Introduction
Intestinal parasitic infections were highly prevalent until 1970s in Korea, especially among residents of major river basins of the country. During the past decade, a rapid decrease in the overall prevalence of intestinal parasites has been observed and is attributed mainly by nationwide control programs and health education undertaken by the Korean government along with increases in living conditions and quality of life among people living in Korea. Despite the remarkable decrease in soil-transmitted nematode infections,
1 however, the prevalence of fish-borne trematode infections such as with
Clonorchis sinensis and
Metagonimus yokogawai has been maintained at relatively high level.
The liver fluke,
C. sinensis, is currently the most important parasite infecting humans in Korea. Egg positive rates of
C. sinensis in general population were 4.6% in 1971, 1.8% in 1976, 2.6% in 1981, 2.7% in 1986, 2.2% in 1992, 1.4% in 1997, and 2.4% in 2004.
2 In 1981, the egg positive rates of people living in those river basins differed considerably; 40.2% in Nakdonggang, 30.8% in Yeongsangang, 17.3% in Seomjingang, 15.7% in Hangang, 15.9% in Tamjingang, 12.0% in Geumgang, and 8.0% in Mangyeonggang Rivers.
3
The infection of
C. sinensis is contracted by eating undercooked fishes that harbour the metacercariae. Contaminated fishes are mainly found from streams and rivers where the freshwater snail
Parafossarulus manchouricus thrive. The liver fluke in the biliary passage provokes severe pathologic changes therein, such as bile duct dilatation, ductal wall thickening, ductal inflammation, biliary mucosal hyperplasia, and biliary cirrhosis. The infection with
C. sinensis was also found to induce cholangiocarcinoma in experimental hamsters acting as a promoter.
4 Clonorchiasis has been recognized as one of several factors of cholangiocarcinoma in humans.
5,6
Although some local data are previously available on this issue, no nationwide surveys have been conducted. The present survey on the prevalence and distribution of human clonorchiasis is one of the prerequisites for the government to implement its national control measures.
Materials and Methods
2.1 Surveyed area
Twenty-nine counties (
Figure 1 and
Table 1) were selected in five major river basins, which were located near or alongside the major rivers of Korea with reference to the nationwide survey in 2004.
2 The study was undertaken for 1 year from January to December 2007.
2.2 Stool collection and examination
Stool specimens were collected in plastic containers and transferred to the laboratory of National Institute of Health. A total of 31,268 subjects were recruited from villages for stool collection by a random household sampling method to assess the prevalence of parasitic infection. The formalin-ether sedimentation technique was used to examine helminth eggs, larvae, and protozoan cysts. One gram of each faecal sample was fixed with 10% neutral formalin in a 10-mL test tube. The formalin-fixed stool specimen was further processed by the formalin-ether concentration technique and examined for parasites in the laboratory. Parasite-positive individuals were treated with praziquantel and other appropriate anti-parasitic drugs at the end of the study.
Results
3.1 Number of positive cases of total intestinal parasites
Of 31,268 stool samples examined, 2957 (9.5%) were found to contain various intestinal parasite eggs, cysts, or larvae. Eggs of
C. sinensis,
Heterophyes spp.,
Echinostoma spp.,
Trichuris trichiura,
Ascaris lumbricoides,
Gymnophalloide seoi, and
Trichostrongylus orientalis were detected from 2542 (8.1%), 255 (0.8%), 36 (0.1%), 30 (0.1%), 8 (0.03%), 7 (0.02%), and 5 (0.02%) individuals, respectively. Protozoan cysts were detected from 133 (1.3%) individuals for
Entamoeba spp. and 50 (0.2%) for
Giardia lamblia (
Table 2).
3.2 Egg positive cases of C. sinensis by river basin and gender
Infection rates of
C. sinensis by river basin, locality, and gender are summarized in
Table 3. The overall egg positive rate of
C. sinensis was 8.1%. The highest rate was observed in the Nakdonggang River basin, which showed an average infection rate of 12.2%, ranging from 2.8% to 23.1% by counties. Yecheon-gun showed the highest rate at 23.1%, followed by Sangju-si (20.8%). The next highest rate was observed in the Seomjingang River basin with an average infection rate of 9.5%, ranging from 6.8% to 14.4%. The lowest infection rate was observed in the Hangang River basin, which showed an average of 1.0%.
The egg positive rate of C. sinensis was higher in male (10.6%) than in female (6.1%). The highest rate of male residents infected with C. sinensis was observed in the Nakdonggang River basin, which showed an average infection rate of 15.3%, ranging from 3.6% to 28.5%. Male residents living in Yecheon-gun showed the highest infection rate at 28.5%, followed by Sangju-si (22.9%). The next highest rate was observed in the Seomjingang River basin (12.7%), followed by the Yeongsangang (5.1%) and the Geumgang (4.6%) River basins. The overall egg positive rate of female residents was 6.1%. The highest female egg positive rate was also observed in the Nakdonggang River basin with an average of 9.7%, ranging from 1.8% to 19.2%. The next highest rate was in the Seomjingang River basin (6.8%), followed by Yeongsangang (2.1%), Geumgang (1.8%), and Hangang (0.6%) River basins.
3.3 Egg positive cases of C. sinensis by age group
Among the 31,268 cases examined by age group, the positive rate of
C. sinensis was highest in the age group between 50 and 59 years (10.4%), followed by the order of 40–49 (8.7%), 60–69 (8.3%), and 70–79 (7.7%) years (
Table 4).
Discussion
The present study indicated that the overall prevalence of intestinal parasites was 9.5% and 2957 individuals were infected with 13 species of intestinal parasites. This result, however, is considerably lower than that of 2006, which was observed a 14.3% positive rate in southern endemic areas.
7
The positive rate of C. sinensis was the highest, covering 85.6% of all positive cases. The egg positive rate of C. sinensis (8.1%) decreased slightly compared with the result of 2006 investigation. In the nationwide surveys in 1997 and 2004, C. sinensis positive rates were 1.4% and 2.4%, respectively, and these results suggested that C. sinensis are prevalent along major river basins. The present study was mainly focused on residents of villages where clonorchiasis has been endemic or suspected. Thus, it is the main reason for the higher egg positive rate of C. sinensis of the present study (8.1%) than that (2.4%) of 2004.
Clonorchiasis has been regarded as a persisting helminthiasis and has largely disappeared throughout the country.
1 In 2006, the total egg positive rate of
C. sinensis was 11.1% among residents living in the river basins of South Korea. The egg positive rates observed in the river basins of Nakdonggang, Seomjingang, Yeongsangang, and Geumgang were 17.1%, 11.2%, 5.5%, and 4.6%, respectively. In 2007, the total egg positive rate of
C. sinensis was 8.1% (
Table 3). The positive rates observed in the river basins of Nakdonggang, Seomjingang, Yeongsangang, Geumgang, and Hangang were 12.2%, 9.5%, 3.3%, 3.1% and 1.0%, respectively (
Table 3). Especially, the river basins of Nakdonggang and Seomjingang Rivers still appear to be endemic.
The average egg positive rate of
C. sinensis among male was 10.6%, which ranged from 0.2% to 28.5% by river basin areas, whereas the average positive rate among female was 6.1% with a range from 0.2% to 19.2% (
Table 3). Infection rates of male and female residents showed between 3.0% and 2.8% decrease compared with results of 2006.
The egg positive rate increased as age increased, reaching a maximum of 10.4% in the age group of 50–59- years (
Table 3), followed by 40–49 (8.4%), 60–69 (8.3%), 70–79 (7.7%), and older than 80 (6.5%) years. The infection status of
C. sinensis among Koreans by age in this study showed the same peak at the 50- to 59-year-old group as compared with the result in 2006.
3 This result appears to be the typical pattern of
C. sinensis infection, showing a peak at the age group of 40–49 or 50–59 years,
7 which is then rapidly decreased afterwards. Because the life span of
C. sinensis is known to be up to 30 years, the infection peak in the age group of 50s is comprehended as an accumulation effect of reinfection or superinfection with age because epidemiologic studies indicate that humans do not develop any resistance to reinfection or superinfection by the parasites.
4 However, the egg positive rate decreased after the peak, in spite of the cumulative effect in the age group of 60s or more. This decreasing phenomenon in senior groups has been commonly observed in previous investigations
4,7–9 and may suggest the possibility of shorter lifespan of residents with clonorchiasis than those of uninfected ones at the endemic areas.
10 This decrease of 27% compared with the previous year appears to be an outcome of low endemicity by praziquantel medication. The clonorchiasis in the endemic area of South Korea is under continuous anthelminthic intervention, and the intervention makes the decrease in positive rate.
11 Our data have been updated the status of clonorchiasis in endemic area of Korea as reduced egg positive rate and lower intensity of infection compared with previous reports.
3,8,9,11,12
In conclusion, the result of this study revealed little decrease in positive rate of C. sinensis compared with the result of southern endemic areas of Korea in 2006. Therefore, continuous and repeated praziquantel medication is necessary in the endemic in Korea.
