Infection status with trematode metacercariae in pond smelts, Hypomesus olidus

Recently, pond smelts, Hypomesus olidus, have become one of the most popular freshwater fish eaten in raw, especially during the winter in Korea. Pond smelts are believed to have no parasites because they live in clean and cold water. However, in Japan, they have been reported as the second intermediate hosts of Clonorchis sinensis and Exorchis oviformis (Morishita et al., 1965). There have been no known reported cases of trematode infection caused by pond smelts in Korea. Therefore, the present study was aimed to investigate the infection status of trematode metacercariae in pond smelts in Korea.

Fish collections and examinations were carried out ten times from January 1998 through February 1999. Among 1,305 H. olidus collected, 459 were purchased from wholesale dealers in Chinchon-gun, Chungchongbuk-do, and the rest of them were caught with a casting net in Soyangho (Lake), Taehoman (Bay), and Paekkokchosuchi (Pond) (Fig. 1). All of the pond smelts were cleanly washed with tap water, ground into small pieces, and digested with pepsin-HCl solution in a 36℃ incubator. Digested samples were washed with 0.85% saline and examined under a stereo-microscope to collect trematode metacercariae.

Fig. 1

The pond smelts, Hypomesus olidus.

Seven species of trematode metacercariae, including two unidentified ones were detected from 1,305 pond smelts. The number of detected metacercariae according to the species are as follow: C. sinensis 8, Holostephanus nipponicus 7, Cyathocotyle orientalis 24, Diplostomum sp. 14, and Metorchis orientalis 7 (Table 1).

Table 1.

The infection status of trematode metacercariae in 1,305 Hypomesus olidus examined

The prevalence of helminthiasis has remarkably decreased in Korea. Especially, infection by soil-transmitted helminths such as Ascaris lumbricoides, Trichuris trichiura, and hookworms is no longer a threat to public health. However, clonorchiasis is still highly prevalent in many endemic areas and is regarded as the most important parasitic disease in Korea (Ministry of Health and Welfare and Korea Association of Health, 1997).

Until now, many types of freshwater fishes have been reported as the second intermediate hosts of C. sinensis in Korea (Kim et al., 1979; Kang et al., 1985; Rim, 1986; Joo, 1988; Sohn and Choi, 1997). However, the role of H. olidus as a intermediate host for parasites has not been reported yet in Korea. In the present study, it was confirmed that the pond smelt indeed plays a role as the second intermediate host for certain types of trematode including C. sinensis. Thus, it is recommended to avoid eating raw pond smelts to prevent possible clonorchiasis.