However, a paper in Japan (Ide, 1935), and another in the Republic of Korea (Nam and Sohn, 2000), have reported the role of the pond smelts as a second intermediate host for C. sinensis. According to the latter report, 8 metacercariae of C. sinensis were found from a total of 1,305 pond smelts collected from several different localities including the Soyang Lake, Taeho Bay and Paekok Water Reservoir (Nam and Sohn, 2000). With regard to the minnow Zacco platypus, natural (Bae et al., 1983) and experimental (Kim, 1964) infections with C. sinensis have been reported. Recently, however, there have been no succeeding reports on the metacercarial infections in pond smelts and minnows. Thus, the present study was undertaken to find the status of infections with C. sinensis and other trematode metacercariae in pond smelts and minnows in the Soyang and Daechung Lakes.

A total of 559 pond smelts (Fig. 1A) and 30 minnows (Fig. 1B) were collected from the Soyang and Daechung Lakes between 22 and 25 January, 2003 (Table 1). The fish were brought to the laboratory and examined for metacercariae by the artificial digestion and muscle compression techniques. In the digestion technique, the fish were ground in a mortar with a pestle, digested at 37℃ for 60 minutes in a 0.6% pepsin-HCl solution, and the metacercariae were sought using a stereomicroscope. In the compression technique, the fish flesh was taken from 4-5 body parts of each fish, compressed between two glass slides, and examined under a stereomicroscope. The metacercariae were identified using light microscopy. However, to confirm the diagnosis, the metacercariae of C. sinensis were artificially fed to 8-week-old Sprague-Dawley rats, and the adult flukes harvested at week 4 post-infection (PI).

From 179 pond smelts collected from the Soyang Lake, on January 22, 2003, just before the "Pond smelt festival", organized by a television broadcasting company, a total of 110 metacercariae (0.61 per fish) of C. sinensis (Fig. 1C) were detected. To support this observation, the next day, 280 pond smelts were collected, but from a different place of the Soyang Lake, and examined by the same methods. Several species of metacercariae were detected; C. sinensis (51 in number; 0.18 per fish), Metagonimus sp. (2) and unidentified species (22). Totally, 161 metacercariae of C. sinensis were harvested from 459 pond smelts collected from the Soyang Lake (Table 1). From 100 pond smelts collected from the Daechung Lake on January 25, 2003, a total of 369 metacercariae of C. sinensis (3.69 per fish) and 51 unidentified species were collected (Table 1).

On the other hand, from 30 minnows (Fig. 1B) collected from the Soyang Lake on 22 January, 2003, a total of 210 metacercariae were collected; C. sinensis (13 in total; 0.43 per fish), Metagonimus sp. (1), Echinostoma sp. (4), Centrocestus armatus (148) and an unidentified trematode (44) (Table 1).

Juvenile (Fig. 1D) and adult flukes of C. sinensis (Figs. 1E & 1F) were recovered from the bile ducts of the infected rats, with the worm recovery rates of 41.5-92.0% at week 4 PI.

The prevalence of C. sinensis infection among the people of the Republic of Korea has decreased since the 1970s (Rim, 1990; Hong et al., 1994). Consequently, a national survey performed in 1997 revealed a 1.4% egg positive rate among 45,832 people random-sampled from all over the country (Ministry of Health and Welfare and Korea Association of Health, 1997). The prevalence of 1.4% for C. sinensis was higher than any other kind of helminthiases diagnosed by fecal examinations (Ministry of Health and Welfare and Korea Association of Health, 1997).

It is generally accepted that the prevalence of human clonorchiasis has been decreasing very slowly, irrespective of the remarkable decrease in the ingestion of raw freshwater fish by the younger people in the Republic of Korea. One of the reasons for the relatively steady prevalence includes the long life span of C. sinensis in the human bile duct; longer than 10 years (Rim, 1990; Hong, 2003), so infected people can appear as egg positive cases for a long time. However, the possibility of new infections occurring should be ruled out.

In the present study, the infection rate of the pond smelts with the metacercariae of C. sinensis was higher than in the previous reports (Nam and Sohn, 2000). This difference may have been due to the different localities of the very big lake where the fish were caught, or to seasonal differences (Kang et al., 1985; Joo, 1988). The Soyang Lake, for example, is very big, and located in four geographical districts; Inje-gun, Yanggu-gun, Hongchun-gun and Chunchon City. It is noteworthy that the metacercarial burden of C. sinensis was higher in the pond smelts caught from the Daechung Lake than those from the Soyang Lake. Further investigations will be required to confirm this result.

It is already known that minnows are an important intermediate host for a variety of human and animalinfecting trematodes. In the present study, the minnows from the Soyang Lake harbored the metacercariae of less than 5 species. In the past, however, metacercariae of C. sinensis, Exorchis oviformis, Metorchis orientalis, Metorchis taiwanensis, Metagonimus yokogawai, Metacercaria hasegawai, Centrocestus armatus, Echinochasmus japonicus and Pseudoxorchis major were found from minnows (Bae et al., 1983; Rhee et al., 1984; Kim et al., 1987).

The present study has confirmed that both the pond smelts and minnows caught from the Soyang and Daechung Lakes are infected with metacercariae of C. sinensis, and could be sources of human infections. To prevent human clonorchiasis, the eating of these fish in the raw state should be avoided.