Abstract: Objective To observe the required conditions of excystment and morphology of metacercariae of Fasciola hepatica. Methods Eggs of F. hepatica from cattle were collected and placed in 28 ℃ water bath for 10 or 11 days to hatch miracidia, which were used to infect Galba pervia for 35-40 days for collection of metacercariae. The metacercariae were assigned to 3 groups with 30 in each group. The metacercariae were placed in bovine bile at 50%, 75% and 100% dilution, respectively. Each group of metacercariae was futher divided into 3 groups and placed in water baths at 37 ℃ and 39 ℃ to observe emerging excysted worms every 30 min for ascertaining the optimal excystment temperature. In addition, 15 experimental groups of metacercariae were set up with 10 in each group to examine the excyst condition. After treatment with artificial gastric juice for 10 min, 30 min, 1 h, 1.5 h and 2 h, the cysts were transferred into 50%, 75% and 100% bovine bile, respectively, and incubated at the optimal temperature for 6 h to observe the exeyt process by inspection every 30 min. The morphology of excysted worms were examined microscopically. For control group, distilled water, normal saline, artificial gastric juice and 50%, 75% and 100% bovine bile were used alone, respectively. For those metacercarae that has not excysted, incubation will be continued in water bath for 24 h upon observation. Results After incubation at 37 ℃ and 39 ℃ separately for 24 h, highest yield of excysted worms was seen at 39 ℃, with 9, 12 and 4 excysted cercariae observed in 50%, 75% and 100% bile, respectively. It was indicated that the optimal incubation temperature for excystment in bile was 39 ℃. The excysted cercariae were earliest seen after incubation in artificial gastric juice for 2 h coupling with 100% bile for 1 h. The highest number of excysted cercariae was found in artificial gastric juice incubation for 1 h coupling with 100% bile 6 h, yieling 8 excysted cercariae. No excyst was observed in the 6 control groups during the observation period, but dead excysted worms were seen after 24 h in different concentrations of bile. Under microscope, the excysted cercariae were shaped in long oval, similar to the cercariae body in morphology, which become the juvenile form of adult worm. The size of excysted cercariae was (228.0-361.0) μm × (114.0-209.0) μm, with an average of 291.0 μm × 163.9 μm, having clearly visible oral and ventral suckers; the intestinal duct is yellowish brown in color, extending from the oral sucker, pharynx and esophagus to two branches in coiled overlapping, being full of both sides of worm body through to the tail. Conclusion F. hepatica metacercariae are unable to excyst spontaneously, which requires gastrointestinal digestive fluid factors to co-activate. The body morphology and size of excysted worms are similar to those of cercariae.

Key words: Fasciola hepatica, Metacercariae, Excystment, Decysticercosis