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Introduction to Freshwater Fish Parasites P2
Increased interest in fish culture has also increased awareness of and experience with parasites that affect fish health, growth, and survival. Information provided on this page is intended for the novice fish hobbyist as a guide to common parasites of freshwater fish. Identification of parasites and their basic treatment is included; however, this information should not be substituted for consultation with an experienced fish health professional.
NOTE: Mention of drugs or chemicals on this page in no way represents a recommendation for their use, nor does it imply that they are approved for use by the Food and Drug Administration or the Environmental Protection Agency.
Flagellated protozoans are small parasites that can infect fish externally and internally. They are characterized by one or more flagella that cause the parasite to move in a whip-
Hexamita is a small (3 -
The recommended treatment for Hexamita / Spironucleus is metronidazole (Flagyl). Metronidazole can be administered in a bath at a concentration of 5 mg/L (18.9 mg/gallon) every other day for three treatments. Medicated feed is even more effective at a dosage of 50 mg/kg body weight (or 10 mg/gm food) for five consecutive days. See also IFAS Fact Sheet VM-
Ichthyobodo, formerly known as Costia, is a commonly encountered external flagellate (Figure 11). Ichthyobodo-
Piscinoodinium is a sedentary flagellate that attaches to the skin, fin, and gills of fish. The common name for Piscinoodinium infection is "Gold Dust" or "Velvet" Disease. The parasite has an amber pigment, visible on heavily infected fish. Affected fish will flash, go off feed, and die. Piscinoodinium is most pathogenic to young fish. The life cycle of this parasite can be completed in 10–14 days at 73–77°F (Figure 12), but lower temperatures can slow the life cycle. Also, the cyst stage is highly resistant to chemical treatment. Therefore, several applications of a treatment (Table 1) may be necessary to eliminate the parasite. For non-
Cryptobia is a flagellated protozoan common in cichlids. They are often mistaken for Hexamita as they are similar in appearance. However, Cryptobia are more drop-
Myxozoa are parasites that are widely dispersed in native and pond-
Clinical signs vary, depending on the target organ. For example, fish may have excess mucus production, observed with Henneguya (Figure 14) infections.
White or yellowish nodules may appear on target organs. Chronic wasting disease is common among intestinal myxozoans such as with Chloromyxum. "Whirling disease" caused by Myxobolus cerebralis has been a serious problem in salmonid culture. Elimination of the affected fish and disinfection of the environment is the best control of myxozoans. There are no established remedies for fish. Spores can survive over a year, so disinfection is mandatory for eradication.
Microsporidians are intracellular parasites that require host tissue for reproduction. Fish acquire the parasite by ingesting infective spores from infected fish or food. Replication within spores (schizogony) causes enlargement of host cells (hypertrophy). Infected fish may develop small tumor-
Clinical signs depend on the tissue infected and can range from no visible lesions to mortalities. In the most serious cases, cysts enlarge to a point that organ function is impaired and severe morbidity and/or mortality results. A common microsporidian infection is Pleistophora, which infects skeletal muscle (Figure 15).
There is no treatment for microsporidian infections in fish. Spores are highly resistant to environmental conditions and can survive for long periods. Elimination of the infected stock and disinfection of the environment is recommended.
Coccidia are intracellular parasites described in a variety of wild-
Clinical signs depend on target organ affected but may include general malaise, poor reproductive capacity, and chronic weight loss. A definitive diagnosis of tissue coccidia should be completed with histologic or electron microscopy. Several compounds have been used to control coccidiosis with some success; however, consultation with an experienced fish health professional is recommended. Maintaining a proper environment and reducing stress appear to be important in preventing coccidia outbreaks in cultured fish.
Monogenean trematodes, also called flatworms or flukes, commonly invade gills, skin, and fins of fish. Monogenean's have a direct life cycle (no intermediate host) and are host-
Freshwater fish infested with skin-
Gyrodactylus and Dactylogyrus are the two most common genera of monogeneans that infect freshwater fish (Figure 17). They differ in their reproductive strategies and their method of attachment to the host fish. Gyrodactylus have no eyespots, two pairs of anchor hooks, and are generally found on the skin and fins of fish. They are live bearers (viviparous) in which the adult parasite can be seen with a fully developed embryo inside the adult's reproductive tract. This reproductive strategy allows populations of Gyrodactylus to multiply quickly, particularly in closed systems where water exchange is minimal.
Dactylogyrus prefers to attach to gills. They have two to four eyespots, one pair of large anchor hooks, and are egg layers. The eggs hatch into free-
Treatment of monogeneans is usually not satisfactory unless the primary cause of increased fluke infestations is found and alleviated. The treatment of choice for freshwater fish is formalin, administered as a short-
Identification of a Parasitic Problem
A common mistake of fish hobbyists 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 hobbyist. For more detailed information about managing fish health please visit;
Tropical Aquaculture Laboratory of the UF|IFAS Extension publication library or click on the name to download the UF|IFAS .pdf white paper.