Studies on intestinal trematodes in Korea VI. On the metacercaria and the second intermediate host of Fibricola seoulensis

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Abstract
This study was carried out to confirm the infection source of the human case of Fibricola seoulensis, and to reveal out a part of its life cycle in Korea. Also the morphological characteristics of the metacercaria were described. The results were summarized as follows: 1. Rana nigromaculata and Natrix tigrina lateralis were found to be infected naturally by the metacercariae(diplostomula) of F. seoulensis. The metacercarial capsule was round to elliptical in tissue of the intermediate hosts with a long diameter 0.232~0.385 mm. Liberated metacercariae were ovoid with small conical posterior body. Body length measured 0.199~0.312 mm and width 0.153~0.252 mm. 2. The infection rate of R. nigromaculata by the metacercariae ranged from 87% to 100% by area, and the number of the larvae ranged from 3 to 390 by frog. The metacercariae were found in skeletal muscle of frogs, from head to hindlegs. 3. All examined N. t. lateralis were found to be infected by the metacercariae with the range of numbers frome 3 to 35,918. The larvae were collected from all viscera and body segments except for the head of the snakes. However, a great majority of the metacercariae were collected from the stomach. 4. Hematoxylin-eosin stained preparations of frog skeletal muscle and snakes stomach revealed that the metacercariae had no cyst wall of worm origin, but encapsulated by the host tissue. Some of them were found in dilated lymphatic vessels. The larval infection was associated with slight or severe inflammatory reaction even with granuloma formation. By above results, it was concluded that the frog, Rana nigromaculata, was the second intermediate host, and the snake Natrix tigrina lateralis was a paratenic host of F. seoulensis in nature in Korea.