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Gill and Skin Flukes

The most common types of worm infections that will be encountered by the koi keeper are Gill Flukes (Dactylogyrus) and Skin Flukes (Gyrodactylus). Although it is commonly known as Skin and Gill Flukes, these parasites are not confined to these specific areas. The skin and gill can be infected by either of the two types. If identified early, they can normally be easily eradicated.

Gill flukes (Dactylogyrus) attack the gills of the koi but can also be found on the skin. They have hooks at the rear end of the bodies that are used to attach to the host and these hooks are arranged in a circular form. Gill flukes are hermaphrodites, so each worm can produce and fertilize a single egg at a time. The egg will develop over a number of days into free-swimming larvae that must find a host within hours. Overcrowding will assist the parasite to find a host more readily and to start reproducing. The adult parasite can produce up to 20 eggs per hour in water with a temperature of 24 degrees Celsius. It is estimated that under ideal circumstances, one adult Gill fluke can produce more than two thousand individuals in only thirty days. Treatment for gill flukes should be aimed at the adult fluke and the free-swimming larvae. The eggs are not sensitive to treatment and consecutive treatments should be done to eradicate the infestation.


Skin flukes (Gyrodactylus) are found on the skin of the fish but can also occur on the gills. The skin fluke has a set of hooks that are used for fastening to the host and under a microscope they appear formidable. When viewing this fluke the developing embryo can be observed inside the adult. If you have a strong enough microscope at your disposal, an embryo can be seen inside the embryo! It is estimated that one adult Gyrodactylus can also produce more than two thousand individuals in 30 days. The live young parasite will set about parasitizing the host immediately. Once attached to the host the skin fluke will live on mucus, skin and blood. The adult skin fluke can survive without a host for up to five days.


The flukes are not visible to the naked eye and must be identified through a microscope. Both types of flukes may appear to be moving across the slide by contracting and expanding. The skin fluke will demonstrate a formidable pair of hooks while the gill fluke will have a few dark spots at the front end, resembling a face.

The fluke is known, regardless of species, to carry and inoculate pathogenic bacteria (Aeromonas and Pseudomonas). It is therefore important to acknowledge the danger posed by these parasites to a fish collection.

Symptoms associated with flukes
  1. Excess mucus production.
  2. Secondary bacterial and fungal infections.
  3. If gill fluke are the problem, the gills may become discoloured.
  4. Large amounts of mucus may cause the gill filament to stick together.
  5. Fish will hang in the water close to air stones and water returns.
  6. Fish may refuse food and may become emaciated
  7. As excess mucus is formed on the gills, the gill covers may appear lifted, as the fish will be unable to close it properly.
  8. If the gill and skin flukes are not identified early, other complications like bacterial and fungal infections will pose a major threat.
  9. Damage to the gills may be irreparable and losses will occur.
  10. Flashing can also cause physical damage to the fish.

Prevention of flukes
  1. Maintain optimum water quality
  2. Ensure optimum oxygen levels
  3. Do regular pond maintenance
  4. Avoid rapid temperature changes

Treatment for flukes
  1. Treatment for skin flukes-normally once
  2. Treatment for gill flukes- a course of treatment may be required because eggs will hatch over time.
  3. Use Malachite Green and Formalin.
  4. Potassium Permanganate
  5. Treat for lesions and bacterial infections

Last Updated on Friday, 05 November 2010 17:32