Histopathology and Mortality in Olive Flounder Infected by Scuticociliatosis caused by Philasterides dicentrarchi

The histophagous scuticociliate, Philasterides dicentrarchi, infects Korean olive flounder, Paralichthys olivaceus, cultured on Jeju Island in South Korea, causing significant economic losses. However, its route of entry is yet unknown. In the present study, we experimentally infected olive flounder by immersion or injection of different concentrations of P. dicentrarchi. Cumulative mortality was highest (95%) in the group of small fish exposed to 2 x 10 ciliates/ml. Ciliates were found in fins, the connective tissues of muscle, and nervous tissues. The major clinico-pathological manifestations were the appearance of bleached spots that coalesced to form brownish patches, hemorrhagic and severe dermal necrotic lesions. The major histopathological changes were degeneration of muscle fibers, necrosis of the hepatic parenchyma, severe edema of the intestinal wall, extensive fouling, necrotic degeneration, and hyperplasia in the gill bronchial epithelium. Many ciliates were ingested in the blood vessels, brain, dorsal fins, muscles, kidneys, and vertebra of infected flounder.