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Costia is a ciliated protozoan that can kill high numbers of fish quickly, just like Chilodonilla. It can be active in cold water down to 45 degrees Fahrenheit. Small quantities of Costia are not uncommon and appear not to be detrimental to the fish’s health. In small numbers, these parasites seem to live on cellular debris in a coexistent relationship with their fish host.

It becomes a severe threat when, for various reasons, the parasite becomes established in large quantities. As with all parasite infestations, large numbers will affect fish health by causing severe tissue damage to both skin and gills and secondary effects such as hyperplasia or secondary infections – particularly of the gill. What makes matters worse is how this parasite reproduces. Since costia multiplies by dividing itself in two, its reproductive rate can be staggering, and the warmer the water, the faster they reproduce.

Costia is detected by a skin scrape or a gill biopsy. It is shaped like a comma and wobbles as it swims. Under the microscope, it can be challenging to find because of its small size. You will probably need a 400x magnification microscope to see these small parasites, as they are only 10 -20 µm long (1µm = 1/1000 millimeter). It sometimes helps to rack down the microscope condenser and add a little more contrast.


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