Ichthyophonus hoferi (Opisthokonta, Ichthyosporea, Ichthyophonida) is a protozoan parasite which causes an internal infection of organs and muscles in rainbow trout (Oncorhynchus mykiss), chinook salmon (O. tshawytscha), Pacific herring (Clupea pallasii), yellowtail (Seriola quinqeradiata) and other Pacific fish.
Previously, Ichthyophonus hoferi was classified into Zygomycetes (Fungi), but now it is a protozoan. Binucleate hyphal bodies grow up into thick walled spherical multinucleate bodies (20-125 μm in diameter). The multinucleate spherical bodies extend the hyphal bodies and produce spores. Fish become infected by ingesting the multinucleate spherical bodies, which germinate and penetrate in the gastric mucosa.
A disease outbreak of Ichthyophonus hoferi occurred in cultured rainbow trout in Japan because fish fed on raw herring infected with the parasite. In North America, wild chinook salmon are affected by I. necator during a spawning migration.
It is occasionally seen in wild fish as well as in aged cultured and aquarium fish. The disease is generally chronic and progressive. It usually is detected on necropsy when the characteristic spherical cyst stages are seen microscopically in the smears of granulomatous lesions of the eye (causing exophthalmos) heart, liver, spleen, kidney, skin, and muscle. Lordosis and scoliosis are occasionally seen in infected rainbow trout.
Culture of suspect lesions is recommended, and Sabaroud’s dextrose agar is acceptable for primary isolation. Preventive measures include removing infected fish and avoiding feeding raw fish products.
Since this parasite is not infectious to human, it is harmless in food hygiene.
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