Sandra Yosha, DVM, PhD

KHV is the most deadly, economically important disease threatening koi in the United States in 2003.

The time interval between exposure and onset of KHV disease is temperature dependent; the lower the temperature, the longer the incubation period (time between a first exposure to the virus and the onset of disease). KHV seems to survive at low temperatures better than high temperatures (>86 F), but disease caused by KHV is generally seen between 68-85 F.

Koi that have been exposed to KHV have 4 possible fates: a) are not infected, i.e., are naturally resistant or are immune b) are infected and die or c) are infected, but survive and eliminate the virus or d) are infected and survive, but become asymptomatic carriers. Asymptomatic carriers are healthy animals that have viral DNA somewhere on or in their body (the site remains to be determined) and when the right conditions (which are not completely known) occur, the viral DNA becomes activated and begins to reproduce more viruses. The conditions under which the virus is eliminated or becomes latent (inactive) is still unknown. Carrier fish contain viral DNA that can become active some time in the future and can spread the infection to previously unexposed fish.

Stress (including transport, parasites, poor water quality and other factors) probably plays a role in causing fish to breakdown from the disease; KHV is often associated with secondary diseases (i.e. Costia).

The virus attacks epithelial cells of the koi, especially skin and gills. This results in excessive mucus shedding, dry feeling skin, dead gill cells (often attacked by fungus, parasites and bacteria). Affected koi do not eat, cannot breathe, and die a slow, painful death, often lasting weeks.

The mortality rates of populations of koi from a pond in which symptomatic koi were diagnosed (confirmed by laboratory analysis) range from 50-100%.

There is clinical evidence that suggests that KHV affects koi, but not goldfish, but it is not known whether or not goldfish can harbor live virus, and if so, for how long.

Diagnosis of KHV

The most effective way to diagnose KHV at the present time in sick fish is by the PCR test for KHV DNA. Virus Isolation and histology (stained slides interpreted by experts) is also possible, but takes longer. Each test has pitfalls, and is only as reliable as the sampling method and interpretation. All of the tests are subject to false negatives; the PCR test can have a false positive results. Histology is not specific for KHV. Accuracy is critical and sometimes multiple tests must be run. This diagnosis is too important to trust to inexperienced hands.

Current tests are not useful in finding KHV in healthy carrier fish. However, directly exposing KHV free koi to healthy suspected carrier fish (at temperatures between 68-80 F) might cause disease in the KHV free koi within a few weeks (or up to 3 months), which can be tested for KHV. The technique is not fail-safe. However, since there is no reliable laboratory method of detecting carriers by any other means, this method could be tried. Obviously, a test that does not cost of lives of innocent koi would be welcome.

Treatment of KHV Disease

There is no cure or treatment for KHV viral infection at this time. Treatments (chloramine T or potassium permanganate treatment for secondary bacterial gill disease, fungal and parasite control; antibiotic treatment for internal secondary bacterial infections; salt for relief of overhydration; vitamin C for immune support), might help koi overcome secondary problems, but they should not be interpreted to mean that they cure KHV disease. There is no cure. At temperatures <65 F, koi generally do not die or show symptoms of KHV infection, but infected fish will develop signs of KHV disease when the temperature reaches optimum for this virus (68-80 F). Raising the temperature to >86 F might increase the number of survivors. There are still no documented scientific reports to establish reliable treatment guidelines at this time, but chloramine T or potassium permanganate treatments may also be beneficial by reducing the number of live viruses in the water. Sick fish might not survive the treatments; surviving fish might have permanent injuries. Treatment can be costly. Individually valuable and privately owned fish might be worth the effort, but it is not cost effective for most dealers. Furthermore, surviving koi must be considered to be carriers until proven otherwise, and therefore, should not be shown, sold, or transferred without advising the new owners that these are KHV survivors.

Prevention of KHV Exposure

It is highly recommended that owners quarantine all new and any returning (i.e., having been outside the system) fish at the owner's home. There is good evidence to assume that all koi have been exposed to one or more disease causing agents, and that through proper care and management, the losses of koi to disease can be greatly reduced. However, unlike other infectious diseases that can be cured or treated effectively, viruses are incurable, and KHV is associated with high mortalities and permanent carrier fish. Therefore, quarantine is the best defense against KHV at this time.

There is good reason to recommend English style koi shows and advise against Japanese style koi shows. Field cases of KHV had a common factor: koi had been in direct contact with other koi (newly purchased, or recently shown in a Japanese style koi show) and then entered a pond without quarantine. Recently transported fish are stressed (which is believed to be underlying facilitator of KHV disease) and koi are moved more often in a Japanese style koi show. KHV is suspected to be present in koi populations from many countries, and current evidence suggests that no county of origin should be considered free of the virus. There seems to be no justification for showing koi in Japanese style koi shows where the fish are in direct contact with many koi from many places. While the tradition and beauty of a Japanese style koi show would be missed by many, preserving tradition is not worth the potential risk of harming all of the beautiful animals that the show was designed to appreciate and honor. Since koi are truly domestic animals and often loved pets, we owe it to them to protect their health.

There is little documented information about how KHV is transmitted (i.e. by water, direct contact with infected fish, through an intermediate host etc.) or about how long, and under what conditions the virus will live away from the host. The virus may be transmitted by water or nets (short term and minimal contact), but disease outbreaks are definitely linked to direct fish to fish contact. Herpes viruses are killed by routine disinfection, so KHV is assumed to be likewise susceptible. Therefore, a good disinfection procedure that is used regularly and correctly is an essential part of a disease prevention program.

Be an educated hobbyist. Quarantine your fish. Ask your dealer about their quarantine and disease control practices, and work with dealers who quarantine and have routine established veterinary care. Each inspection, quarantine and disease prevention program reduces the risk of KHV. Watch for updates about the virus and control measures such as vaccinated fish, resistant fish or populations of KHV free fish, which might become available in the near future. Attend seminars about KHV or work with qualified professionals who are up to date on this disease. If KHV is suspected, get professional help. This disease is too deadly to be missed and too costly to be wrong. The best way to control KHV is by having breeders, dealers, veterinarians, researchers and hobbyists working together so that the hobby remains fun for all.


Quarantine alone will not prevent the spread of KHV, but it will reduce the risk. Quarantine, when used as part of a comprehensive disease prevention program, is the single best current method of preventing many different types of infectious diseases from entering a koi pond. The following quarantine protocol represents current thinking and experience of several knowledgeable experts in the scientific, veterinary and hobby community and will likely uncover most KHV infected koi. It is not fail-safe, and all koi owners are strongly advised to seek professional advice when dealing with this disease. However, it is a very important first step in reducing the risk of introducing KHV into a pond.


Quarantine is forced isolation. Therefore, a suitable place for this isolation needs to be prepared. Excellent conditions and good water quality is essential because no quarantine is better than quarantine in a system that has poor conditions. Details of how to build several different quarantine systems will be given in the May/June '03 issue of KoiUSA.

The quarantine system needs to:



Be adequate in size and shape - round is better than square


Be free of initial pathogens - sanitize the system prior to use, or use a system known to be disease free.


Be in a safe place - the environment should be tranquil and not subject to extremes


Have a working bio filter - nearly a must and a definite advantage - otherwise daily water changes and ammonia binders are required.


Have the capacity to heat the water to 70-75 degrees F.


Be located a sufficient distance or have barriers such that other areas are not easily contaminated by water or equipment used in and/or around the system.


Have adequate disinfection - see separate disinfection section.


If the 'heat method' is intended to be employed in an attempt to increase survivors of KHV, provisions to heat the tank water to 86 F need to be available.


Under ideal conditions, the water in the quarantine tank should be the same temperature and pH as the wat4er in which the koi has been swimming over the last couple of weeks. If this is not possible, then try to get as close to the temperature as practical, remembering that koi tolerate a rise in water temperature better than a drop.

The koi to be isolated should be contained in a transport bag with adequate water and oxygen and floated in the isolation tank water for 20 to 30 minutes to allow for temperature equilibrium between the water in the bag and in the tank.

Fish (but not transport water) should be released into a properly sized container (koi tub) of clean quarantine tank water and subsequently the fish should be quickly transferred to the tank. If the hobbyist is not competent to handle the fish with bare hands, the transport bag should be used to transfer the koi and should contain as little water as possible, the goal being to minimize the amount of transport water put into the quarantine tank.

The fish should be kept quiet and under cover of Styrofoam or pvc pipes

The temperature of the water in the Q-tank should be adjusted and held between 70 to 75 F for a period of no less than three weeks. During the temperature adjustment period, the temperature should not drop or raise more than 5F per day.

Do not feed the koi for the first few days. Then, if the fish appear hungry, start feeding a little twice per day.

Over the entire period of quarantine, be sure to monitor water parameters, particularly ammonia, nitrite, pH alkalinity, and temperature and make any necessary adjustments and/or take appropriate measures.

Water changes are usually beneficial, are recommended and a minimum of 10% per day is advisable. Always dechlorinate new water used for water changes and use an ammonia binder if the water contains chloramines and you are changing over 20% or if you do not have a working bio-converter on the system.

The fish should be maintained at 70-75 F for no less than three weeks and a much better period would be three months. The longer the quarantine period, the more likely infected fish will be revealed.

At some time during the quarantine period, a fish of "lesser desirability" from your pond should be placed in the tank with the new arrival(s). Koi are social animals, and need companions. However, if there are more than one koi in quarantine, the pond fish can be added after 1-3 weeks in quarantine, and the population kept for 3 weeks to 3 months more. By waiting until 3 weeks, many diseases, including KHV will become obvious, and can be eliminated before any pond fish are at risk.

During the period of quarantine, periodically check (or have a professional check) for any other abnormal or disease conditions. It is advisable to have a complete and accurate diagnosis of conditions found in quarantine, make sure that the quarantine system has good water quality and follow expert advice for keeping koi healthy. Monitor the tank daily and if any fish (new or old) shows the symptoms typical of KHV, call a qualified veterinarian to take samples and submit them to a qualified lab for testing. For a list of qualified veterinarians, see the AKCA web site at: www.akca.org and follow the "KHA" link then the "Referral Veterinarians" link. If maintained at the 70-75 F temperature with no attempt to otherwise support the fish or suppress opportunistic pathogens, about 10% of the fish should be expected to survive.

KHV infected koi may be euthanized (humanely killed) and disposed of properly to prevent spread, or an attempt may be made to save the fish. One method reported successful in saving a substantial percentage of KHV infected koi is to elevate the temperature of the tank water to 86 F until the fish recovers. Others (see previous section) involve adding chemicals to the water to treat the symptoms of sick fish and possibly to prevent transmission of virus. Remember there is no cure for KHV.


Using separate equipment for separate systems is best, but when that is not possible or practical or if disease has invaded a system and ponds and equipment need to be disinfected, the following should prove helpful.

Household bleach, a sodium hypochlorite solution (usually 5.25%), is a good disinfectant particularly for ponds (without fish) and for infrequent or once-off disinfecting of larger equipment. The recommended concentrations are 200 ppm (~145 ml or 5 oz per 10 gallons) for one hour and 10 ppm (~725 ml or 3 cups per 1,000 gallons) for 24 hours. Bleach is toxic to fish but can be neutralized with sodium thiosulfate ("STS" - a dechlorinator). Use STS at the rate of 1 gram per 10 liters of water (~380 grams or 0.81 pounds per 1,000 gallons of water) to neutralize sodium hypochlorite at 100 ppm. Bleach can quickly degrade nets and other equipment. A material that is much gentler on equipment and good choice for general sanitizing of nets, tubs and smaller equipment is benzalkonium chloride ("BZK"), a quaternary ammonium chloride compound.

For some years, many koi shows have used BZK to disinfect equipment. The typical dose used there is about 200 ppm. That dosing was chosen because it is effective against many pathogens and is not lethal to koi in short term dips of that concentration. If the risk of accidentally placing koi in a disinfecting tank is small, a higher concentration can be used to provide faster action, i.e., 250 to 500 ppm for 20 to 30 minutes or 1000 ppm for one minute. If 50% BZK is purchased, a disinfection solution of ~400 ppm may be prepared by placing 3.0 ml of 50% BZK in each gallon of water. To avoid large amounts of foam, the BZK concentrate should be added to the water and not vice versa. Containers for dipping equipment should be large enough to handle any and all equipment anticipated to need disinfecting. If there is a risk of significant contaminants entering the dipping container, care should be taken to cover the container when not in use. BZK is available in a 50% solution from Argent Labs. Western Quat (quaternary ammonium chloride, ~10% solution) is available from Aquatic Eco-Systems. Or, see your local veterinarian for specific uses and disinfection protocols. Disinfection does not work if not used properly; it works best when used as part of a comprehensive disease prevention program.

Equipment that is dirty or has organics on its surface should be thoroughly cleaned prior to disinfecting as contaminants can protect pathogens from the disinfectants and organics tend to combine with and "use up" the active ingredients of disinfecting solutions or protect pathogens from it.

Dispose of plants from infected KHV infected ponds.

Contributing authors:

Rob Hildreth, DVM, Aquatic Veterinary Services, 4840 Irvine Blvd. #108, Irvine, CA 92620, (714) 838-3433, Thefishdoc@aol.com

Tim Miller-Morgan, DVM, Extension Veterinarian/Assistant Professor, Ornamental Aquaculture, Oregon Extension Sea Grant, College of Veterinary Medicine, Hatfield Marine Science Center, Oregon State University, Newport, Oregon, (541) 574-6534 (voice), (541) 265-3887 (fax), (541) 270-0223 (cell), tim.miller-morgan@hmsc.orst.edu

Allen C. Riggs, DVM, MS, Aquatic Animal Health, Dept. of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, PO Box 100136, Gainesville, Florida 32610-0136, Telephone 352-392-4700 ext. 5686, Fax: 352-392-8289, riggsa@mail.vetmed.ufl.edu

Sandra Yosha, DVM, PhD, Private Practice and Consulting Veterinarian for the Koi Health Advisor Program of the AKCA, Lakeland, FL, Cell: 863-838-4282, dvm4fish@aol.com