Many fish parasites do not actually invade the tissues, but feed off the mucus, bacterial and sloughed epithelial cells on the surface, or have attachment organs that anchor the parasite in place on the surface of the skin and feed on bacterial, protozoan and other material in the passing water. The small, cylindrical-shaped organism penetrates the epithelial tissues and continues migrating along the fascial planes of the underlying muscles to invade many of the internal organs. This parasite causes "tet" or "guppy disease" in a number of species of tropical aquarium fishes, especially guppies, neon tetras and mollies. Another ciliate that occasionally causes problems by invading the deeper tissues of fish is Tetrahymena pyriformis. This parasite is another relatively large ciliate, but does not have the large C-shaped nucleus that is characteristic of Ichthyophthirius. Cryptocaryon irritans is the saltwater equivalent of the freshwater Ichthyophthirius organism and causes similar clinical signs. The parasite is covered with an external surface layer of cilia and can be identified by the presence of a relatively large horseshoe-shaped or C-shaped macronucleus. This large parasite causes multiple small raised, white lesions that develop as a result of the parasite residing in the skin, fins and gill tissue of the host. Perhaps the most readily recognized protozoan fish parasite is Ichthyophthirius multifiliis, commonly referred to as "Ich" or "white spot disease". Numerous protozoan parasites penetrate the epithelial tissues of the skin and gills of fish. Parasitic infections in fish are diagnosed by direct observation, wet mount preparations of skin, gill and fins, fecal samples, tissue squashes, blood smears, and histopathology. As a result, parasites can be found infesting the outer skin surfaces, inhabiting the lumen of any organ, or deeply embedded within the parenchyma of any tissue of the host. Fish can serve as an intermediate, transport and/or definitive host for the various parasites.
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