| Turbot (Scophthalmus maximus ) is one of the most important cultured fish species in Europe, from where it was introduced into China in 1992. With the consequential successes in artificial breeding from 1997, the commercial culture has been rapidly spreaded along the coast of North China, becoming an important industry. Every year about 50 million juveniles were produced, and yielded more than 20,000 MT marketable fish which totally valued about 250 million USD. However, the rapid expansion of turbot culture led to the occurrence of diseases, in which scuticociliatosis has been considered as a serious disease in the industry.An epidemiological study was conducted along the coast of Shandong Province from 2003 to 2004, during the outbreaks of scuticociliatosis. In most cases, scuticociliatosis resulted in high infection rate and strong pathogenicity. The mortalities in some cases even reached up to 90%. The disease normally occurred during spring and summer seasons, as the temperature around 14 to 20℃. In addition, juvenile and young adult were sensitive to the infection than the adults. The infected individuals showed darkened skin, poor feeding, slow growth, and sometimes haemorrhage and ulceration on the whitish foci.Upon the isolation and cultivation of the ciliates, morphological studies were carried out by means of smear observation, staining with the silver carbonate method, Chatton-Lwoff silver impregnation techniques as well as scanning electron microscopy. The ciliates were rather variable in size, usually (28~48) μm×(15~29)μm, elongatedand ovoid with a pointed anterior and a rounded posterior. A contractile vacuole and a caudal cilium were clearly shown, while 11 bipolar somatic kineties were arranged in non-spiral all over the body. The buccal cavity was located in anterior and occupied nearly 1/3 of the body length. There were three membranes (Mi, M2, M3) and a paroral membrane (PM) within the buccal cavity. PM extended from the posterior end of M2 to the end of cytostome. There were two nuclei, one large round to slightly oval macronucleus and an adjacent small micronucleus, located in the middle of the body. Based on the above characteristics, the histophagous ciliate was identified as Mesanophrys carcini.The optimum growth temperature, pH and salinity of the isolated ciliate were tested to be 10 to 20°C, 15.0 to 30.0 and 5.09 to 8.03 respectively, which means that the ciliate have a wide-ranging temperature, salinity, but being acid tendency. Therefore, it was difficult to control/inactivate the ciliate by artificial changing the environmental factors during the culture conditions.Histopathological studies revealed that there were massive ciliates around the foci, and the ciliates could cause cellular degeneration, hyperplasia and tissue necrosis. The susceptible tissues were operculum, gills, fins, skin, intestine, brain and the cavities of the internal organs. The challenge test indicated that the mortalities of infected turbots could reach up to 100% after either intraperitoneal injection or abrasion-immersion, and the affected turbots showed the same symptoms as naturally infected ones. Different organs and tissues were dissected from moribund fish and examined by smear preparation and histopathology to detect ciliates. We found the ciliates can invade host tissue within internal organs such as muscles, gills, brain and heart, causing severe damage and eventually lead to death. The results demonstrated that the ciliate was the causative pathogen of the scuticociliatosis in farmed turbot.Inactivation test to the ciliate by using antiprotozoals were carried out upon the isolation of the ciliate. The results showed that CuSO4 (3.1ppm) , formalin (31ppm), H2O2 (62ppm) could kill the ciliate within 24h, 7h, and 0.5h, respectively. The mixture of formalin and f^C^both in 31ppm could increase the efficency to kill the ciliate. Inaddition, based on the tests in culture sites, a herbal compound was found to be effective to cure the scuticociliatosis, in which the drug was added into the feeds with a rate of 5g/kg. This result provides the correct application for these drugs, is also useful to supervise the healthy management during turbot culture.This is the first report to find Mesanophrys carcini in farmed turbot in China, therefore, the host range of this ciliate has extended from shellfish to marine fish. This work would be scientifically significance in ecological studies of the scuticociliates, and meanwhile it is helpful in the prevention and treatment of the scuticociliatosis. |
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