Costia – microscope view

Costia or Ichthyobodo necator as known scientifically, is an external parasite which is active in a range of temperatures from 2°c to 29°c.

Costia is occasionally found in small numbers on the skin and gills of healthy koi without causing any harm and lives on cellular debris. The natural defences of the koi appear to keep the population reduced to an acceptable level.

Costia is a serious threat when it becomes established in large numbers and reverts to a parasitic existence. This normally happens when the koi are under stress where the koi’s defences are overwhelmed by other factors such as poor system maintenance, overcrowding, low oxygen, poor water quality or other parasitic infestation.

It is vital that good husbandry techniques are employed such as regular water changes, filter discharges and water testing.  It is also beneficial to install some form of heating to prevent temperature fluctuations.

Because it attacks the gills costia can kill quickly. Being a cold water parasite it is particularly dangerous in the spring when the immune system is weaker after the winter.

Costia has a very simple life cycle, using a process known as binary fission, meaning each parasite divides into two and is temperature dependent. It is a cold water parasite that reproduces rapidly at temperatures between 10°c & 25°c but does not apparently survive above 30°c. The life cycle is completed in 10 – 12 hours at 25°c.   At temperatures below 8°c costia encysts, reverting to trophozoites when conditions become more favourable. The trophozoites must then find a host within a short time to survive.


This koi parasite is extremely small (10-20 microns long) and a magnification of 100 times is the absolute minimum required to identify costia, 400 times is ideal. When taking a skin scrape and looking for costia, try to avoid using a thick layer of mucus and add a drop or two of pond water to the slide.

Costia is a very fast moving parasite and looks in shape similar to a comma, search the entire slide carefully but you will often find it easiest to identify this parasite along the edge of the mucus layer where this meets water.


As costia has a short lifespan when not attached to a koi, a one off treatment will normally be sufficient to get rid of the infection.  An effective treatment is the use of malachite green and formalin at the recommended dose rate stated by the manufacturer as the rate is dependant on the concentration of the mix used.

Alternative treatments include Potassium Permanganate, Chloramine T or simply to raise the temperature to at least 30°c although this may prove stressful for the koi.