Ichthyobodo necatrix, also known as Costia, is a microscopic protozoan parasite. It causes great harm, it will even kill a Koi if left unchecked. Costia is the hardest parasite to see, for it is one of the smallest parasites that attack Koi. It has a very short life cycle, but it can still be active in cold water. Look for a group of swarming commas on the microscope slide. Costia, like any other parasite, can cause a secondary infection like Aeromonas.
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