Histopathological changes of Korea cultured olive flounder, Paralichthys olivaceus due to scuticociliatosis caused by histophagous scuticociliate, Philasterides dicentrarachi.

Recently, mass mortality due to histophagous scuticociliate, Philasterides dicentrarachi in olive flounder, Paralichthys olivaceus occurred in aquaculture farms of Jeju Island, South Korea. We have artificially induced infection through immersion on day 12, the density of ciliates slowly decreased in the experimental tanks; on day 28 to 2300cells/ml in 3cm group and 1800cells/ml in 5cm group. The survival rate of P. olivaceus on days 24 and 28 of infection in the 3cm group was decreased to 4 and 6, respectively; however, no mortality was recorded in the 5cm group. In the control group 100% survival in the chosen period of both groups occurred except in the 3cm group whereas on days 24 and 28 the death rate was 1 and 2, respectively. Many ciliates with ingested red blood cells were observed in the gills, skeletal muscle, skin, fins and brains of infected flounder. The parasites were also observed in the lamina propria of the digestive tract, pharynx, peritoneal fluid, cerebrospinal fluid and cornea. Histopathological and inflammatory changes were characterized by swelling and generalized erythema and muscle tissue necrosis. Affected fish showed varying levels of tissue damage including severe epidermal and dermal necrotic lesions. Loss of scales, appearance of bleached spots that coalesced to form brownish patches, haemorrhagic and severe dermal necrotic lesions were the major clinico-pathological manifestations. The parasite was also seen distributed extensively in the entire brain causing widespread nerve necrosis after a prolonged period of infection. These results appear to support the hypothesis that the P. dicentrarchi may be penetrating via the gills or the skin, and then travel via the bloodstream to other parts of the body, including the body cavity.