Epidemiological studies of Clonorchis sinensis in vicinity of river Taewha, Kyungnam province, Korea

Current status of Clonorchis sinensis and clonorchiasis in Korea: epidemiological perspectives integrating the data from human and intermediate hosts

Clonorchis sinensis is a carcinogenic liver fluke that causes clonorchiasis in humans. Clonorchiasis is prevalent in East Asian countries, and approximately 15–20 million individuals are estimated to be infected with this fluke globally. This review highlights the current status of C. sinensis and clonorchiasis in Korea from the epidemiological perspective involving the analysis of humans and intermediate hosts. Despite the recent decline in C. sinensis infection rate in Korea, C. sinensis infections remain endemic in 5 major river basins (Han-gang, Geum-gang, Seomjin-gang, Yeongsan-gang and Nakdong-gang; gang means river) with a high incidence of cholangiocarcinoma. A noticeable pattern involves increasing mild infections among patients diagnosed positive for C. sinensis eggs. The infection rate of C. sinensis metacercariae in the second intermediate host, freshwater fish, is also maintained at a substantial level. Thus, the One Health approach integrating different sectors and disciplines is recommended to accelerate and sustain control of C. sinensis, thereby leading to successful eradication. Health promotion via information dissemination and health education should be extended to prevent the consumption of raw freshwater fish by residents living in high-risk areas.

Infection Characteristics of Clonorchis sinensis Metacercariae in Fish from Republic of Korea

The infection nature of Clonorchis sinensis metacercariae (CsMc) in freshwater fish hosts is closely related to the transmission of human clonorchiasis. This article reviewed the infection characteristics of CsMc in freshwater fish in the Republic of Korea (Korea). The status of CsMc infection was examined in a total of 17,792 cyprinid fish of 49 species in 9 water systems, which included Hantan-/Imjin-gang, Han-gang, Geum-gang, Mangyeong-gang, Yeongsan-gang, Tamjin-gang, Seomjin-gang, Nakdong-gang, and streams in the east coastal areas from 2010 to 2020. The infection status of CsMc was examined according to cyprinid fish species and water systems, after which analyzed by endemicity and susceptibility index. The high endemicity was shown in the cyprinid fish from 3 regions (6.1%) in the upper reaches of Nakdong-gang, such as Banbyeon-cheon (stream), Yongjeon-cheon, and Wi-cheon. The moderate levels were observed in fishes from 8 regions (16.3%), and low endemicity was shown in fishes from 20 regions (40.8%). No CsMc were detected in fish from 18 regions (36.7%). The susceptibility of CsMc in index fish, Puntungia herzi, was found to be a reliable index without examination of other fish species. CsMc infection rates were closely related to subfamily groups in the cyprinid fish hosts in a highly endemic area. In Korea, a total of 58 fish species in 10 families has been listed as the second intermediate hosts for C. sinensis. This review provides several novel features of CsMc infection and clarifies the species of second intermediate freshwater fish host in Korea.

Insights on foodborne zoonotic trematodes in freshwater snails in North and Central Vietnam

Foodborne zoonotic trematode (FZT) infections are common neglected tropical diseases in Southeast Asia. Their complicated life cycles involve freshwater snails as intermediate hosts. A cross-sectional study was conducted in Yen Bai and Thanh Hoa provinces in North and Central Vietnam, to investigate the diversity of cercariae of potential FZT and to construct the phylogenetic relationship of trematode cercariae based on the Internal Transcribed Spacer 2 (ITS2) region. Among 17 snail species collected from various habitats, 13 were infected by 10 cercarial groups among which parapleurolophocercous, pleurolophocercous, and echinostome cercariae were of zoonotic importance. The monophyletic tree separated cercarial sequences into different groups following the description of the cercariae families in which Haplorchidae, Opisthorchiidae, Echinochasmidae, and Echinostomatidae are important families of FZT. The overall prevalence was different among snail species and habitats and showed a seasonal trend. Parapleurolophocercous and echinostome cercariae emerged as the most common cercariae in snails in Yen Bai, while monostome, echinostome, and megalura cercariae were most common in Thanh Hoa. Using a molecular approach, we identified Parafossarulus striatulus as the first intermediate snail host of Clonorchis sinensis in Thac Ba Lake. Melanoides tuberculata and Bithynia fuchsiana were we identified preferred intermediate snail hosts of a diverse range of trematode species including intestinal flukes (i.e., Haplorchis pumilio and Echinochasmus japonicus) in Yen Bai and Thanh Hoa, respectively.

Prevalence of Clonorchis sinensis metacercariae in freshwater fish from three latitudinal regions of the Korean Peninsula

A large-scale survey was conducted to investigate the infection status of fresh water fishes with Clonorchis sinensis metacercariae (CsMc) in 3 wide regions, which were tentatively divided by latitudinal levels of the Korean peninsula. A total of 4,071 freshwater fishes were collected from 3 regions, i.e., northern (Gangwon-do: 1,543 fish), middle (Chungcheongbuk-do and Gyeongsangbuk-do: 1,167 fish), and southern areas (Jeollanam-do, Ulsan-si, and Gyeongsangnam-do: 1,361 fish). Each fish was examined by the artificial digestion method from 2003 to 2010. In northern areas, only 11 (0.7%) fish of 2 species, Pungtungia herzi and Squalidus japonicus coreanus from Hantan-gang, Cheolwon-gun, Gangwon-do were infected with av. 2.6 CsMc. In middle areas, 149 (12.8%) fish were infected with av. 164 CsMc. In southern areas, 538 (39.5%) fish were infected with av. 159 CsMc. In the analysis of endemicity in 3 regions with an index fish, P. herzi, 9 (6.2%) of 146 P. herzi from northern areas were infected with av. 2.8 CsMc. In middle areas, 34 (31.8%) of 107 P. herzi were infected with av. 215 CsMc, and in southern areas, 158 (92.9%) of 170 P. herzi were infected with av. 409 CsMc. From these results, it has been confirmed that the infection status of fish with CsMc is obviously different among the 3 latitudinal regions of the Korean peninsula with higher prevalence and burden in southern regions.

Distribution of freshwater snails in family-based VAC ponds and associated waterbodies with special reference to intermediate hosts of fish-borne zoonotic trematodes in Nam Dinh Province, Vietnam

Fish-borne zoonotic trematodes, such as Clonorchis sinensis, heterophyids and others, constitute a public health concern in parts of northern Vietnam and infections with these trematodes are often thought to be linked to fish culture. One common fish culture system is the integrated fish-livestock (VAC) ponds where individual households have 1 or more ponds. Fish fry, mainly of various carp species, produced in hatcheries, not necessarily local, are introduced into nursery ponds and after approximately 6 weeks, juvenile fishes are transferred to household ponds, referred to as grow-out ponds. Grow-out ponds are usually fertilized with organic debris, including animal excreta, to stimulate algal growth and subsequently fish growth. This paper describes the distribution of freshwater snails and occurrence of trematode infections in these in VAC ponds and associated habitats as part of a major study on risk factors of FZT infections in cultured fish in two communes, Nghia Lac and Nghia Phu, Nghia Hung District, Nam Dinh Province. The area is under intense rice cultivation with an extensive canal network supplying fields and also household VAC ponds. A total of 16 snail species was found and four were widely distributed i.e. Angulyagra polyzonata, Melanoides tuberculata, Bithynia fuchsiana and Pomacea insularum. Snail diversity and counts were higher in nursery ponds than in grow-out ponds. Species of the families Thiaridae and Viviparidae were more abundant than other species in VAC ponds while species of the Bithyniidae, Stenothyridae and Planorbidae dominated in rice fields and small canals. Trematode infections were found in eight snail species and among these M. tuberculata had the highest overall prevalence of infection (13.28%). No trematode infections were found in species of the Viviparidae and Ampullaridae except for metacercariae. Parapleurolophocercous and pleurolophocercous cercariae constituted the most common type of cercariae recovered, contributing 40.6% of all infections followed by echinostome cercariae (35.0%) and xiphidiocercariae (17.3%). Bithynia fuschiana and M. tuberculata had the most diverse trematode fauna. C. sinensis was not recorded in this study. The VAC pond system in this area, is very important for transmission of minute intestinal trematodes while they play little role in transmission of C. sinensis as its intermediate hosts, bithynid snails, rarely occur in these ponds. From a public health perspective this is positive as the effects of infections with intestinal trematodes are considered mild. On the other hand it is possible that even such subtle effects could have importance in public health as transmission is very intense in the area. And this in combination with the aquaculture importance, reduced marketability of fishes with high metacercariae loads, warrants that control efforts against these trematodes are initiated to reduce transmission in this production system.

Fish-borne zoonotic trematode metacercariae in the Republic of Korea

The prevalence of fish-borne trematodes (FBT), including Clonorchis sinensis, is still high in riverside areas of the Republic of Korea. The author reviewed the detection and identification methods, differential keys, fish intermediate hosts, and morphological characteristics of FBT metacercariae. FBT metacercariae found in freshwater fish are classified mainly into 4 families, i.e., Opisthorchiidae, Heterophyidae, Echinostomatidae, and Clinostomidae. The metacercariae of C. sinensis, found in 40 species of freshwater fish, are elliptical and 0.15-0.17 x 0.13-0.15 mm in size, have nearly equal sized oral and ventral suckers, brownish pigment granules, and an O-shaped excretory bladder. Their general morphologies are similar to those of Metorchis orientalis (except in the thickness of the cyst wall). Metagonimus spp. (M. yokogawai, M. takahashii, and M. miyatai) metacercariae are subglobular or disc-shaped, and 0.14-0.16 mm in diameter. They have yellow-brownish pigment granules, a ventral sucker deflectively located from median, and a V-shaped excretory bladder. The metacercariae and fish intermediate hosts of Centrocestus armatus, Clinostomum complanatum, and 3 echinostomatid flukes (Echinostoma hortense, E. cinetorchis, and Echinochasmus japonicus) were summarized. FBT metacercariae detected in brackish water fish are mainly members of the Heterophyidae. The morphological characters, identification keys, and fish intermediate hosts of 7 species (Heterophyes nocens, Heterophyopsis continua, Pygidiopsis summa, Stellantchasmus falcatus, Stictodora fuscata, Stictodora lari, and Acanthotrema felis) were also reviewed. The contents treated in this study will provide assistance at the laboratory bench level to those working on recovery of metacercariae from fish hosts and identifying them.

A bile based study of Clonorchis sinensis infections in patients with biliary tract diseases in Ulsan, Korea

Stool examination is believed to be the most reliable method for detecting Clonorchis sinensis (CS) eggs. However, it has limited value for diagnosing clonorchiasis when the biliary tract is obstructed or when there is a light infection. We evaluated the infection states of CS in patients with biliary tract diseases using a bile sample. From January 2001 to August 2003, 238 patients who had undergone endoscopic biliary drainage were prospectively included in the study. The patients' bile samples were obtained directly from the nasobiliary drainage tube and then analyzed to detect CS eggs. The overall CS egg positive rate was 28.2% (35.4% in males, 19.4% in females). The egg positive rate was similar in all age groups examined: 26.7% in 30-39 years, 25.0% in 40-49 years, 24.4% in 50-59 years, 30.2% in 60-69 years, 35.3% in 70-79 years, and 25.0% in 80 years of age and over. There were no significant differences in the egg positive rate between the disease groups: 32.6% in bile duct cancer, 38.5% in gallbladder cancer, and 26.4% in gallstone diseases. Our results show that the CS infection rate was very high, regardless of the age, gender, and type of diseases of the patients. Although the study population was limited to patients with biliary tract diseases, it is assumed that clonorchiasis is still an endemic disease in Ulsan, Korea.

Changing patterns of Clonorchis sinensis infections in Kyongbuk, Korea

Studies were conducted from May, 1993 to April, 1995 to determine the changing patterns of infection by the liver fluke, Clonorchis sinensis, among residents and fish hosts in Kyongbuk Province. The infection rate among residents was 7.7% by stool examination. The rate in males (11.3%) was significantly higher than females (4.1%). Positive rate of intradermal test was 27.6% in the same population. The special type of a simple catalytic model was applied for the analysis of intradermal positive reactors by age and sex, and the equation was y = 0.4776 (1 - e-0.0375t) for males and, y = 0.2085 (1 - e-0.0138t) for females. Analysis of stool examination data by two-stage catalytic model revealed y = 0.025 (e-0.00471 - e-0.0235t). The annual Clonorchis infection rate was 4.7 per 1,000 susceptibles and the annual loss rate was 23.5 per 1,000 infected. The frequency distribution by the eggs per gram (EPG) was calculated as well as the cumulative percentages of positives. The regression equations were y = 0.929 + 1.506 log x for males and, y = 0.473 + 1.767 log x for females. Of the 25 fish species, 7 species were infected with Clonorchis metacercariae. Infection rates varied by the species, and ranged from 2.8% in Puntungia herzi to 30.0% in Pseudorasbora parva. Average number of the matacercariae per gram of flesh was 58.1 in P. parva, followed by 10.2 in Gnathopogon atromaculatus, 7.0 in Saurogobio dabryi, and 3.0 in Paracheilognathus rhombea. The present study indicates that clonorchiasis in Kyongbuk Province is less prevalent than that of several decades ago.

The infestation states and changing patterns of human infecting metacercariae in freshwater fish in Kyongsang-do and Kyonggi-do, Korea

The infestation rates and abundances of human infecting metacercariae (Clonorchis sinensis, Metagonismus spp., Centrocestus armatus, Echinostoma hortense, Echinochasmus japonicus, Clinostomum complanatum) in freshwater fish were investigated at the three river areas-Taewhagang (river), Hyongsangang (river), Nakdonggang (river) -in Kyongsang-do and at four streams-Yonpungchon, Munsanchon, Kyonganchon, and Konjiamchon-in Kyonggi-do, Korea in 1994-1995. The fish caught at Taewhagang were heavily infested with metacercariae of Clonorchis sinensis and Centrocestus armatus. At Hyongsangang, Zacco platypus and Z. temmincki were moderately infested with metacercariae of C. armatus. Chomanpo, at the basin of Nakdonggang, was still endemic for C. sinensis. In the fish caught at four streams of Kyonggi-do, metacercariae of C. sinensis exhibited the highest infestation rate and intensity out of 6 species of metacercariae. The infestation intensity of C. sinensis metacercariae in fish flesh was markedly different according to each division of flesh. The cause of this difference was conjectured as a result of larval behavior. The metacercariae of C. armatus were found in almost all parts, except scales and fins, of fish. The infestation rates and intensities of C. sinensis and C. armatus metacercariae in Taewhagang greatly increased as compared with those of previous reports. Rhinogobius brunneus and Acanthorhodeus macropterus are newly recorded intermediate hosts of Echinostoma hortense. The reason of large differences from previous data was discussed and the standard method of metacercaria examination was proposed.

Epidemiological studies of digenetic trematodes in Yongyang County, Kyungpook Province

The present study was undertaken to determine the endemicity of trematode infections in Yongyang County, Kyungpook Province, Korea. One hundred and seventy-seven out of 955 residents examined were found to be infected with one or more kinds of helminths. The prevalence rate was 18.5%. Among them, 70 were found to be infected with Clonorchis sinensis, 74 with Metagonimus sp. and 24 with both flukes. The eggs of Fasciolidae were demonstrated from two specimens. The prevalence rate of clonorchiasis in males was 12.6%, while that in females was 3.6% (p less than 0.05). The prevalence of metagonimiasis in males was 12.0% and was also significantly higher than 6.1% in females (p less than 0.05). The intensity of both infections was significantly heavier in males than in females. The prevalence of both flukes was higher in residents of over 30 years of age than in those below that age. No correlation was found between the intensity and the age group. Among eight species of the fresh-water fish collected at the Panbyon River, Zacco temmincki was found to be the most frequently and heavily infested with metacercariae of Metagonimus sp., and Gnathopogon atromaculatus alone was found to be infested with metacercariae of Clonorchis sinensis. The present results suggest that Yongyang area still remains endemic with Clonorchis sinensis and Metagonimus sp. infection.

Changing patterns of infection with digenetic larval trematodes from fresh-water fish in river Taewha, Kyongnam province

Recent patterns of infection with digenetic larval trematodes in fresh-water and brackish-water fish were studied in three locations of the river Taewha during the period from April to October 1988, and compared with the data reported previously in the same river. Of 16 species of fish examined, the encysted larvae of Cyathocotyle orientalis were found most frequently from 9 species of fresh-water fish. The metacercariae of Echinochasmus sp., Metacercaria hasegawai and Metagonimus yokogawai were found 8 species, those of Clonorchis sinensis from 7 species, and Exorchis oviformis and Metorchis orientalis from 5 species of fish. The infection rates of fish with C. sinensis metacercariae were not lower than those reported in 1980, wheres their intensity of infection was found lowered in 3 species, Coreobagrus brevicorpus, Gnathopogon atromaculatus, and Puntungia herzi. The infection rates of 3 species of fish with M. yokogawai metacercariae were lower than the results in 1982, while the rate was higher in 2 species, Zacco platypus, and Z. temmincki, and rather stationary in Plecoglossus altivelis. The intensity of infection in several species of fish appeared rather higher than in 1980. the encysted larvae of C. orientalis, Echinochasmus sp., E. oviformis and Metacercaria hasegawai showed variations in infection rates of fish in 1980 and in the present study. It was found that the rate of infection with digenetic larval trematodes in fresh-water fish was still relatively high in the river Taewha, and the metacercarial burden in the fish varied greatly by different fish in 1980 and in the present study.

[Infestation of larval trematodes from fresh-water fish and brackish-water fish in River Hyungsan, Kyungpook Province, Korea]

In order to determine the infestation patterns for metacercaria of digenetic trematodes, several kinds of fresh-water and brackish-water fish were caught in River Hyungsan, Kyungpook Province, Korea and examined from March to October in 1983. They were collected by netting, fishig with rod and line, or using the bait of crushed oil cake in transparent plastic bowls, and were dissected into the flesh, scales and fins with tail, and then examined for the presence and infestation density of larval trematodes. Of the 11 kinds of fish examined, the encysted larvae of Exorchis oviformis were found all of them, Metagonimus yokogawai from 9 kinds, Cyathocotyle orientalis, Metacercaria hasegawai and Metorchis orientalis from 6 kinds, and Clonorchis sinensis from 4 kinds in the present study. In the intensity of infestation with Clonorchis sinensis, the Korean shiner, Gnathopogon atromaculatus Nichols et Pope, was the most heavily infected species, the average number of metacercariae per gram of fish flesh being 45.5, followed by the striped shiner, Puntungia herzi Herzenstein 11.6, and Southern top-mouthed minnow, Pseudorasbora parva (Temminck et Shlegel), 9.7. The intensity of infestation with the Metagonimus larvae in 8 species of fresh-water fish was relatively low, and the average number per gram of flesh varied from 0.23 to 9.1. In comparison, the brackish-water fish were more heavily infected and the mean number of cysts per gram of flesh in the sweetfish, Plecoglossus altivelis, was 32.9. The results of this study generally indicate that Clonorchis sinensis and Metagonimus yokogawai infection may occur by consuming raw fresh-water fish caught in the river Hyungsan, Kyungpook Province, Korea.

Infestation With Larval Trematodes From Fresh-Water Fish In Natural And Fish Breeding Ponds

The present study was undertaken to determine the pattern of infestation with the encysted larvae of digenetic trematodes in freshwater fish collected in both the natural and fish breeding ponds in Kyungpook Province, Korea. The fish collected in the two natural ponds, Nass-chil, and Sa-il, and three fish breeding ponds, Deok-dong, Mo-gok, and Seongryu-tunnel, were examined from July 1980 to August 1981. The data were compared with the results obtained from freshwater fish caught in the river Kumho by Hwang and Choi(1980). Ten species of fish caught in the ponds were examined for the presence of the larval trematodes. Seven species of the larvae, Clonorchis sinensis, Cyathocotyle orientalis, Echinochasmus species, Exorchis oviformis, Metacercaria hasegawai, Metagonimus yokogawai, and Metorchis orientails and two kinds of undetermined cyst A and B were found. The infestitation rate for Clonorchis sinensis metacercaria in the Korean shiner, Gnathopogon atromaculatus collected in the natural ponds was higher than the rate in the river Kumho by Hwang and Choi(1980), but lower in the fish collected in the breeding ponds. By contrast, the infestation rate in the flat bitterling, Paracheilognathus rhombea in ponds was higher, while there was no change of the rate in the southern top-mouthed minnow, Pseudorasbora parva. The intensity of infestation, expressed as the average number of Clonorchis sinensis metacercaria per gram of flesh, were lower than in the bullhead, Coreobagrus brevicorpus, and Gnathopogon atromaculatus in the ponds in 1981. The intensity was higher in Paracheilognathus rhombea, but no fluctuation in the intensity of infestation in Pseudorasbora parva was observed. The infestation rates and densities for the encysted larvae of digenetic trematodes except for Clonorchis sinensis varied from fish to fish. However, it was found that the rate and intensity of the larval trematodes from all the fish in the ponds was lower than from those in the rivers in Kyungpook Province, Korea.