Tetraonchus awakurai

From Fish
Tetraonchus spp parasitizing the gill filaments
Adult T. awakurai

Tetraonchus awakurai is a monogenean parasite (Taxonomy: Plathyhelminthes, Monogenea, Monopisthocotylea) that infects the gills of Masu salmon (Oncorhynchus masou masou), red-spotted masu salmon (Oncorhynchus masou ishikawai) and rainbow trout (Oncorhynchus mykiss).

Life cycle

The elongate body is about 1 mm long. T. awakurai attaches by a haptor with 2 pairs of anchors and connecting bars at the posterior end. It possesses 2 pairs of eye spots at the anterior body. T. awakurai transfers from fish to fish (no intermediate hosts).

Clinical signs

The parasite (ca. 1 mm) is visually observed in the gill filament. Heavily infected fish exhibits excessive mucus secretion in the gill.

Masu salmon and red-spotted masu salmon are highly susceptible to this parasite, whereas rainbow trout is resistant.

Since this parasite is not infectious to human, it is harmless in food hygiene[1].


Flatten the parasite and observe the morphology. Tetraonchus awakurai can be morphologically distinguished from T. oncorhynchi infecting masu salmon and red-spotted masu salmon. T. oncorhynchi is larger in length (ca. 1.5 mm) and possesses larger testis.


No control methods are available, but increase of water exchange rates in the pond may be effective for reducing the infection intensity.


  1. Ogawa, K. (2006) Tetraonchosis. New atlas of fish diseases (ed. by Hatai, K. and K. Ogawa), Midori Shobo, p:41