Ecological Studies of Food-Borne Trematode Infections in Cheongdo County, Kyongbuk Province, Korea

Abstract

In order to determine the epidemiological patterns of fish-borne trematodes in the vicinity of the river Cheongdo, Kyongbuk Province, Korea where the snails, Parafossarulus manchourichus and Semisulcospira libertina, are abundant, the infection rates for the cercarial and encysted larvae of digenetic trematodes in the snail and fish hosts, and prevalence of Clonorchiasis and Metagonimiasis among the residents were studied during the period of 1998 and 2002. Among the Parafossarulus snails examined, four species of larval trematodes were found; Loxogenes liberum was found to most frequently liberate cercaria with 1.81 per cent, followed by Cyathocotyle orientalis with 0.88 per cent, by Exorchis oviformis with 0.34 per cent and the least frequently by Clonorchis sinensis with 0.19 per cent. Among 2,179 Semisulcospira snails, seven species of larval trematodes and 3 kinds of undetermined cercariae were found in 85 snails, which comprises 3.9 per cent overall infection rate. Of these, the most frequently emerged cercaria was Cyathocotyle orientalis with 1.37 per cent, followed by Cercaria nipponensis with 0.55 per cent, Cercaria incerta with 0.5 per cent, and Metagonimus species with 0.41 per cent. In nineteen species of fish, Clonorchis sinensis, Cyathocotyle orientalis, Echnochasmus species Exorchis oviformis, Metacercaria hasegawai, Metagonimus species, Metorchis orientalis, Centrocestus armatus, Clinostomum complanatum and some kinds of undetermined larvae were found. Of these, the encysted larvae of Cyathocotyle orientalis was found most fequently in 18 species of fresh-water fish, followed by Metacercaria hasegawai in 17 species, Clonorchis sinensis and Exorchis oviformis in 12 species, and Clinostomum complanatum in 8 species of fish. In general, the infection rates of digenetic larval trematodes varied greatly from fish to fish. As for the intensity of Clonorchis sinensis, Sarcocheilichthys wakiyae was the most heavily infected species with on average number of 42.8 cysts per gram of flesh, followed by Gnathopogon atromaculatus with 42.7 and Pungtungia herzi with 17.6 cysts. In the case of Metagonimus infections, the intensity was relatively low and the average number of cysts varied from 0.01 in Zacco temmincki to 3.9 in Microphysogobio yaluensis. The intensity of the other digenetic trematodes showed variations depending on the fishes. The infection rates of Clonorchis sinensis among the residents in the vicinity of the river Cheongdo were relatively high, being overall 4.3 per cent with a prevalence of 6.3 per cent in males and 2.5 per cent in females. The difference in the rates of infection between males and females was found to be statistically significant (t >2). As for the age-specificity, the rate was 0.9 per cent in the 10-19 year age group and 5.7 per cent in the 30-39 year age group. The rate increased as age increased, reaching a maximum of 13.6 per cent in the 40-49 year age group, followed by a decrease. Regarding the intensity of Clonorchis infections, approximately 80 per cent of the residents with clonorchiasis had less than 1,000 eggs per gram of feces. In summary, this study showed that endemic foci of Clonorchis sinensis exist in the vicinity of the river Cheongdo and Clonorchiasis and metagonimiasis among the residents are still highly prevalent.