Introduction to Freshwater Fish Parasites
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A common mistake of fish culturists is misdiagnosing disease problems and treating their sick fish with the wrong medication or chemical. When the chemical doesn't work, they will try another, then another. Selecting the wrong treatment because of misdiagnosis is a waste of time and money and may be more detrimental to the fish than no treatment at all. The majority of fish parasites can only be identified by the use of a microscope. If a microscope is unavailable, or the person using it has no previous experience with one, the diagnosis is difficult and questionable. Successful fish culturists learn by experience. Newcomers to the field need to learn the fundamentals of diagnostic procedures and how to use a microscope to identify parasites by attending short training courses. The following descriptions of common parasites can be used as references for understanding a professional diagnostic report or as a quick reference for the experienced fish culturist.

Most of the commonly encountered fish parasites are protozoans. With practice, these can be among the easiest to identify, and are usually among the easiest to control. Protozoans are single-celled organisms, many of which are free-living in the aquatic environment. Typically, no intermediate host is required for the parasite to reproduce (direct life cycle). Consequently, they can build up to very high numbers when fish are crowded causing weight loss, debilitation, and mortality. Five groups of protozoans are described in this publication: ciliates, flagellates, myxozoans, microsporidians, and coccidians. Parasitic protozoans in the latter three groups can be difficult or impossible to control as discussed below.

Most of the protozoans identified by aquarists will be ciliates. These organisms have tiny hair-like structures called cilia that are used for locomotion and/or feeding. Ciliates have a direct life cycle and many are common inhabitants of pond-reared fish. Most species do not seem to bother host fish until numbers become excessive. In aquaria, which are usually closed systems, ciliates should be eliminated. Uncontrollable or recurrent infestations with ciliated protozoans are indicative of a husbandry problem. Many of the parasites proliferate in organic debris accumulated in the bottom of a tank or vat. Ciliates are easily transmitted from tank to tank by nets, hoses, or caretakers' wet hands. Symptoms typical of ciliates include skin and gill irritation displayed by flashing, rubbing, and rapid breathing.

Ichthyophthirius multifiliis
The disease called "Ich" or "white spot disease" has been a problem to aquarists for generations. Fish infected with this organism typically develop small blister-like raised lesions along the body wall and/or fins. If the infection is restricted to the gills, no white spots will be seen. The gills will appear swollen and be covered with thick mucus. Identification of the parasite on the gills, skin, and/or fins is necessary to conclude that fish has an "ich" infection.

Figure 1: An illustration of a I. multifiliis parasite.

The mature parasite ( Figure 1 ) is very large, up to 1000 m in diameter, is very dark in color due to the thick cilia covering the entire cell, and moves with an amoeboid motion. Classically, I . multifiliis is identified by its large horseshoe-shaped macronucleus. This feature is not always readily visible, however, and should not be the sole criterion for identification. Immature forms of I . multifiliis are smaller and more translucent in appearance. Some individuals have suggested that the immature forms of I . multifiliis resemble Tetrahymena . Fortunately, scanning the preparation will usually reveal the presence of mature parasites and allow confirmation of the diagnosis.

Figure 2: The life cycle of I. multifiliis

If only one parasite is seen, the entire system should be treated immediately. "Ich" is an obligate parasite and capable of causing massive mortality within a short time. Because the encysted stage ( Figure 2 ) is resistant to chemicals, a single treatment is not sufficient to treat "Ich". Repeating the selected treatment ( Table 1 ) every other day (at water temperatures 68--77F) for three to five treatments will disrupt the life cycle and control the outbreak. Daily cleaning of the tank or vat helps to remove encysted forms from the environment. For more information, see Extension Circular 920 , Ichythyophthirius multifiliis (White Spot) Infections of Fish .

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