Ichthyobodo

Ichthyobodo necator (Costia necatrix) is a common ectoparasitic pathogen of freshwater fishes. The parasite exhibits very broad host and geographic distribution, infecting many species in freshwater and occasionally in seawater. Crosstransmission studies and recent molecular studies indicate that the parasite actually represents an assemblage of morphologically indistinguishable species with narrower host specificities.

Ichthyobodo necator is a small, actively swimming flagellated protozoan (10-15 µm in length and about 5 µm wide) which is an obligate parasite of fish. It is one of the smallest ectoparasites that infect fishes. In crowded conditions, Ichthyobodo spreads rapidly infecting the gills and body surfaces, including the fins. It is more common in coldwater species.

Two forms are observed: pyriform stages attached to the surface of gills and skin, and oval or bean-shaped freeswimming forms. The flagellate has a single, centrally located nucleus which contains a large Nucleolus. It has two unequal flagella that extend from a deep flagellar pocket. These flagella are easily visualised by phase contrast microscopy. Each cell bears 2–4 flagella that are used for locomotion while not attached to the host.

Ichthyobodo parasitises many species of fish and death from infestation is more likely to occur in larvae and juveniles than adults, and if untreated, mass mortalities may occur within a few days. Poor water quality and other stresses (especially crowding) may allow this normally mutualistic parasite to reproduce rapidly and overwhelm the host. Transmission is direct from fish to fish, and no cyst stages have been detected. The duration that the parasite remains viable off its host has yet to be determined.

Ichthyobodo attacks the whole surface of the host. Skin or gill infestations are often very intense, and the action of the flagellate results in disruption of the epithelial or epidermal cells, causing widespread pathological changes. Studies have shown degenerative changes and severe hyperplasia in epidermal cells, exhaustion of goblet cells at the site of infestation, and hyperplasia of malpighian cells.

Affected fish exhibit laboured breathing and may be listless and position themselves at the surface, all signs of respiratory distress. Infestations of the body surface are associated with flashing behaviour and a greyish cast on the skin. Heavily infested fish cease feeding, become emaciated and anorexic, and swim near the surface. The gills become clogged with mucus and the filaments may appear swollen.

Diagnosis The infestation is identified by observing numerous flagellate parasites, with the morphology described above, in wet mounts of the skin or gills. Ichthyobodo can also be identified on gill surfaces in histological sections, but wet mounts are preferred because parasites may become dislodged from the gills during processing of tissues, and the small flagellates are more easily identified when they are actively motile. They may be attached to host tissue or swimming free.

Treatment External treatments with formalin and malachite green have been used successfully to treat Ichthyobodo infections in fresh water. Although bath treatments with formalin have long been the treatment of choice for Ichthyobodo, oral treatments with metronidazole, secnidazole, benzimidazole, or triclabendazole have been reported as 100% effective.