Centrocestus armatus (Digenea: Heterophyidae) is a small intestinal fluke bearing 42–48 circumoral spines, which mainly inhabits in the small intestines of birds and mammals. This fluke was first recovered from dogs, cats, rabbits, rats and mice experimentally fed with cyprinoid fish with C. armatus metacercariae (CaMc) in Japan [1]. Human infections were experimentally proven in Japan, and a natural human case was reported in the Republic of Korea (Korea) [1,2]. The freshwater snails, Semisulcospira spp., are known to be the first intermediate hosts in Japan and Korea [3,4]. Various species of freshwater fish, including Zacco platypus and Z. temminckii, were reported as the second intermediate hosts in Japan and Korea [5–13]. As the natural definitive hosts of this fluke, the large egret, Egretta alba modesta, and stray cat, Felis catus, were recorded in Korea [14–17].

This species of heterophyid fluke is not clinically important unlike Clonorchis sinensis and Metagonimus spp. as fishborne zoonotic trematodes. And so the epidemiological study on the metacercarial infections in fish hosts has been rarely done in Korea [6–8,10,11]. In Korea, Chun [6] described more than 11 species of digenetic trematode metacercariae (DTM) together with CaMc detected in 16 fish species from streams and ponds in adjacent areas of Nakdong-gang. Rhee et al. [8] detected CaMc in 7 out of 32 fish species from Mangyeong-gang and they also found them in 5 out of 33 fish species from Dongjin-gang [9]. In 1988, Hong et al. [10] investigated the infection status of CaMc in 2 species of chub, Z. platypus and Z. temminckii, collected from 19 sites in 6 major rivers of Korea. Recently, Cho et al. [12] reported the infection status of zoonotic trematode metacercariae including CaMc in fishes from the water systems in Gangwon-do. Sohn et al. [13] surveyed the infection status of DTM in fishes from the water systems of Hantan-gang and Imjin-gang. However, most of previous studies were performed in limited areas except for Hong et al. [10]. Even in Hong et al. [10], only 2 species of chub, Z. platypus and Z. temminckii, collected from 19 sites were examined. Therefore, the present study was performed to estimate the endemisities of CaMc in the freshwater fishes nationwidely and moreover, to expand the fauna on the fish hosts of C. armatus in Korea.

In the present study, we investigated the infection status with CaMc nationwidely in the freshwater fishes from the water systems of major rivers in Korea. Total 89 times in 50 sites (Han-gang: 11 times in 8 sites; Geum-gang: 10 times in 6 sites; Mangyeong-gang and Dongjin-gang: 5 times in 2 sites; Yeongsan-gang and Tamjin-gang: 10 times in 5 sites; Seomjin-gang: 13 times in 8 sites; Nakdong-gang: 29 times in 13 sites; streams in the east coast: 11 times in 8 sites) were surveyed. However, the data from the 12 times surveys performed in 10 sites (Hangang: 9 times in 7 sites; streams in the east coast: 3 times in 3 sites) were already reported in Cho et al. [12]. At any rate, total 14,977 fish including 2,200 Z. platypus, 1,732 Z. temminckii and 297 O. uncirostris amurensis were examined in this study. The proportion of Zacco spp. is more or less different by the surveyed sites (19.8–33.0%: 26.3% in average) and fish species (Z. platypus: 9.2–22.1%; Z. temminckii: 9.1–14.4%). These differences are due to the fish ecology in surveyed rivers.

The prevalences with CaMc were very high, 93.7–100% (97.1% in average), in Zacco spp. from all surveyed sites. However, their densities were more or less different by the surveyed areas, and slightly higher in Z. platypus (2,109 in average) than in Z. temminckii (1,567). They were most high in Nakdong-gang in Gyeongsangnam-do, (4,201 in average), and followed by Geum-gang (2,343), Nakdong-gang in Gyeongsangbuk-do (1,623), Han-gang (1,564), Tamjin-gang (1,550), Yeongsan-gang (1,493), streams in the east coast (1,028), Seomjin-gang (488) and Mangyeong-gang and Dongjin-gang (170) (Table 5). The prevalence in another species of rasborinid fish, O. uncirostris amurensis, was 74.8% (222 out of 297 fish examined) and their density was 278 per fish infected. On the other hand, Hong et al. [10] reported 86.8% and 78.5% prevalences in Z. platypus (164 out of 189 fish examined) and Z. temminckii (142 out of 181 fish examined), from 19 sites in the 6 major rivers of Korea. They also reported that densities with CaMc were more higher in Z. platypus (224 in average) than in Z. temminckii (131) [10]. In Hong et al. [10], the prevalences with CaMc in Zacco spp. were more or less different by the surveyed rivers, 60.6–100% (82.7% in average), and their densities were most high in Nakdong-gang (347 in average) and followed by Seomjin-gang (120), Tamjin-gang (59), Yeongsan-gang (44), Han-gang (29) and Geum-gang (27). The endemicities of CaMc in Zacco spp. in our study is much higher when we compared with those of Hong et al. [10]. However, the infection trend by fish species, more higher endemicity in Z. platypus than in Z. temminckii, is the same in Hong et al. [10]. Therefore, it appears that the endemicity of CaMc is currently much higher than in the past. It’s reason why is obscure, but we can only suppose with general knowledges in the Parasitology textbook. Recently, the ecological conditions for this heterophyid fluke will be much better than in the past in Korea.

Chun [6] detected CaMc in 9 fish species, i.e., Z. platypus, Z. temminckii, P. altivelis, P. esocinus, P. parva, P. herzi, T. hakonensis, S. gracilis maejimae, S. japonicus coreanus, from the water systems of Nakdong-gang in Gyeongsangnam-do. Rhee et al. [8] found CaMc in 7 fish species, i.e., Aphyocypris chinensis, C. auratus, C. splendidus, Microphysogobio yaluensis, Rhodeus uyekii, P. parva and Z. platypus, from Mangyeong-gang and they also detected them in 5 fish species, i.e., Gnathopogon similis (=Squalidus intermedius), Pseudobagrus fulvidraco, P. parva, Rhodeus ocellatus ocellatus and Z. platypus, from Dongjin-gang [9]. Cho et al. [12] reported 15 fish species, i.e., A. rhombeus, A. signifier, C. herzi, G. brevibarba, H. longirostris, H. mylodon, K. rotundicaudata, L. andersoni, M. longidorsalis, O. uncirostris amurensis, Orthrias toni, P. esocinus, P. herzi, R. oxycephalus, and T. hakonensis, from the water systems of Gangwon-do as the new second intermediate hosts of C. armatus. Recently, Sohn et al. [13] detected CaMc in 5 fish species, Z. platypus, Z. temminckii, A. rhombeus, A. yamatsutae, and A. macropterus from the water systems of Hantan-gang and Imjin-gang. In this study, we found CaMc in total 44 fish species including 3 rasborinid fish, Z. platypus, Z. temminckii and O. uncirostris amurensis. Among them, 25 species were already reported in the previous studies [6–13]. Remain 19 species, i.e., A. springeri, A. gracilis, A. koreensis, A. majusculus, C. urotaenia, C. lutheri, C. kawamebari, G. strigatus, H. labeo, H. eigenmanni, M. salmoides, O. platycephala, O. masou masou, P. phoxinus, S. nigripinis morii, S. variegatus wakiyae and S. asotus, are to be the new second intermediate hosts of C. armatus in Korea. Accordingly, total 50 fish species (33 genera) are listed as the second intermediate hosts of C. armatus in Korea.

In the genus Centrocestus, the number of circumoral spines is considered to be the most reliable character in the species identification. C. armatus, having 42–48 circumoral spines, was first described by Tanabe [1] in Japan. Kobayashi [18] classified members in the genus Centrocestus into 4 groups on the basis of the number of circumoral spines. The group of C. yokogawai has less than 30 circumoral spines (26–30), C. formosanus has 32–40 ones, C. armatus has 42–48 ones, and C. polyspinosus has more than 50 ones (50–60) respectively. Among Centrocestus species with 42–48 circumoral spines, C. armatus is differentiated by characteristic morphological features include a small number of intrauterine eggs, the median location of the ovary, and the side by side location of the 2 testes [2,20]. On the other hand, Chai et al. [21] proposed to consider 6 species, i.e., C. cuspidatus, C. armatus, C. formosanus, C. kurokawai, C. polyspinosus, and C. asadai, in the genus Centrocestus as the valid ones.

In Korea, 3 species of Centrocestus fluke, i.e., C. armatus, C. asadai and C. formosanus, were reported in the literatures [2,4, 6,22,23]. Choi et al. [22] described C. asadai adults, which were recovered in the small intestines of mouse experimentally infected with the metacercariae from Tribolodon taczanowskii. Cho et al. [23] reported only the presence of C. formosanus cercariae in Korea. They found these cercariae from Semisulcospira libertina snails collected around the Jinyang-ho (ho means lake) in Gyeongsangnam-do, Korea. In regard to C. armatus, a lot of findings on the intermediate and definitive hosts were revealed by many Korean workers [2,4,6–17]. Adult worms were recovered from a human, the large egret (Egretta alba modesta) and stray cats (Felis catus) naturally infected [2,14–17]. Cercariae of this fluke were detected from Semisulcospira snails collected in Judongcheon in Dalseong-gun, Gyeongsangbuk-do [4]. Metacercarial infections were verified in the surveys of freshwater fish including Z. platypus and Z. temminckii [5–13]. Accordingly, we can know that C. armatus is predominant species in Korea and the presence of another 2 species, C. asadai and C. formosanus, is obscure, so there are no reports about those flukes since the first records [22,23].

Conclusively, it is confirmed that rasborinid fish, Z. platypus, Z. temminckii, and O. uncirostris amurensis, are highly and heavily infected with CaMc and the densities with CaMc are more or less different by the surveyed areas and fish species. Additionally, 19 fish species are to be the new second intermediate hosts of C. armatus in Korea.