Ichthyophthirius multifiliis
Andrew Moss

Nearly every keeper of freshwater fish, whether amateur or professional, has encountered Ichthyophthirius multifiliis at some point in time. Ichthyophthirius multifiliis, or Ich for short, is a ciliated protozoan that parasitizes fish. It is the largest known parasitic protozoan found on fishes and the most common aquarium fish disease. It is also responsible for the largest number of captive fish fatalities. In the wild, Ich can be found in virtually any environment suitable for freshwater fish.

Ich is particularly dangerous for enclosed tanks, as the recirculation of water allows the parasite to move more easily between hosts. A fish is infected when the parasite burrows through the mucus layer and settles under the fish’s scales. Ich then encases itself in a hard shell, called a cyst. These cysts appear as small white spots on the surface of the fish. Once mature, they are between 0.5 and 1.0 mm across. At this point, the cyst breaks out of the skin, leaving an open wound and settling at the bottom of the tank. Later, it bursts open, releasing hundreds of microbes into the water. These Ich then attempt to find a new host for reproduction.

Fish infected with Ich can display a wide array of symptoms: loss of appetite, hiding abnormally, rubbing against objects, droopy fins and fast gill movement. A diagnosis of Ich can be confirmed with the appearance of small, grainy white spots. Ich damages fish by restricting gill movement, creating open sores and irritating the epidermis by increasing mucus production. If left untreated, it can cause 100% mortality in a fish population within a relatively short amount of time.

Several effective treatments are available online or at pet stores (formalin, malachite green, copper sulphate, etc), but Ich must be caught early in its development cycle if the fish are to be saved. Treatment is most effective when the water temperature is above 82ºF, as it increases Ich’s development cycle and leaves it more vulnerable to the treatment. Increasing the salt content of the water can also be an effective treatment, as the parasites are less tolerant than fish of higher salinity. Overcrowding and poor water chemistry are significant risk factors for Ichthyophthirius multifiliis and should be avoided. Susceptibility to Ich can be prevented by sustaining an environment well-suited to the captive fish. By maintaining a low-stress environment, the likelihood of an outbreak of Ich can be significantly reduced, keeping fish healthy and fish-keepers happy.







*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.


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