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Koi & Goldfish Diseases

General: ALL health problems that arise with fish are due to STRESS! This is pointed out by the following diagram (called the Venn Diagram):

  • Common problems that arise involve Parasites, Bacteria, Fungi, and Viruses.

    Web Sites

    Parasites: We are likely to encounter any one of six types of parasites as follows:

    Chilodinella (Ciliated Protozoan)Return To List

    • Under the microscope it has the shape of a man’s ear or a modified heart shape and tends to spin around as you observe it. You need to be quick, though, because it only lasts about 4 minutes once placed on a slide – then dies. Once it dies it appears as a large round organism full of tiny bubbles (may resemble Ich but does not have a crescent-shaped nucleus – nor is there any motion).
    • “This organism is one of the fastest fish killers there is.” (References made in quotes are taken from Koi Health and Disease by Erik L. Johnson, D.V.M.) It should be suspected if a number of fish are dying on the surface or if the fish roll over on their sides (except when disturbed – which causes the Koi to dash madly).
    • Chilodinella does well in cold water (as low as 5 degrees Celsius) so it is able to attack fish in the spring when their immune systems are suppressed. They can be active right on into the summer, however.
    • Treatment is with 0.3% salt solution that is maintained for 14 days. Aeration needs to be better than usual because the survivors may have severe gill damage.

    Costia (Ciliated Protozoan) Return To List

    • Under the microscope it appears as a small swarming “comma.”
    • It is a “parasite of freshwater fish that has the capability to kill fish in great numbers, and in no short time-span.”
    • Infections most often occur in the spring since the organism does well in cold water (as low as 2 degrees Celsius). Remember that is the time when fish are weak and their immune systems are suppressed. It continues to be active clear up to a temperature of 29 degrees Celsius.
    • Costia can be attached or free swimming. The little “comma” shaped parasite attaches to the skin or gills of fish by the thin end of the comma. It can cause a large percentage of fish to die, fins may be reddened, and the fish appear not able to breath very well. Spider-web lesions and excess mucus are characteristic in rapidly dying fish.
    • Treatment is with a 0.3% salt solution, which readily kills the parasite.

    Ichthyophthirius (“Ich”) (Ciliated Protozoan): Return To List

    • Under the microscope it appears as what Dr. Johnson calls “ Planet U.” It is large, dark colored, nearly non-mobile, horseshoe shaped, with a light colored macronucleus. Protoplasm can be observed moving on the inside.
    • Generally Ich causes white spots all over the body and fins of a fish. However, it is rare that the white spots are seen on Koi and Goldfish, even though the fish may be dying.
    • May kill smaller fish while sparing larger ones.
    • “Damage to the gills is the primary way it kills, but damage to the skin with secondary bacterial infection may also figure prominently.” Some fish are resistant “carriers” of the parasite and some fish that have survived an attack without treatment can have strong immunity to re-infection.
    • Treatment with a 0.3% salt solution with heating is usually preferred and is successful under normal circumstances (exceptions do occur). You can refer to “Ich Control With Minimal Fuss” in Dr. Johnson’s book on Koi Health and Disease for details. The salt attacks the swarming stage (Tomites) of the parasite, as other stages are protected by the skin of the fish or by enclosing cysts. Treatment can take up to 21 days.
    • Remember that salt damages and/or kills plants and those plants need to be treated with Formalin, if possible, before being re-introduced to the main system.

    Trichodinia (Ciliated Protozoan): Return To List

    • Under the microscope it appears as a “Frisbee” or a “Flying Saucer” that tumbles around and can move rapidly through the field of vision. The cilia are clearly visible.
    • Dr. Johnson feels this parasite is possibly over-rated in its pathogenicity and, on its own, probably could not kill a big Koi (though it could cause “spider-web lesions in the skin).
    • Causes scratching in Koi that can result in irritation and/or breaks in the cuticle or skin, thereby allowing bacteria to enter and to cause ulcers. Flashing (not Flash Dancing!) of the fish is an indication of the scratching. The fish might come up out of the water and dive down to the bottom in one fluid motion. Basically, it’s trying to find a “tree” to rub against!
    • The presence of Trichodinia is a strong indicator of a water quality problem, especially the buildup of mulm (solid waste materials such as plant parts, fish ecrement, and leftover food in various stages of decomposition).
    • In most cases, a 0.3% salt treatment, usually only for a few days, will kill the organism.

    Flukes (Trematodes): Return To List

    • These microscopic organisms occur in two major classes: the Gill Fluke (class Dactylogyrus) and the Skin Fluke (class Gyrodactylus). Dr. Johnson has noted, however, that there is a great deal of crossover of these organisms with Gill Flukes being found on skin and Skin Flukes being found on gills.
    • Members of class Dactylogyrus lay eggs (about 2 per hour in cold water and up to 20 per hour in warm water) which take up to a month to hatch in the winter and only 4 days to hatch in the summer. After hatching, the larva is free swimming and can infect a host fish and will have the capability of producing eggs about ten days after attaching to the host. An adult lives from two weeks to a month before dying of old age (although in cold water they can survive in hibernation for five to seven months). One adult can produce up to 2,320 individuals within 30 days during the summer.
    • Members of class Gyrodactylus produce live young which can immediately parasitize a host fish. One adult can produce up to 2,452 individuals within 30 days during the summer. Life span is about the same as Gill Flukes.
    • All flukes have suction cups on their anterior (front) end and hooks (haptens) on their other end. They can attach with either end, but most often they are deeply hooked into the skin with their haptens and are exploring the surface of the fish with their suction cups.
    • They can kill fish through the accumulation of large numbers on any one fish (especially when the fish are small). Dr. Johnson points out that they also inject pathogenic bacteria when they attach their haptens to the skin or gills of a fish and thereby cause Ulcer Disease that involve mortal infections of Aeromonas and Pseudomonas bacteria.
    • Treatment can involve using the following (in order of preference):
      • Fluke Tabs R (Aquarium Products, Glen Burnie MD)
      • Potassium Permanganate
      • Praziquantel

    Argulus (Fish Lice – A Crustacean): Return To List

    • On the fish this organism appears as a “freckle” that moves. It can be transparent, tan, or green in color. It can parasitize other animals in the pond – such as amphibians (e.g. frogs).
    • The fish do not need to be weakened for this parasite to attack. It the organism gets into the pond, it will attach to fish and proliferate. However, it may take a long time before the organism is visible and able to be diagnosed. One adult might lay about 500 eggs on the surface of a rock. It takes 4 weeks for the eggs to hatch. The immature form attaches to fish and begin feeding right away, but it can take about 5 to 6 weeks before they become visible. In the meantime fish will “flash” due to the irritation caused by the immature organism.
    • Fish Lice have tiny stiletto-like mouth parts that puncture the fish’s skin and allow the organism to feed on blood. Where they bite into fish they can infect the skin with bacteria (such as Aeromonas and Pseudomonas) with ulcers resulting.
    • Treatment is with Dimilin (a type of insecticide), which usually kills the organism in 3-4 days.

    The following sites have additional information onKoi and Goldfish Parasites


    Return To List

    Bacteria: Diagnosing and treating bacterial diseases of fish is not as straightforward as with parasites. This Information Sheet will not provide details on diagnosis or treatment. We recommend that if you are interested in more detail than provided here or than we provide to you in a consultation that you purchase reference texts (such as Dr. Johnson’s book, Koi Health and Disease) and study the topic on your own. We have training and experience that we can share with you but want to be clear that we are not doctors and therefore do not have the credentials to diagnose disease or provide treatment. You, as the owner of the fish, must ultimately decide what course of action you wish to take in caring for your diseased fish.

    • To diagnose the specific bacterial organism causing a problem with your fish it would be necessary to take a culture that would then be subjected to bacterial identification procedures. Those procedures go beyond simply looking under the microscope. Slides of cultures that are stained with particular stains can give some information (whether the organism is gram negative or gram positive, for example) but genus and species identification can become quite technical. Basically, it is usually not necessary to worry about such detailed identification under normal circumstances.
    • Bacterial diseases of fish are usually secondary infections resulting from injury to the fish (as occurs with parasites, for example) or weakness caused by stress (due to such things a poor water quality, crowding, or poor nutrition). Potentially dangerous bacteria are usually plentiful in any pond water but do not cause a problem with healthy fish.
    • Most often a bacterial infection will show as an ulcer on the fish. Sometimes the ulcer can become quite “gross” as it eats away the mouth or fins. Since the bacteria are inside the fish to one degree or another, water treatment is usually not effective. The fish need to be fed a medicated food, swabbed with a medicated ointment (on the sore), and/or given injections of antibiotics. Sources of stress or injury need to be eliminated and the temperature of the water needs to be elevated.
    • Entry of the bacteria into the blood and organs of a fish can result in deadly damage to organs and/or severe swelling. Bodies of infected fish can swell to such an extent that scales stick out (giving a pinecone type of appearance) as with Dropsy.
    • Unfortunately, there are times when treatment is not feasible (from a practical and/or financial standpoint). It could be more important to try to keep the other fish in the pond healthy than to expend tremendous resources on one fish.

    Return To List

    Fungi: The presence of fungi is usually indicated by what appears to be cotton-wool coming out of a lesion. Before treatment with anti-fungal agents, microscopic verification of the fungus is necessary because there is a bacterium ( Flexibacter columnaris) that has the appearance of a fungus.

    Return To List

    Viruses: Please see a technical manual (such as Dr. Johnson’s book) or for information on viruses. Some can be treated successfully (e.g. KHV – Koi Herpes Virus).


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