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Koi Medicine

Greg Lewbart M.S., V.M.D., Dipl. ACZM

Each year more veterinarians are seeing ornamental fish patients and our knowledge of how to manage the pet fish patient is increasing exponentially. A large percentage of these patients are nishikigoi (Cyprinus carpio) which are taxonomically an ornamental carp. In this country and in other parts of the world we simply refer to these fish as koi (not coy). Koi have been selectively bred in Asia for hundreds of years with qualities such as color, pattern, size, and confirmation of utmost importance in terms of desirability and value. In Japan, these beautiful fish are symbols of masculinity and longevity. In the United States it is not uncommon for a koi owner to have between $1,000.00 and $25,000.00 worth of fish in a pond, and this does not include the value of the pond and filtration equipment.

The following notes include the basics on how to approach and manage a koi case. A formulary with pertinent drugs and dosages and a reference section with valuable text book and periodical resources are included.

An accurate and concise history is essential when working up a koi or koi pond case. Table 1 contains a list of pertinent questions to include in a pet koi history. Obtaining an accurate history should come before any water quality testing or diagnostic procedures.

Before beginning time consuming water quality and biopsy diagnostic procedures, the common problem of pond predation must be ruled out. This leading cause of acute mortality and morbidity among koi can be identified by close monitoring of the pond for predatory birds and mammals like raccoons. Injuries sustained as a result of contact with a predator commonly lead to life threatening bacterial, fungal, and parasitic disease.

Since these notes focus on koi medicine, the emphasis of this section will be placed on tests which can be performed in the field and will yield immediate results. Several companies manufacture test kits which accurately and inexpensively test the appropriate water quality parameters (Hach Company, P.O. Box 389, Loveland, CO 80539, 1-800-227-4224; Gilford Instrument Labs, Inc., Ovelin, OH 44074, 216-774-1041; La Motte Chemicals, P.O. Box 329, Chestertown, MD 21260, 301-778-3100; Orion Research, Inc., 840 Memorial Drive, Cambridge, MA 02139, 617-864-5400). The basic koi diagnostic laboratory must be equipped to test for temperature, ammonia, nitrite, nitrate, pH, dissolved oxygen and total alkalinity (buffering capacity of the water). Kits that also measure copper and chlorine are desirable.

Once the history has been taken and the water tested, it may be time for biopsy and/or necropsy procedures. As a rule, koi are strong, fractious animals that usually require sedation with tricaine methanesulfonate (100-150 ppm for 3-5 minutes) before attempting biopsy procedures. A practical alternative to MS-222 is clove oil. Several studies have investigated clove oil as a fish anesthetic. Clove oil is available on an over-the-counter basis at most pharmacies. Clove oil, also known as eugenol, is distilled from the clove tree, Eugenia aromatica. Eugenol is not completely soluble in water and should be diluted with ethanol at a ratio of 1:10 (clove oil:ethanol) to yield a working stock solution of 100 mg/ml since each ml of clove oil contains approximately 1 gram of drug. Concentrations of between 40 and 120 mg/liter are effective in freshwater and marine species and results are comparable to MS-222, except that recovery may be prolonged.

Antemortem biopsy samples can be obtained with a sterile scalpel blade, scissors, and forceps. Suspect areas of skin and fins can be scraped or removed and placed on a glass slide containing a drop of water. Once a coverslip has been applied the sample can be examined under the microscope for the presence of pathogens. A small gill biopsy can usually be safely taken by removing a few millimeters of gill lamellae from a single gill arch. Blood can be collected from the caudal vein (hemal arch). Antech diagnostics will evaluate koi blood for CBC and plasma chemistry. Be certain to collect the blood with a heparinized needle and syringe. A recent publication provides details on this process as well as reference range values for koi and goldfish (Groff & Zinkl, 1999).

Poor water quality is a leading killer of koi. Good, clean water is the first step in keeping koi healthy. Adequate water temperature is critical to fish health and is an easy parameter to control. Koi thrive at temperatures close to 20 degrees C (68 degrees F) but can be kept between 24 and 27 degrees C (76 and 80 degrees F). See the ACVC Proceedings article on water quality for more details.

Most bacterial pathogens of koi are Gram negative rods and include such genera as Aeromonas and Pseudomonas. Flavobacterium columnaris has also been identified as a lethal problem in koi ponds. Bacterial infections can be severe and lethal bacteremias and septicemias are not uncommon. Antemortem blood cultures can be diagnostic and if a necropsy is performed the kidney should be sampled for culture and sensitivity. There is at least one bacterin vaccine available on the market through Aqua Health Limited in Canada.

Once a diagnosis of bacterial disease has been made or is at least suspected, a treatment plan should be formulated. Larger koi may be injected intracoelomically or intramuscularly with antibiotics which are effective against gram negative pathogens (see Table 2). Once culture and sensitivity results have been obtained, the antibiotic regimen may be altered.

An alternative to injectable antibiotic therapy is utilizing the oral route. Antibiotics may be mixed into a gelatinized food or given as a bolus via a gastric tube if the fish is refusing food. There is also a koi paste food on the market which can be mixed to include oral chemotherapeutics. Koi are quite easy to force feed since there is no risk of aspiration. A flexible blunt tube such as a red rubber catheter can be used to administer a gruel which should be fairly viscous. Commercial flake or pelleted food can be mixed with small amounts of water to formulate the tube feeding gruel. Special care should be taken when passing the gastric tube to avoid perforation of the gastrointestinal lining since koi do not have a true stomach. The clinician will benefit by being familiar with the general internal anatomy of the koi patient.

A third and less desirable approach to chemotherapy is to administer the treatment as a bath. Antibiotics and other compounds can be added directly to the water. This type of treatment is more appropriate for ectoparasite infections. Fish treated in this manner should be removed from the pond and placed in a hospital tank or pool. The treatment tank should be well aerated and any carbon filtration should be discontinued. Table 2 contains doses and duration of treatment for a number of compounds.

Fungal diseases are usually external and are most always secondary to a break in the integrity of the epidermis and associated mucus coating. The most common pathogen is Saprolegnia. Fungal hyphae and spores can be observed in a skin or fin biopsy sample under the microscope. If the infection is not severe many fish will heal with supportive care. The fungal colony can be gently removed with a cotton swab and the underlying wound may be treated topically with a disinfectant or antibiotic cream. Green algal dermatitis has also been reported in koi.

An overwhelming protozoal disease can constitute an emergency situation for a koi pond. Some protozoans such as Ichthyopthirius ("Ich") have an encysted stage which is resistant to chemotherapeutic treatment. When faced with a protozoal outbreak, the clinician must look for a source. This will most commonly be the addition of an unquarantined animal to the aquarium or the presence of a stressor such as overcrowding or poor water quality. Several antiprotozoal compounds appear in Table 2 .

Adding salt to the water in the right quantities will kill some protozoans and can help reduce stress on koi by decreasing the osmotic gradient they are forced to deal with every day. Since freshwater fishes are hyperosmotic to their environment water tends to "leak" into them. The kidneys and gills remove this excess water but debilitated fishes have a problem if their normal water removing mechanisms are working poorly. Koi will readily tolerate between 0.1 and 0.3% salt in their water. This works out to between 1 and 3 grams of salt (artificial sea salt is best) per liter of water. A good ballpark salt dose for the koi client is 1 pound of salt per 100 gallons of water (works out to 1.2 ppt or 0.12%).

The helminths are a broad group of parasites which includes the skin and gill flukes (monogeneans), cestodes, nematodes, trematodes, and crustacean parasites. Koi are most commonly affected by monogenean trematodes and crustacean ectoparasites. Gross observation for the presence of fish lice (Argulus sp.) or anchor worms (Laernea sp.) along with skin and gill biopsies will diagnose ectoparasitic problems. Antemortem fecal examination or a thorough necropsy will diagnose an internal helminth problem.

Koi are also susceptible to a variety of miscellaneous problems including trauma, egg retention or urogenital prolapse, idiopathic scoliosis, and neoplasia. Based on anecdotal reports, it appears that ovarian tumors, which can get quite large, are fairly common in koi.  

Further Reading:

1.  Blasiola, GC II: Koi: A Complete Owner's Manual. Barron's, Hauppauge, NY, 1995, 104 pp.

2.  Gratzek, JB: Aquariology: The Science of Fish Health Management. Morris Plains, NJ: Tetra Press, 1992, 330 pp.

3.  Groff J.M. and J.G. Zinkl. 1999. Hematology and clinical chemistry of cyprinid fish. The Veterinary Clinics of North America; Exotic Animal Practice, 2(3):741-776.

4.  Hikasa Y, Takase, K, Ogasawara, T, and Ogasawara S: Anesthesia and recovery with tricaine methanesulfonate, eugenol and thiopental sodium in the carp, Cyprinus carpio. Japanese Journal of Food Microbiology, 4:161-166, 1986.

5.  Johnson, EL: Koi Helath and Disease. Johnson Veterinary Services, 3805 Robinson Rd., Marietta, GA 30068, 1997, 141 pp.

6.  Lewbart, GA: Emergency pet fish medicine. In Bonagura JD and Kirk RW (eds.): Current Veterinary Therapy XII: Small Animal Practice. Philadelphia, WB Saunders Co, 1995, 1369-1374.

7.  Lewbart G.A. Clinical nutrition of ornamental fish. Seminars in Avian and Exotic Pet Medicine, 1998 7(3):154-158.

8.  Lewbart G.A. Koi Medicine and Management. Suppl Compend Contin Educ Pract Vet, 1998, 20(3A):5-12.

9.  Noga, EJ: Fish Disease: Diagnosis and Treatment. Mosby, 1996, 340 pp.

10.  Stoskopf, MK: Fish Medicine. Philadelphia: W.B. Saunders Company, 1993, 882 pp.


1.  Koi Health Quarterly, Futura Printing Limited, The Koi Health Group, 3 Sunnydale Avenue, Brighton, East Sussex, BN1 8NR, England.

2.  Koi USA, Associated Koi Clubs of America, Midway City, CA 92655

3.  Koi Carp Magazine, Freestyle Publications, LTd., Alexander House, Ling Road, Tower Park, Poole, Dorset, BH12 4NZ

4.  Tropical Fish Hobbyist Magazine, T.F.H. Publications, Neptune, NJ, 908-988-8400.

5.  Freshwater and Marine Aquarium, PO Box 487, Sierra Madre, CA 91025, 818-355-6415.



1.  How long have you been keeping koi?

2.  What are the problems with the fish today?

3.  When did you first notice these problems?

4.  How long have you owned the sick koi and where did they come from?

5.  Are there other fish in the same tank or pond with the sick fish and if so, how are they doing?

6.  What is the size (volume) of the pond and how is it heated, filtered, and aerated?

7.  Do you have a water test kit and if so, how often do you test the water? What are your most current results?

8.  What and how often do you feed your fish?

9.  Have the fish already been treated? If so, by whom, and with what medications?

10.  Is there a possibility that the fish were exposed to some type of toxin?






Common when fish have been inappropriately transported to clinic. Place affected fish(es) in plastic bag containing 1/3 volume of water and fill rest of bag with 100% oxygen from anesthesia machine. Closebag tightly with rubberband or surgical tape. Keep fish(es) in bag until they have resumed normal swimming and respiratory behavior.

Elevated Ammonia

Water changes. Examine filtration, biological load and feeding

Nitrite, or Nitrate

practices.Measure pH to help determine how toxic the total ammonia is in the aquarium or pond.

Abnormal pH

Gradual water changes to bring pH back into normal range. Measure total alkalinity (buffering capacity). If low (below 50 ppm) add crushed coral or dolomite to the system to increase buffering capacity.

Chlorine/Chloramine Toxicity

Water containing the fishes should be immediately treated with a dechlorinating agent or the fish should be removed to a pool, bucket, or cooler containing clean dechlorinated water. Chlorine causes gill necrosis resulting in decreased respiratory surface area. Use ice packs in water to lower temperature which will increase amount of dissolved oxygen (DO) in the water. Koi and goldfish may be lowered to 60 degrees Fahrenheit. In addition, an oxygen bottle may be used to bubble 100% oxygen into the water to help increase DO levels. Artificial sea salt at a dose of 1-2 grams/L of water should be added to treatment tank as well. Ip or IV dexamethasone at a dose of 2 mg/kg q24 h for 3 days may also increase chance of survival. This treatment regimen may be necessary for several days or more depending on the severity of the toxicity.

Topical Bacterial

Apply Panalog or Silvadene cream directly to wound q 12 h. Allow affected area to remain out of water for 30-60 seconds while medication is absorbed. Gills should remain submerged. Parenteral antibiotic therapy may be warranted (see below).

Systemic Bacterial Disease

Antibiotic therapy options: Enrofloxacin, 5 mg/kg given IM or ICe q 48 h for 15 days; 5 mg/kg PO for 10-14 days or 0.1% in food and feed to fish for 10-14 days; 2.5 mg/L as a 5 hour bath, repeated q 24h for 5-7 days, 50-75% water change between treatments. Trimethoprim Sulfamethoxazole, 30 mg/kg given IM or ICe q 24 h for 7-10 days; 30 mg/kg PO q 24 h for 10-14 days or 0.2% in food and feed to fish for 10-14 days. Amikacin, 5.0 mg/kg IM or ICe q 72 h for 2-3 weeks. Culture and sensitivity results may indicate a more appropriate antibiotic. As a last resort, some antibiotics can be effective as a bath treatment. Most can be dissolved in water at a concentration of between 10 and 20 mg/L. Treatment should last for 6-12 hours and should be followed by a 50% water change. Discontinue carbon filtration and water flow through biological filter during bath treatment. Keep water well aerated. Repeat bath treatments at least 3-5 times. Many koi owning clients have access to human drugs and AzactamR, as well as several other antimicrobials, are commonly used by advanced hobbyists.

Protozoal Disease

Ectoparasitic protozoans must be treated with a dip or bath. Options: Most koi and goldfish will tolerate a 4-5 minute dip in full strength (30-35 g/liter) seawater. Aerate well and monitor very closely. Formaldehyde, 20-25 mg/l for 12 to 24 hours followed by a 50% water change. Encysted parasites like "Ich"will require several treatments. Always change water between treatments. Metronidazole, as a bath treatment will kill some external flagellates, 10-20 mg/liter, q 24 h for 3 consecutive days, water changes between treatments; for internal flagellates use in food at 0.2 % (20 mg/g food) and feed for 10 days.

Trematode Disease

Ectoparasitic monogeneans can be serious. Found on skin and gills. Options: Praziquantel, 5-10 mg/L as a 3-6 hour bath., repeat in 7 days. Remove fish to treatment tank if possible and aerate water well. May not kill all species of monogeneans. This treatment will kill most internal cestodes. Seawater dips may be effective against some parasites. See protozoal protocol above. Acetic acid, 2 ml/L glacial acetic acid as a 30-45 second dip, safe for goldfish and koi but test one fish first (biotest).

Nematode Disease

Fenbendazole, combine in food at a concentration of 0.2% and feed for 3 days and repeat in 14-21 days. Levamisole, 2.0 ppm as a bath for 12-24 hours.

Cestode Disease

Praziquantel, bath treatment (see trematode protocol above); 5 mg/kg fish PO in food, repeat in 14-21 days; 5 mg/kg given IP, repeat in 14-21 days. Injectable treatment may also work for internal digenean trematodes.

Crustacean Disease

Common ectoparasites of goldfish and koi (fish louse, anchorworm). More common in ponds than in aquarium. Dimethyl phosphonate, 0.5 mg/liter, 3 treatments 10 days apart, 20-30% water change 24-48 hours following treatment. Use extreme caution when handling these organophosphate compounds. Liquid form commonlyused to kill cattle grubs is easy to handle, measure, and dispense. Acetic acid, dip procedure described above for trematodes is effective for treating new arrivals to a pond for crustacean parasites. Always test with one fish first, if possible. Dimilin (diflubenzuron), a chitin syntehsis inhibitor, can be used at a concentration of 0.1 mg/L. This is an environmental toxin in that it will kill native arthropods. Use may be restricted. Consult EPA guidelines. An alternative to diflubensuron is lufenuron (ProgramR). Anecdotal reports indicate that this drug is effective for treating ectoparasitic crustaceans in koi at a dose of 0.1-0.2 mg/L. The veterinarain should be aware of a possible risk to native aquatic arthropods.

* Portions of these notes are modified from: Lewbart G.A. Koi Medicine and Management. Suppl Compend Contin Educ Pract Vet, 1998, 20(3A):5-12.

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