Ichthyophonus

Ichthyophonus sp. in Adult Chinook Salmon Oncorhynchus tshawytscha

Dr Diane Elliott
Western Fisheries Research Center
U.S. Geological Survey
Seattle
USA

(Note: Histological slides were provided by Dr George Sanders, Western Fisheries Research Center and Department of Comparative Medicine, University of Washingon, Seattle, Washington, and all histological photos were taken by Diane Elliott. Gross photo taken by Dr Richard M. Kocan, School of Aquatic and Fishery Sciences, University of Washington, Seattle, Washington.)

The parasite Ichthyophonus sp. belongs to an unusual group of protists placed evolutionarily between fungi and animals (Ragan et al. 1996). Parasites identified as Ichthyophonus infect a variety of species of freshwater and marine fishes over a wide geographic range. Many organs can be infected. In salmonids, heaviest infections are frequently found in the heart, skeletal muscle, liver, kidney, and spleen, and are often characterised by the appearance of whitish nodules in these organs (Fig 1).

In 1985, Ichthyophonus sp. was first reported in a few adult chinook salmon (Oncorhynchus tshawytscha) captured in the Yukon River (Alaska and Canada) during their spawning migration. Now, the parasite infects more than 40% of the returning adult chinook salmon in the river (Kocan et al. 2004), and has been associated with significant pre-spawn mortality in this economically important population. Ichthyophonus sp. becomes more disseminated throughout the tissues of the fish (including the gonads) as they migrate upriver, and as the spawning migration progresses from mid-July through mid-August. Fish presumably become infected with Ichthyophonus by ingestion of spores, but the source of infection for Yukon River chinook salmon has not been determined.

The stage most commonly observed in tissue sections is the multinucleate ‘resting spore’ (Figs. 2-5), which appears roughly circular, can measure from 10 to over 300 µm in diameter, and has a thick fibrous wall that stains PAS-positive (Figs. 5-7). The presence of hyphae protruding through the outer spore wall is a definitive characteristic (Fig 6); these are often visible in tissues sampled post-mortem and in chronic infections.

In acute Ichthyophonus infections in Yukon River chinook salmon, host tissue reaction is minimal to absent. When present, the host reaction is primarily peripheral necrosis without fibrosis (Figs. 2-3). In subacute infections, the minimal to moderate host response is characterised by peripheral necrosis with fibrosis. In chronic infections, a moderate to severe host response may be present, characterised by peripheral necrosis, fibrosis, and granulomatous inflammation (Figs 4 and 6).

References
Kocan R, Hershberger P and Winton J (2004). Ichthyophoniasis: An emerging disease in chinook salmon in the Yukon River. Journal of Aquatic Animal Health 16, 58-72.

Ragan MA, Goggin CL, Cawthorn RJ, Cerenius L, Jamieson AVC, Plourde SM, Rand TG, Söderhäll K and Gutell RR (1996). A novel clade of protistan parasites near the animal-fungal divergence. Proceedings of the National Academy of Sciences, USA 93, 11907-11912.

List of Figures
Fig 1. Gross appearance of the heart of an adult chinook salmon severely infected with Ichthyophonus sp..
Fig 2. Severe infection of Ichthyophonus sp. in cardiac muscle of an adult chinook salmon.
Fig 3 Higher magnification photo of Ichthyophonus in the heart of an adult chinook salmon showing multiple nuclei in a spore.
Fig 4. Chronic Ichthyophonus infection in the liver of an adult chinook salmon.
Fig 5. Ichthyophonus spores in the heart of a chinook salmon.
Fig 6. Germinating Ichthyophonus spore in the heart of an adult chinook salmon.
Fig. 7. Ichthyophonus spore in the testis of an adult chinook salmon surrounded by a granulomatous inflammatory response.

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