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Costia (Ichthyobodo necatrix), also known as “Killer Don” is a ciliated protozoan that has the capability to kill fish in great numbers. It can only be identified through a skin scrape and a microscope and is active and proliferates in water temperatures from as low as 2 degrees Celsius to 25 degrees Celsius. The parasite cannot survive temperatures of 28 degrees Celsius and above. Below 10 degrees Celcius, Costia may form into a cyst for protection. Costia can survive as a free swimming organism or attached to a host. Costia has been described as misshapen circles, bean or comma shaped with two hair-like flaggela of unequal length that enable it to swim. It cannot survive long without a host. Costia usually inhabits the skin as well as the gills. Once attached to the host, it actually destroys tissue at that site. Koi suffering from a heavy infestation of Costia may not make it through the treatment as is the case with many parasitic infections.

Symptoms associated with Costia
  1. Bluish or white film on the body because of increased mucus production.
  2. Penetrates the epithelium causing tissue irritation.
  3. Open wounds or ulcers.
  4. Reddening of the skin and fins.
  5. Koi may become lifeless and hang in the water with clamped fins.
  6. Frequent flashing/flicking.

Prevention of Costia
  1. Costia mostly become a problem when fish are stressed. It may be environmental factors or frequent netting or presence of predators.
  2. Maintain good water quality.
  3. Avoid frequent changes in temperature by heating the pond during Spring and Autumn
  4. Do frequent system maintenance such as water changes and filter discharges.
  5. Avoid introducing new koi without quarantine.

Treatment for Costia
  1. Costia has a relative short life span and a once off treatment is usually enough.
  2. Malachite Green and Formalin.
  3. Potassium Permanganate.
  4. Salt. (Some salt resistant strains do occur!)
  • My personal choice would be Malachite Green and Formalin

Updated 23 January 2009

Last Updated on Friday, 23 January 2009 19:37