Fish can serve as an intermediate, transport and/or definitive host for the various parasites. 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. 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.
External protozoan parasites
Numerous protozoan parasites penetrate the epithelial tissues of the skin and gills of fish. Perhaps the most readily recognized protozoan fish parasite is Ichthyophthirius multifiliis, commonly referred to as "Ich" or "white spot disease". 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. 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. Cryptocaryon irritans is the saltwater equivalent of the freshwater Ichthyophthirius organism and causes similar clinical signs. This parasite is another relatively large ciliate, but does not have the large C-shaped nucleus that is characteristic of Ichthyophthirius. Another ciliate that occasionally causes problems by invading the deeper tissues of fish is Tetrahymena pyriformis. This parasite causes "tet" or "guppy disease" in a number of species of tropical aquarium fishes, especially guppies, neon tetras and mollies. 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.
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. Of the protozoans that infest the surface of the skin of fish is a group of flattened, discoid ciliates of the genera Trichodina, Trichodinella and Tripartiella. These parasites of freshwater, brackish and marine species are readily identified by their internal circular denticular ring that has both an inward and outward facing ring of tooth-like projections. Those that parasitize the gill are generally most pathogenic where they cause significant tissue irritation, hyperplasia of epithelial tissues and respiratory problems. Chilodonella sp. is a dorsoventrally flattened, oval protozoan that has cilia located in distinct bands along the surface of the body. This freshwater parasite causes irritation of the gills and fins by its feeding activity which results in hyperplasia and fusion of gill lamellae and hyperplasia of fin tissue. Ichthyobodo sp. (~ Costia sp.) is an extremely small pyriform-shaped flagellate of freshwater fish. This parasite can be found either attached by one of its two flagella to the skin and gill tissues or in the water displaying a characteristic spiraling swimming behavior.
A diverse group of ciliated parasites that attach directly to the skin of the fish and do not actually feed on the fish but obtain food from the water as it passes by the fish. These include Ambiphyra sp., Trichophyra sp., Epistylis sp. and Heteropolaria sp., all of which have a disk for attaching to the surface of the epithelial cells of the skin or gill. Though these parasites do not normally cause a problem in small numbers, the health of the fish can be affected if large numbers of the parasites are present on the gills where they can disrupt diffusion of respiratory gasses and excretory products.
Dinoflagellates of the genera Piscinoodinium (Oodinium), Amyloodinium, and Crepidoodinium are parasitic on freshwater and marine species of fish. These parasites cause a disease commonly called "velvet disease" or "rust disease" due to the reddish coloration the parasites impart to the affected body, fins or gills. The parasite has finger-like projections (rhizoids) that penetrate the epithelial cells and act as holdfast organs and obtain nutrients from the cells. Infestations of the gill tissue cause epithelial hyperplasia and fusion of the lamellae resulting in secondary hypoxia and osmoregulatory compromise.
Internal protozoan parasites
Spironucleus sp. and Hexamita sp. are small flagellate parasites that are frequently found in the lumen of the intestinal tract of freshwater and marine tropical fish. Mild infestations of the intestinal tract are generally asymptomatic, while heavy infestations can be pathogenic causing necrosis and sloughing of the intestinal epithelium. The resulting clinical signs include inappetence, unthriftiness, mucoid or pale stool, poor condition, emaciation and death.
Microsporidean parasites (now classified with the fungal organisms) produce a spore-filled cyst within almost any tissue of many freshwater and marine species. As the cysts gets larger, infected muscle tissue becomes displaced and turns white in color. Pleistophora hyphessobryconis, causes a syndrome called "neon tetra disease" in zebra danios, cichlids and cyprinids. Myxosporideans are a common sporozoan parasite of many species of fish. These parasites form a spore-filled tissue cyst that displaces or disrupts the function of the infected tissue.
Coccidial (Eimeria sp. and Goussia sp.) infections of the intestinal tract and kidney, respectively, are a common occurrence in young fish where infection can cause poor growth, emaciation and death. Infections can also be found in adult fish, but the resulting pathology is generally less severe.
Both freshwater and marine fishes have a number of flagellated hemoprotozoans that can cause health problems. Trypanosoma sp. has be described from the blue-eyed plecostomus (Panaque suttoni) imported from South America, while Cryptobia iubilans has been described as causing a granulomatous disease in African cichlids and discus.
Monogeneans are parasitic flatworms that infest the external surfaces of many species of freshwater, brackish or marine fish. The monogeneans have an anterior oral sucker used for feeding on mucus and sloughed epithelial cells, while the posterior end has a holdfast organ for attaching to the host. These parasites cause focal irritation, increased mucus production and hyperplasia of the epithelial tissues due to their the feeding activity around the central point of attachment.
Many species of trematodes, cestodes, nematodes and acanthocephalans use fish as an intermediate host for the developing parasites. These larval stages generally cause minimal pathology, but heavy infestations can result in tissue or organ displacement, stunted growth, and emaciation. Fish can also serve as a definitive host, and can harbor a variety of adult digenetic trematodes, cestodes, nematodes and acanthocephalan parasites in the lumen of their intestinal tracts. As in mammals, these helminthes generally cause minimal pathology in the host fish, though heavy infestations can cause poor growth and emaciation.
A number of crustacean parasites infect the skin and gills of tropical and ornamental fishes. Lernaea sp, or "anchor worm", is a copepod crustacean of pond-reared fish, especially goldfish, carp and koi. The adult female parasite develops an anchor-shaped anterior end that is embedded in the muscle of the fish, while the posterior portion of the female’s body hangs along the outside of the fish. Ergasilus sp. is a copepod parasite in which the antennae are modified into specialized pincers used to grasp onto the gill filaments of pond and ornamental fish. The "fish louse", Argulus sp., is a crustacean parasite of many species of pond and ornamental fish. This dorsoventrally-flattened, oval parasite has eight short legs, a dorsal carapace covering the body, two ventrally-located circular sucking discs, two dark eyespots, and a ventral piercing stylet for feeding. This parasite causes a severe inflammatory reaction at the site of stylet penetration and has been implicated in the transmission of several bacterial, viral and hemoprotozoal diseases. Leeches will also occasionally infest ornamental and pond species of fish. Though generally not associated with any significant pathology, large numbers of leeches on an individual can sometimes cause severe anemia and death.
Treatment of the external parasites is fairly simple since most are susceptible to various water-borne chemotherapeutic compounds such as salt, formalin or copper, but treatment of the internal parasites can be difficult to impossible. In addition to the use of chemotherapeutic agents for the treatment of parasites, good husbandry practices are essential for reducing further parasite infestations.