Fig. 2. Trichodina parasitized the fin of chum salmon.

Fig. 3. SEM of Trichodina on the gill.

Fig. 1. Tail of chum salmon fry infected with Trichodina.

(Photos by T. Nomura)

Parasite Trichodina spp.
Taxonomy Ciliophora, Oligohymenophorea
Hosts Many freshwater and marine fishes
Infection site Skin, fin, gill
Clinical signs Infected fish usually exhibits no external abnormality. Wheel-like parasites are observed under a stereomicroscope (Figs. 1 and 2).
Parasitology The parasite body is semipherical and several tens to several hundreds mm in diameter (Fig. 3). Trichodinaattaches on the fish host by the denticles at the central part of the adhesive disc and the marginal cilium. It feeds on bacteria and detritus particles from the fish surface, and proliferates by binary fission (Lom and Dykova, 1992; Ogawa, 2004). Well-known species are Trichodina reticulate from common carp and goldfish, T. truttae from salmonids, etc.
Pathology Trichodina is the opportunistic parasite because it proliferates when hostfs immune system is deteriorated. In heavily infected fish, the skin is injured and mucus secretions are evident. Mass mortalities (cumulative loss of 56 %) occurred in juvenile chum salmon Oncorhynchus keta infested with T. truttae in a transmission experiment (Urawa, 1992). Disease outbreaks caused by T. murmanica were reported in Atlantic cod Gadus morhua (Khan, 2004). Diseased cod showed epidermal and fin erosion, and tail necrosis.
Health hazard Since this parasite is not infectious to human, it is harmless in food hygiene.
Diagnosis Observe the shape and number of the denticles in a preparation stained by the silver impregnation technique.
References Khan, R. A. (2004): Disease outbreaks and mass mortality in cultured Atlantic cod, Gadus morhua L., associated with Trichodina murmanica (Ciliophora). J. Fish Dis., 27, 181-184.

Lom, J and I. Dykova (1992) Protozoan Parasites of Fishes, Developments in Aquaculture and Fisheries Science, 26, Elsevier, pp. 315.

Ogawa, K. (2004): Protozoan diseases. Infectious and parasitic diseases of fish and shellfish. (ed. by Wakabayashi, H. and K. Muroga), Koseisha koseikaku,  pp.285-338. (In Japanese)

Urawa, S. (1992): Trichodina truttae Mueller, 1937 (Ciliophora: Peritrichida) on juvenile chum salmon (Oncorhynchus keta): pathogenicity and host-parasite interactions. Fish Pathol., 27, 29-37.