Abstract:Epitheliocystis is a kind of fish disease which is caused by bacteria in the order of Chlamydiales. Its typical characterization is epithelial hypertrophy in gills and skin of diseased fish. Fish epitheliocystis has not been reported in China. As a newly cultured marine fish species, spotted knifejaw (Oplegnathus puncatus) has high economic value. O. puncatus juveniles of 15 cm in total length died continuously in a hatchery located in Shandong Province in 2015. And the cumulative mortality was more than 40% in 15 d. The field investigation recorded that the water temperature was 21℃ and the water salinity was 30 during epidemics. Diseased O. puncatus usually scattered in the tank. They stagnated in the water with little vitality and/or lost balance even drifted with the water flow. Clinical signs of diseased fish contained debility, anorexia and dyspnea. Diseased fish kept mouth open with operculum opening and closing at a high rate. They did not response to feeding. Clinical examination and necropsy revealed that gills of diseased fish were covered by a lot of mucus. The gill filaments were damaged and the intestinal tract was empty. Many cysts with 30–70 μm in diameter, round or ovoid in shape and light yellow-brown in color were observed on wet mounts of gill filaments from diseased fish under the light microscope. The histopathology by hematoxylin and eosin staining indicated that the ends of secondary lamellae adhered each other and many epithelial cells enlarged and formed cysts. These cysts were basophilia and homogeneous. The examination of scanning electron microscope showed that the gill filaments of diseased fish were clavate. The secondary lamellae were covered by a lot of mucus and embedded by many cysts with smooth surface. Based on the field investigation, clinical examination and necropsy of diseased fish and microscopic observation of gills, this disease was considered as epitheliocystis of O. puncatus. This is the first report of epitheliocystis in cultured O. puncatus in China.
