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External Protozoan Diseases of Fish
by Robert B. Moeller Jr., DVM

California Animal Health and Food Safety Laboratory System
University of California

Ichthyophthirius multifiliis ("Ich" or White Spot Disease)

  1. The largest protozoan parasite of fish. The trophozoite are up to 100 microns diameter, ciliated and contain an oval horseshoe shaped nucleus.
  2. This is a disease of aquarium and hatchery reared fish.
  3. Clinically fish become hyperactive with fish flashing and cutting against rocks or sides of aquariums. As the trophozoites enlarge they cause hyperplasia of the epidermis with white spots forming on the skin and gills. Severely infected fish may have respiratory problems and die. Histologically there is epidermal hyperplasia with the encysted trophozoite present in the epidermis.
  4. The life cycle is direct. Encysted trophozoites (trophonts) leave the fish and settle to the bottom of the tank. The trophozoites (tomonts)divide into numerous tomites (theronts) that are released to infect the skin of the fish. The life cycle takes approximately 4 days to complete. However, it can be sped up by increasing the water temperature.
  5. The only way to treat the disease is by interrupting the life cycle of the parasite. Removal of fish from the infected water for 3 days (25°C) will usually interrupt the life cycle (Tomites live only 48 hours at 26°C). One must treat the water to kill the tomites to prevent spread of the disease (Malachite green, formalin, methylene blue, or KMnO4). Remember, these treatments only kill the tomites and not the trophozoites that are encysted in the fish.
  6. Cryptocaryon irritans is the salt water equivalent to Ichthyophthirius.

Ichthyobodo necator (Costiasis)

  1. Piriform shaped protozoa 6-12 microns long with two short and two long flagella. These are stalked protozoa that attach to the skin or gills.
  2. This disease is observed in most aquariums and hatchery raised fish. This disease occurs primarily in cold waters (10°C) and affects very young fish when they are just beginning to eat food.
  3. Clinically the fish may flash, produce abundant mucus over the skin (blue slime disease) and/or show respiratory distress (flaring of gills). Histologically the parasites are attached to the epithelial surface of the skin or gills.
  4. Transmission of the parasite is by direct contact with the protozoa. This protozoon is a free swimmer so it can swim and then attach to the host where it undergoes binary fusion for reproduction.

Trichodina sp. (Trichodiniasis)

  1. This disease is caused by a group of peritrichal ciliated protozoans. The organisms are saucer shaped, 50 microns diameter, with rows of cilia at both ends and a macro and micronucleus. When viewed dorsoventrally, the parasite appears as an ornate disk with a characteristic ring of interlocking denticles forming a circle in the middle of the organism. (Trichodina truttae is considered to be a specific pathogen for salmonids).
  2. These are observed on most fresh and saltwater fish. This protozoon is relatively common on many fish and is not always associated with disease.
  3. Clinically fish usually exhibit flashing and become lethargic. There is an increase in mucus production causing a white to bluish haze on the skin. The skin may develop ulcers and the fins may fray. If the gills are involved, the fish may have severe respiratory distress. Histologically, masses of organisms are attached by adhesive discs and denticles of exoskeleton to the epidermis. The underlying epithelial cells undergo necrosis. There is secondary hyperplasia and hypertrophy of the gill epithelium.
  4. Transmission is by direct contact with infected fish and or contaminated water.

Tetrahymena corlissi and Tetrahymena pyriformis

  1. Normally a free living oval ciliated 50-70 micron long protozoa.
  2. The organism has been known to affect the fry of various cultured fish (Guppy "Guppie killer" and Northern pike).
  3. Clinically, one may observe necrosis and hemorrhage of the skin. In severe cases the fish have rupture of the body walls and the fish eviscerate. Histologically one observes massive invasion of the musculature by this organism. (The ventral abdominal wall is severely affected.)
  4. This is a free living protozoan that only becomes a problem at times of overcrowding and poor water quality.(water having a high organic matter content)

Dinoflagellates (Velvet disease, Coral fish disease)

  1. Dinoflagellate 100 microns diameter containing chromatophores and a single eccentric nucleus. When free swimming they are 20 microns diameter contain a transverse flagellum in the transverse furrow and a longitudinal flagellum in the longitudinal sulcus. Several species of dinoflagellate are involved:
    1. Oodinium (Velvet disease)
    2. Amyloodinium (Coral fish disease)
  2. Problem in aquarium and cultured fish.
  3. Clinically, fish flash in the water and become depressed with lateral opercular movement. A shimmering heavy yellow colored mucus secretion over the skin and gills is observed. Histologically, large oval organism (80 microns diameter) with multiple chromatophores and a single eccentric nucleus are attached to epithelial cells by pseudopodia.
  4. Transmission is by direct contact with infected fish, and contaminated water.

Epistylis (Heteropolaria sp.; Red sore disease)

  1. Branched stalked ciliated protozoan (Heteropolaria colisarum).
  2. Found primarily in wild populations of scaled fish.
  3. Clinically, one observes ulcers or cotton-like growth on the skin, scales and spine resulting in a red colored lesion. In catfish the lesion involves the spines and bones that underlie the skin of the head and pectoral girdle. This protozoan parasite has also been observed on eggs.
  4. This ciliated protozoan is primarily a free-living protozoan that lives on aquatic plants and is believed to be an opportunist. Outbreaks have occurred in catfish and salmon that have been maintained in water high in organic content.


  1. This disease is caused by the ciliated protozoan Apiosoma that has a barrel shaped body with cilia at the distal end and a large rounded macronucleus.
  2. This organism usually is not a problem but can affect many species of fish.
  3. The organism can appear on the gills or skin causing increased mucus production and hyperplasia. Severe infections of the gills will cause respiratory problems.
  4. This disease is a problem when fish are exposed to poor water quality.
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16. Bruno D.W., Poppe T.T., A color atlas of Salmonid Diseases. Academic press, 1996.
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