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Costia on koi fish

The Costia fish parasite is another microscopic organism. The malicious Costia parasite on koi fish is extremely small and quite difficult to diagnose due to its size. It is basically impossible to determine the presence of Costia parasite on koi fish without the aid of a microscope. Costia is comparable in size to a red blood cell but its erratic spinning motion is unmistakable.

Costia is shaped like a bean or comma. It has two long uneven flagella, or hair-like appendages which are used for mobility.  The Costia parasite on koi fish will feed on its host by spearing the cells of the skin and gills and sucking out the content. In response to the vicious parasitic attack koi will become lethargic and may rest on the pond bottom. Also, the koi may increase the amount of mucus on its skin which can make the area appear slightly grayish.  As with many other parasites damage done to the host fish by Costia will enable bacteria to invade the skin and encourage an outbreak of skin lesions and ulcers.

Water temperatures from 39⁰-85⁰F are ideal for Costia, though it really prefers the lower end of this range. Every Costia parasite must be attached to a host to sustain life; it dies within a matter of hours if it becomes free-swimming.

What are koi Costia symptoms?

Look for Costia in koi fish that exhibit these signs: Lethargy, redness or irritated skin, excessive mucus covering on skin, isolation and weakness.