Infection Status with Clinostomum complanatum Metacercariae in Fish from Water Systems of Nakdong-gang (River) in Korea.

The infection status of Clinostomum complanatum metacercariae (CcMc) was broadly surveyed in fishes from water systems of Nakdong-gang (River) in the Republic of Korea (Korea) for 5 years. All 4,468 fishes collected were individually examined by the artificial digestion methods. No CcMc were detected in fishes from Naeseongcheon in Yecheon-gun, Gigyecheon in Gyeongju-si, Gyeongsangbuk-do, and Hamancheon in Haman-gun, Gyeongsangnam-do. In fishes from Wicheon in Gunwi-gun, Gyeongsangbuk-do, CcMc were detected in 180 (15.4%) out of 1,168 fishes and their intensity was 5.8 per fish infected. The prevalences with CcMc in positive fish species from 3 other surveyed areas, i.e., Banbyeoncheon in Yeongyang-gun, Nakdong-gang in Sangju-si and Hoecheon in Goryeong-gun, in Gyeongsangbuk-do were 19.3%, 33.3%, and 19.0% and their intensities were 1.5, 17.6, and 2.6 per fish infected respectively. In fishes from Yangcheon in Sancheong-gun, Gyeongsangnam-do, CcMc were detected in 811 (45.6%) out of 1,779 fishes examined and their intensity was 9.8 per fish infected. The prevalence was most high in Squalidus spp. (97.7%) and followed by acheilognathinid fish (66.8%), Pungtungia herzi (52.0%), rasborinid fish (39.6%) and Hemibarbus spp. (25.9%) from Yangcheon. The intensity was also most high in Squalidus spp. (27.0). The endemicity with CcMc was very low in fishes from water systems of Nakdong-gang in the east coast of Korea. Conclusively, it is confirmed that various fish species act as the second intermediate hosts of C. complanatum, and the endemicities of CcMc are more or less different by the fish species and group from Yangcheon, in Sancheong-gun, Gyeongsangnam-do, Korea.

INTRODUCTION

Clinostomum complanatum (Digenea: Clinostomidae) is a laryngial fluke in avian and mammarian hosts. This species of trematode is sometimes to be infected in humans through the consumption of raw fish meat. More than 25 human cases were mainly reported in Japan and the Republic of Korea (Korea) [1-8]. In Korea, total 6 cases were reported since the first occurrence in 1994 [3-8].

Clinostomum spp. laryngial flukes commonly have 2 intermediate hosts in the life cycle. The freshwater snails in the family Lymnaeidae, i.e., Lymnaea spp. and Radix spp., serve as the first intermediate hosts, and freshwater and brackish water fish act as the second intermediate hosts. In Korea, Chung et al. [9] recorded 12 species of freshwater fishes as the second intermediate hosts of C. complanatum, and Chung et al. [10] also described the cercariae and redia of C. complanatum, which were detected in Radix auricularia coreana from a pond, an enzootic focus of this trematode, in Uiseong-gun (gun=county), Gyeongsangbuk-do (do=Province). Rim et al. [11] detected C. complanatum metacercariae (CcMc) in 2 fish species, i.e., Pseudorasbora parva and Squalidus chankaensis tsuchigae, from the streams of Taewha-gang (gang means river) and Hyeongsan-gang located in the southeast regions of Korea. Recently, Sohn et al. [12] found CcMc in 2 fish species, Punctungia herzi and Hemibarbus longirostris, from Chatancheon (a stream of Hantan-gang) in Yeoncheon-gun, Gyeonggi-do.

On the other hand, the infection status of zoonotic trematode metacercariae (ZTM) in the second intermediate hosts are used as the important epidemiological indices. Thus, many Korean workers have investigated the infection status with ZTM in fishes from various endemic areas to estimate the endemicities of specific zoonotic trematode, i.e., Clonorchis sinensis, Metagonimus spp. including M. yokogawai, Centrocestus armatus and Isthmiophora hortensis, infections [13-19]. However, the infection status of CcMc was rarely surveyed and has not been broadly investigated in freshwater fish from the water systems of Korea. Therefore, in the present study, we are going to investigate the infection status of CcMc in freshwater fish from the water systems of Nakdong-gang in Korea.

MATERIALS AND METHODS

Fish collection (Fig. 1)

Water systems of Nakdong-gang in Gyeongsangbuk-do: Fishes from Naeseongcheon (① in Fig. 1) in Yecheon-gun (2014: 103 fishes in 15 spp.), Banbyeoncheon (②) in Yeongyang-gun (2015: 161 fishes in 12 spp.), Wicheon (③) in Gunwi-gun (2013: 107 fishes in 12 spp.; 2014: 338 fishes in 24 spp.; 2015: 245 fishes in 19 spp.; 2016: 279 fishes in 24 spp.; 2017: 199 fishes in 17 spp.) Nakdong-gang (④) in Sangju-si (si=city), (2017: 143 fishes in 14 spp.) and Hoecheon (⑤) in Goryeong-gun (2013: 165 fishes in 13 spp.) were examined.

Fig. 1

Surveyed areas in water syatems of Nakdong-gang (River) in Korea: ① Naeseongcheon in Yecheon-gun, ② Banbyeoncheon in Yeongyang-gun, ③ Wicheon in Gunwi-gun, ④ Nakdong-gang in Sangju-si, ⑤ Hoecheon in Goryeong-gun, Gyeongsangbuk-do, ⑥ Yangcheon in Sancheong-gun, ⑦ Yeongdeok in Jinju-si, ⑧ Hamancheon in Haman-gun, Gyeongsangnam-do, ⑨ Wangpicheon in Uljin-gun, ⑩ Osipcheon in Yeongdeok-gun, ⑪ Gigyecheon in Gyeongju-si, Gyeongsangbuk-do, ⑫ Cheokgwacheon, and ⑬ Taehwa-gang in Ulsan-si (Metropolitan City).

Water systems of Nakdong-gang in Gyeongsangnam-do: Fishes from Yangcheon (⑥) in Sancheong-gun (2013: 644 fishes in 20 spp.; 2014: 291 fishes in 14 spp.; 2015: 183 fishes in 15 spp.; 2016: 253 fishes in 16 spp.; 2017: 408 fishes in 22 spp.), Jisucheon (⑦) in Jinju-si (2014: 94 fishes in 11 spp.) and Hamancheon (⑧) in Haman-gun (2014: 73 fishes in 9 spp.) were examined.

Water systems of Nakdong-gang in the east coast of Korea: Fishes from Wangpicheon (⑨) in Uljin-gun (2015: 239 fishes in 13 spp.), Osipcheon (⑩) in Yeongdeok-gun (2015: 122 fishes in 11 spp.) and Gigyecheon (⑪) in Gyeongju-si (2015: 111 fishes in 11 spp.), Gyeongsangbuk-do, Cheokgwacheon (⑫) (2015: 221 fishes in 10 spp.) and Taehwagang (⑬) (2015: 89 fishes in 14 spp.) in Ulsan Metropolitan City were examined.

Examination methods

All collected fishes with ice were transferred to the laboratory of the Department of Parasitology and Tropical Medicine, Gyeongsang National University College of Medicine, Jinju, Korea. After the identification of fish species, they were individually ground with a mortar. Each ground fish meat was mixed with artificial gastric juice and the mixture was incubated at 36°C for 2 hr. The digested material of each fish was washed with 0.85% saline until the supernatant is cleared and examined with naked eyes and under a stereomicroscope. The excysted CcMc (Fig. 2) were separately collected [20] and were counted to get hold of infection rates (%) and densities (No. of CcMc per fish infected) by fish species.

Fig. 2

Metacercariae of Clinostomum complanatum collected in Squalidus japonicus coreanus from Yangcheon in Sancheong-gun, Gyeongsangnam-do. (A) A fresh unstained metacercaria shows the intestinal ceca with characteristic contents. (B) A metacercaria stained with Semichon’s acetocarmin, which has the larger ventral sucker (VS) closely located to the oral sucker (OS) and the primordia of testes (arrows). The scale bar is 1 mm.

RESULTS

Infection status with CcMc in fish from Nakdong-gang in Gyeongsangbuk-do

No CcMc were detected in fishes from Naeseongcheon in Yecheon-gun. In fishes from Wicheon in Gunwi-gun, CcMc were detected in 180 (15.4%) out of 1,168 fishes and their intensity was 5.8 per fish infected. The prevalences by the year examined were 15.0–27.0% (20.5% in average) in the positive fish species from Wicheon. The infection status with CcMc by the fish species and examined year in Wicheon was detailedly shown in Table 1. In positive fish species from another surveyed areas, i.e., Banbyeoncheon in Yeongyang-gun, Nakdong-gang in Sangju-si and Hoecheon in Goryeong-gun, the prevalence with CcMc was 23.8% in average and their intensity was 9.2 per fish infected. The infection status with CcMc by the fish species and survey areas was detailedly revealed in Table 2.

Table 1

Infection status of Clinostomum complanatum metacercariae by the fish species and examined year in Wicheon in Gunwi-gun, Gyeongsangbuk-do, Korea

Year and fish examined	No. of fish examined	No. of fish infected (%)	No. of CcMc detected
			Range	Average
2013
Pungtungia herzi	20	1 (5.0)	-	2.0
Acheilognathus koreensis	10	4 (40.0)	1–7	3.5
Squalidus gracilis majimae	9	3 (33.3)	-	1.0
Subtotal	39	8 (20.5)	1–7	2.4
2014
Zacco platypus	60	4 (6.7)	1–7	3.0
Zacco temminckii	50	2 (4.0)	1–3	2.0
Pungtungia herzi	47	16 (34.0)	1–34	7.1
Acheilognathus koreensis	29	13 (44.8)	1–32	6.1
Acheilognathus yamatsutae	25	1 (4.0)	-	1.0
Squalidus japonicus coreanus	14	11 (78.6)	1–48	7.9
Carassius auratus	13	1 (7.7)	-	2.0
Squalidus gracilis majimae	13	2 (15.4)	-	1.0
Subtotal	251	50 (19.9)	1–48	6.0
2015
Zacco platypus	50	2 (4.0)	2–7	4.5
Zacoo temminckii	49	1 (2.0)	-	1.0
Pungtungia herzi	41	8 (19.5)	1–11	3.6
Acheilognathus koreensis	29	20 (69.0)	1–20	4.3
Acheilognathus yamatsutae	16	1 (6.3)	-	1.0
Squalidus japonicus coreanus	15	4 (26.7)	8–47	21.0
Squalidus gracilis majimae	11	4 (36.4)	1–16	5.8
Abbottina springeri	4	1 (25.0)	-	3.0
Subtotal	215	41 (19.1)	1–47	5.7
2016
Zacco platypus	63	1 (1.6)	-	5.0
Acheilognathus koreensis	55	31 (56.4)	1–27	5.1
Pungtungia herzi	38	11 (28.9)	1–8	2.8
Acheilognathus yamatsutae	29	2 (6.9)	2–4	3.0
Acheilognathus rhombeus	15	7 (46.7)	1–3	2.1
Squalidus japonicus coreanus	9	3 (33.3)	8–34	19.0
Squalidus gracilis majimae	2	2 (100)	5–59	32.0
Subtotal	211	57 (27.0)	1–59	5.9
2017
Zacco platypus	50	3 (6.0)	2–3	2.3
Squalidus japonicus coreanus	31	7 (22.6)	1–33	8.6
Acheilognathus yamatsutae	27	2 (7.4)	-	1.0
Pungtungia herzi	26	5 (19.2)	1–13	4.6
Acheilognathus koreensis	26	7 (26.9)	1–42	8.6
Subtotal	160	24 (15.0)	1–42	6.3
Total	876	180 (20.5)	1–59	5.8

Table 2

Infection status of Clinostomum complanatum metacercariae by the species of fish from water systems of Nakdong-gang in Gyeongsangbuk-do

Locality and fish sp.	No. of fish examined	No. of fish infected (%)	No. of CcMc detected
			Range	Average
Banbyeoncheon in Yeongyang-gun
Pungtungia herzi	26	2 (7.7)	1–2	1.5
Acheilognathus koreensis	18	7 (38.9)	1–5	1.6
Squalidus gracilis majimae	8	1 (12.5)	-	1.0
Abbottina springeri	5	1 (20.0)	-	1.0
Subtotal	57	11 (19.3)	1–5	1.5
Nakdonggang in Sangju-si
Acheilognathus lanceolatus	31	9 (29.0)	1–11	4.0
Carassius auratus	9	2 (22.2)	86–147	116.5
Acheilognathus rhombeus	9	5 (55.6)	3–26	11.6
Hemibarbus labeo	6	1 (16.7)	-	4.0
Hemibarbus longirostris	2	2 (100)	1–2	1.5
Subtotal	57	19 (33.3)	1–147	17.6
Hoecheon in Goryeong-gun
Hemiculter eigenmanni	34	3 (8.8)	1–3	1.7
Pseudorasbora parva	16	5 (31.3)	1–10	3.2
Hemibarbus labeo	7	2 (28.6)	-	3.0
Squalidus japonicus coreanus	1	1 (100)	-	1.0
Subtotal	58	11 (19.0)	1–10	2.6
Total	172	41 (23.8)	1–147	9.2

Infection status with CcMc in fish from Nakdong-gang in Gyeongsangnam-do

No CcMc were detected in fishes from Hamancheon in Haman-gun. In fishes from Yangcheon in Sancheong-gun, CcMc were detected in 811 (45.6%) out of 1,779 fishes examined and their intensity was 9.8 per fish infected. The prevalences by the year examined were 38.6–62.8% (51.4% in average) in the positive fish species from Yangcheon. The infection status with CcMc by the fish species and examined year in Yangcheon was detailedly designated in Table 3. Only one CcMc was found in a S. gracilis majimae from Jisucheon in Jinju-si. The infection tendency with CcMc by the fish groups was somewhat revealed in fishes from Yangcheon. The prevalence was most high in Squalidus spp. (97.7%) and followed by acheilognathinid fish (66.8%), P. herzi (52.0%), rasborinid fish (39.6%) and Hemibarbus spp. (25.9%). The intensity was also most high in Squalidus spp. (27.0) and followed by acheilognathinid fish (8.9), P. herzi (6.6), rasborinid fish (3.7) and Hemibarbus spp. (2.5) (Table 4).

Table 3

Infection status of Clinostomum complanatum metacercariae by the fish species and examined year in Yangcheon in Sancheong-gun, Gyeongsangnam-do, Korea

Year and fish examined	No. of fish examined	No. of fish infected (%)	No. of CcMc detected
			Range	Average
2013
Pungtungia herzi	142	86 (60.6)	1–34	5.9
Zacco koreanus	94	49 (52.1)	1–22	3.3
Acheilognathus majusculus	85	66 (77.6)	1–22	6.8
Zacco platypus	55	35 (63.6)	1–8	2.9
Squalidus chankaensis	51	51 (100)	1–143	32.6
Hemibarbus longirostris	34	7 (20.6)	1–5	1.9
Acheilognathus koreensis	31	28 (90.3)	1–14	6.5
Squalidus japonicus coreanus	30	30 (100)	2–46	19.5
Carassius auratus	20	5 (25.0)	1–5	2.4
Acanthorhodeus gracilis	20	5 (25.0)	1–6	3.2
Acanthorhodeus macropterus	17	1 (5.9)	-	1.0
Acheilognathus rhombeus	8	5 (62.5)	1–7	2.2
Acheilognathus yamatsutae	1	1 (100)	-	5.0
Subtotal	588	369 (62.8)	1–143	10.0
2014
Pungtungia herzi	65	31 (47.7)	1–14	63.1
Acheilognathus majusculus	40	22 (55.0)	1–18	3.5
Zacco koreanus	40	15 (37.5)	1–6	1.9
Coreoperca herzi	30	2 (6.7)	-	1.0
Zacco platypus	25	10 (40.0)	1–6	2.8
Squalidus japonicus coreanus	25	25 (100)	7–67	23.2
Acheilognathus koreensis	13	10 (76.9)	1–17	7.4
Odontobutis platycephala	13	2 (15.4)	5–10	7.5
Acheilognathus yamatsutae	10	7 (70.0)	1–6	2.9
Hemibarbus longirostris	7	2 (28.6)	-	1.0
Subtotal	268	126 (47.0)	1–67	7.3
2015
Pungtungia herzi	40	19 (47.5)	1–45	9.9
Zacoo temminckii	40	8 (20.0)	1–12	4.1
Zacco platypus	40	20 (50.0)	1–22	5.2
Hemibarbus longirostris	25	6 (24.0)	1–4	3.3
Acheilognathus majusculus	9	2 (22.2)	14–40	27.0
Acheilognathus koreensis	7	7 (100)	3–74	16.1
Acheilognathus yamatsutae	3	1 (33.3)	-	1.0
Squalidus japonicus coreanus	2	1 (50.0)	-	12.0
Subtotal	166	64 (38.6)	1–74	8.2
2016
Acheilognathus koreensis	40	34 (85.0)	1–25	6.2
Pungtungia herzi	33	13 (39.4)	1–8	2.8
Zacco platypus	32	9 (28.1)	1–10	3.2
Acheilognathus majusculus	23	15 (65.2)	1–16	4.1
Carassius auratus	22	1 (4.5)	-	1.0
Zacco temminckii	21	2 (9.5)	1–3	2.0
Zacco koreanus	20	11 (55.0)	1–8	3.7
Acheilognathus rhombeus	20	15 (75.0)	1–13	5.1
Acheilognathus yamatsutae	13	9 (69.2)	1–14	4.6
Squalidus japonicus coreanus	5	5 (100)	6–64	28.8
Hemibarbus longirostris	4	1 (25.0)	-	7.0
Subtotal	233	115 (49.4)	1–64	5.7
2017
Pungtungia herzi	53	24 (45.3)	1–73	12.9
Zacco platypus	38	6 (15.8)	1–15	7.7
Acheilognathus majusculus	37	27 (73.0)	1–52	14.9
Acheilognathus yamatsutae	31	23 (74.2)	1–28	5.6
Zacco temminckii	30	6 (20.0)	1–7	3.0
Carassius auratus	28	1 (3.6)	-	1.0
Acanthorhodeus gracilis	21	10 (47.6)	16–253	57.1
Zacco koreanus	20	9 (45.0)	1–14	7.7
Coreoperca herzi	18	1 (5.6)	-	1.0
Squalidus japonicus coreanus	15	15 (100)	1–108	31.4
Hemibarbus labeo	8	2 (25.0)	2–5	3.5
Hemibarbus longirostris	7	4 (57.1)	1–2	1.3
Acheilognathus rhombeus	7	3 (42.9)	4–50	24.0
Acheilognathus koreensis	5	3 (60.0)	2–33	14.0
Squalidus gracilis majimae	3	1 (33.3)	-	1.0
Acanthorhodeus macropterus	2	2 (100)	8–9	8.5
Subtotal	323	137 (42.4)	1–253	15.8
Total	1,578	811 (51.4)	1–253	9.8

Table 4

Infection status of Clinostomum complanatum metacercariae by the fish groups from Yangcheon in Sancheong-gun, Gyeongsangnam-do, Korea

Fish examined	No. of fish examined	No. of fish infected (%)	No. of CcMc detected
			Range	Average
Squalidus japonicus coreanus	77	76 (98.7)	1–108	23.6
Squalidus chankaensis	51	51 (100)	1–143	32.6
Squalidus gracilis majimae	3	1 (33.3)	-	1.0
Subtotal	131	128 (97.7)	1–143	27.0
Hemibarbus longirostris	77	20 (26.0)	1–7	2.4
Hemibarbus labeo	8	2 (25.0)	2–5	3.5
Subtotal	85	22 (25.9)	1–7	2.5
Acheilognathus majusculus	194	132 (68.0)	1–52	7.9
Acheilognathus koreensis	96	82 (85.4)	1–74	7.6
Acheilognathus rhombeus	35	23 (65.7)	1–50	7.0
Acheilognathus yamatsutae	58	41 (70.7)	1–28	4.8
Acanthorhodeus gracilis	41	15 (36.6)	1–253	39.1
Acanthorhodeus macropterus	19	3 (15.8)	1–9	6.0
Subtotal	443	296 (66.8)	1–253	8.9
Zacco platypus	190	80 (42.1)	1–22	3.9
Zacco koreanus	174	84 (48.3)	1–22	3.6
Zacco temminckii	91	16 (17.6)	1–12	3.4
Subtotal	455	180 (39.6)	1–22	3.7
Pungtungia herzi	333	173 (52.0)	1–73	6.6
Carassius auratus	70	7 (10.0)	1–5	2.0
Coreoperca herzi	48	3 (6.3)	-	1.0
Odontobutis platycephala	13	2 (15.4)	5–10	7.5
Total	1,578	811 (51.4)	1–253	9.8

Infection status with CcMc in fish from the streams in the east coast

No CcMc were detected in fishes from Gigyecheon (a branch of Hyeongsangang) in Gyeongju-si, Gyeongsangbuk-do. In fishes from water systems of Nakdong-gang in the east coast of Korea, the endemicity with CcMc was very low. The infection status with CcMc by the survey areas and fish species examined was detailedly revealed in Table 5.

Table 5

Infection status of Clinostomum complanatum metacercariae by the fish species and survey area in the water systems of Nakdong-gang in the east coast of Korea

Locality and fish sp.	No. of fish examined	No. of fish infected (%)	No. of CcMc detected
			Range	Average
Whangpicheon in Uljin-gun
Pungtungia herzi	47	3 (6.4)	2–6	3.7
Squalidus gracilis majimae	6	1 (16.7)	-	6.0
Subtotal	53	4 (7.6)	2–6	4.3
Osipcheon in Yeongdeok-gun
Squalidus gracilis majimae	14	5 (35.7)	1–5	2.2
Cheokgwacheon in Ulju-gun
Zacco temminckii	50	1 (2.0)	-	1.0
Pungtungia herzi	47	13 (27.7)	1–13	3.8
Rhynchocypris oxycephalus	20	1 (5.0)	-	3.0
Acheilognathus rhombeus	20	5 (25.0)	1–4	1.8
Acanthorhodeus gracilis	17	7 (41.2)	3–9	4.9
Zacco platypus	15	2 (13.3)	-	1.0
Subtotal	169	29 (17.2)	1–13	3.4
Taehwa-gang in Ulju-gun
Carassius auratus	14	1 (7.1)	-	1.0
Total	250	39 (15.6)	1–13	3.3

DISCUSSION

In the present study, CcMc were found in total 23 species of freshwater fishes from the water systems of Nakdong-gang in Korea. Among these fish hosts, 10 ones, A. koreensis, A. rhombeus, A. yamatsutae, C. auratus, H. longirostris, P. parva, P. herzi, S. chankaesis tsuchigae, S. gracilis maejimae, and Z. temminkii, have been previously reported in Korea [9,11,12]. Therefore, a total of 26 fish species including 3 ones, Cobitis sinensis, Microphysogobio yaluensis and Rhodeus uyekii, formerly reported in Chung et al. [9] are to be listed as the second intermediate hosts of C. complanatum in Korea. In Japan, less than 8 fish species, i.e., Carassius spp. including C. auratus, Cyprinus carpio, P. parva, Pseudogobio esocinus, Rhodeus ocellatus, and R. lanceolatus (syn. Acheilognathus lanceolatus), were reported as the second intermediate hosts of C. complanatum until 1992 [21].

The prevalence of CcMc was most high in the positive fish group from Yangcheon (51.4%) in Sancheong-gun, Gyeongsangnam-do, followed by that from Nakdong-gang (33.3%) in Sangju-si, Gyeongsangbuk-do, and those from remain other areas were relatively low (7.1–20.5%). Even no CcMc were detected in fishes from 3 survey areas, Naeseongcheon in Yecheon-gun, Gigyecheon in Gyeongju-si, Gyeongsangbuk-do and Hamancheon in Haman-gun, Gyeongsangnam-do. The intensity of infection with CcMc was most high in the positive fish group from Nakdong-gang (17.6 per fish infected) in Sangju-si, followed by that from Yangcheon (9.8) in Sancheong-gun, and those from remain other areas were relatively low (1.0–5.8). The reason why the more higher intensity in fish from Nakdong-gang in Sangju-si is 2 crucian carps, C. auratus, heavily infected (86 and 147 CcMc). Above findings suggested that the endemicities of CcMc are relatively high in fish from Yangcheon in Sancheong-gun and Nakdong-gang in Sangju-si, and relatively low in fish from other areas like in previous studies in Korea [9,11,12].

In the highly endemic area of CcMc, Yangcheon in Sancheong-gun, Gyeongsangnam-do, the endemicity was revealed a certain tendency by the subfamily groups in cyprinid fish. The endemicity (average CcMc intensity×prevalence/100) was 8.47 in gobioninid (Gobioninae) fish group, i.e., Squalidus spp., P. herzi and Hemibarbus spp., 5.95 in acheilognathinid (Acheilognathininae) fish, Acanthorhodeus spp. and Acheilognathus spp., and 1.47 in rasborinid (Rasborininae) fish group, Zacco spp. The endemicity in the gobioninid fish was most high in Squalidus spp., 26.38, followed by in P. herzi, 3.43, and in Hemibarbus spp., 0.65. From the above findings, we could know that the susceptibility of C. complanatum cercariae is relatively high in Squalidus spp. and the endemicities of CcMc are more or less different by the fish group and species from Yangcheon in Sancheong-gun, Gyeongsangnam-do, Korea.

Two types of CcMc were reported in India. Non-encysted ones are usually found in the body cavity of Trichogaster fasciatus with high infection rates [22]. They are known to be the pathogenic agents in the viscera and musculature of many fish species [23]. Meanwhile, encysted ones are detected in the body cavitiy of Channa punctatus and they also act as the etiologic agents in their habitat organs of fish [24]. However, we only found non-cysted CcMc in this study. Generally, we have been thought that the cyst wall of CcMc is so feeble and then it is easily bursted by the process of artificial digestion method. The further studies are needed whether CcMc is to be non-encysted or excysted in the digestion process of fish meat in Korea.

Several species of ardeiid birds have been reported as the natural definitive hosts of C. complanatum in the world. Even in Japan, C. complanatum adults were recovered from 4 species of wild birds, i.e., Nycticorax nycticorax, Ardea cinerea, Egretta garzetta, and E. intermedia, in Tottori prefecture [25]. However, only chick was proved as the experimental definitive host in Korea [9]. Accordingly, we should pay attention to the recovery of C. complanatum adults in the survey of wild birds.

More than 50 species have been reported as the members of genus Clinostomum by a variety of authors, even though their taxonomic validity is quite controversial. Among them, C. complanatum is the type-species and the most widely distributed in the world including Korea [3,9,20-24]. It is questionable that every Clinostomum specimens collected in various species of fish from various geographical areas of Korean peninsula are CcMc, although specimens from human cases and experimental chick were already named as C. complanatum. The molecular study on the Clinostomum specimens from various origins should be carried out in the near future in Korea.

Conclusively, it is confirmed that the variety of fish species act as the second intermediate hosts of C. complanatum in the water systems of Nakdong-gang, and the endemicities of CcMc are more or less different by the fish group and species from Yangcheon, most endemic area, in Sancheong-gun, Gyeongsangnam-do, Korea.