[Experimental life history of Echinostoma hortense]

The complete life cycle of Echinostoma hortense has been maintained in the laboratory, using Lymnaea pervia snails and Rana nigromaculata tadpoles as the first and second intermediate hosts. ICR mice was used as the definitive host. Within the egg of E. hortense, the miracidium was fully matured in 13 days of incubation at 29-30 degrees C. The miracidium was 93.8 x 53.6 microns in average size, covered with numerous cilia of 7-11 microns length. The epidermal plates were arranged in 6-8-4-2 formula. The first generation rediae (1.19 x 0.27 mm in average size) were observed in 14 days after miracidial challenge to the snails, and the second generation rediae (1.40 x 0.26 mm in average size) in 30 days. The average size of the cercaria was 295.5 x 145.0 microns. Their head crown was poorly developed, and collar spines were not yet observed. After a cercarial challenge to the tadpoles, all of the tadpoles became infected and the average worm recovery rate was 88.5%. The majority of the metacercariae (75.5%) were recovered from the muscle of the tadpole's posterior body and the rest (24.3%) from their gills. The metacercariae from the tadpoles were elliptical, and 167.7 x 129.9 microns in average size. The recovery rate of adults from the mice was different by the age of the metacercariae grown in the tadpoles. The metacercariae younger than 5 hrs could not infect mice whereas those older than 6 hrs could infect mice. The recovery rate became higher as the metacercaria matured, with the peak recovery rate of 90.0% at the metacercarial age of 9 days. Thereafter the recovery rate decreased to 55.0% at the age of 50 days. As shown by the above results, the whole life cycle of E. hortense has been completed in the laboratory. At least 55-58 days were required to maintain one egg-to-egg cycle of E. hortense.