Ichthyobodo (Costia)

From Fish
Haemorrhagic lesions caused by I. necator
I. necator (arrow) on the gill filament of masu salmon.
Wet mount of I. necator.

Ichthyobodo (Costia) is a protozoan flagellate parasite, found on the gills and skin of fish. All freshwater fish species are susceptible to this parasite.

Life cycle

The parasite body is pyriform and 8-13 mm in size. I. necator attaches on the host by the attachment plate and feeds by the cytostome and the cytopharyngeal canal protruding into the host cell[1]. The free, non-feeding form has an oval body and swims by 2 (rarely 4) unequal flagella. Though many Ichthyobodo which infect freshwater or marine fishes were described as Ichthyobodo necator, it was indicated by transmission experiments and gene analyses that most of them are different species from I. necator[2].

Clinical signs

Infected fish will commonly flash or scrape against objects and in more extreme cases stop eating and gasp at the water's surface. A characteristic sign of infection with this parasite is excess mucus production. Heavily infested fish exhibits anorexia and petechial haemorrhagic lesions in the skin.


Check the flagella in a wet-mount preparation. A morphological observation by the Diff-Quik stain and an analysis for SSU rDNA are needed for the identification to a species level[3].


A new control method using green tea extract and its active ingredient (a kind of catechin) is investigated because the pharmaceutical law in Japan prohibited the therapeutic use of formalin for food fish[4].


  1. Lom, J and I. Dykova (1992) Protozoan Parasites of Fishes, Developments in Aquaculture and Fisheries Science, 26, Elsevier, pp. 315
  2. Urawa, S., N, Ueki and E. Karlsbakk (1998) A review of Ichthyobodo infection in marine fishes. Fish Pathol 33:311-320.
  3. Isaksen, T. E., E. Karlsbakk and A. Nylund (2007) Ichthyobodo hippoglossi n. sp. (Kinetoplastea: Prokinetoplastida: Ichthyobodonidae fam. nov.), an ectoparasitic flagellate infecting farmed Atlantic halibut Hippoglossus hippoglossus. Dis Aquat Org 73:207-217.
  4. Suzuki, K., N. Misaka and D. K. Sakai (2006) Efficacy of green tea extract on removal of the ectoparasitic flagellate Ichthyobodo necator from chum salmon, Oncorhynchus keta, and masu salmon, O. masou. Aquaculture 259:17-27