Echinostoma hortense and Heterophyid Metacercariae Encysted in Yellowfin Goby, Acanthogobius flavimanus, from Shinan-gun and Muan-gun (Jeollanam-do), Korea

More than 50 million people are currently estimated to be infected with intestinal trematodes, including at least 18 million people infected by fishborne trematodes (FBT) around the world. Heterophyid (Heterophyidae) and echinostomatid flukes (Echinostomatidae) are regarded as the most important groups found in the intestinal tract of humans [1,2]. With regard to heterophyids, 10 species in 7 genera, i.e. Metagonimus yokogawai, Metagonimus takahashii, Metagonimus miyatai, Heterophyes nocens, Heterophyopsis continua, Stellantchasmus falcatus, Pygidiopsis summa, Centrocestus armatus, Strictodora fuscata, and Stictodora lari, have been reported as human-infecting species in the Republic of Korea [2]. Among them, 6 species, namely, H. nocens, H. continua, P. summa, S. falcatus, S. fuscata, and S. lari, are infected by consumption of raw flesh of estuarine fish [1,2]. Among these fish hosts, yellowfin goby, Acanthogobius flavimanus, are sometimes heavily infected with the metacercariae of heterophyid flukes, and are popularly eaten raw by many residents in coastal areas of Korea [3-7]. However, echinostomatid metacercariae have never been reported from yellowfin gobies in the Republic of Korea. Therefore, we performed an epidemiological survey to know the infection status of FBT metacercariae in yellowfin gobies collected in Shinan-gun and Muan-gun, Jeollanam-do (province), the Republic of Korea.

On September 2006, 14-15 gobies were collected, and examined by artificial digestion method (Table 1). Trematode metacercariae were segregated, collected under a stereomicroscope, and categorized according to measurements and morphological characteristics. Infection rates and intensities were then calculated. A total of 8 Echinostoma hortense metacercariae collected in gobies from Jido-myeon in Shinan-gun were used to experimentally infect a rat to obtain adult worms. Two weeks after the infection, 4 adult flukes were recovered from the small intestine of the rat, which were fixed in 10% neutral buffered formalin, and stained with Semichon's acetocarmine in order to observe their morphological characteristics.

Table 1.

The yellowfin goby, Acanthogobius flavimanus, collected in Shinan-gun and Muan-gun (Jeollanam-do), Korea

Metacercariae of H. nocens, H. continua, and Stictodora spp. were detected in all of 15 gobies caught from Aphae-myeon in Shinan-gun. The average numbers of metacercariae per infected fish were 153 (H. nocens), 20 (H. continua), and 334 (Stictodora spp.) respectively. A total of 11 P. summa metacercariae were collected in only 1 goby from Aphae-myeon (Table 2). In 2 of 14 gobies from Jido-myeon in Shinan-gun, 8 E. hortense metacercariae were detected; however, no other kinds of metacercariae were found in 14 gobies from Jido-myeon. In 15 gobies from Haeje-myeon in Muan-gun, the metacercariae of H. continua were found in 100%, Stictodora spp. in 86.7%, and H. nocens in 6.7% of gobies examined. The average numbers of metacercariae per infected fish were 23.3 (H. continua), 416 (Stictodora spp.), and 2.0 (H. nocens), respectively (Table 2).

Table 2.

Infection status of heterophyid metacercariae in gobies from Aphae-myeon, Shinan-gun and Haeje-myeon, Muan-gun (Jeollanam-do), Korea

The metacercariae of E. hortense found in gobies were elliptical, 140-163 × 130-143 µm (mean 150 × 138 µm) in size, and had 27 collar spines on the head crown, and excretory granules were arranged posteriorly in V-shape from the pharynx level (Fig. 1). Adults of E. hortense recovered from an experimental rat at 2 week after infection were 8.20-9.05 × 1.35-1.63 mm (mean 8.72 × 1.51 mm) in size, had 27 collar spines, and the other morphological characteristics, for example, the right-side deviated round ovary, lobular testes, diffusely distributed vitellaria, and intrauterine eggs, were compatible with previous studies. One adult specimen (Semichon's acetocarmine stained) of E. hortense is deposited in Invertebrate Research Division, National Institute of Biological Resources, Korea (NIBRIV0000128379).

Fig. 1

The metacercariae of Echinostoma hortense detected in an yellowfin goby, Acanthogobius flavimanus, from Jido-myeon, Shinan-gun (Jeollanam-do), Korea. They are elliptical with 150 × 138 µm average size, and had an oral sucker (OS) and 27 collar spines (arrow marks) in the head crown, and excretory granules (EG) located posteriorly in V-shape from the pharynx level. Scale bar = 50 µm.

It has been proved for the first time in this study that the yellowfin goby, A. flavimanus, acts as a second intermediate host for E. hortense. As the second intermediate hosts of this fluke, several species of freshwater fish, i.e. muddy loach (Misgurnus anguillicaudatus), Chinese muddy loach (Misgurnus mizolepis), dark sleeper (Odontobutis interrupta), Chinese minnow (Rhynchocypris oxycephalus), Japanese aucha perch (Coreoperca kawamebari), short barbel gudgeon (Squalidus japonicus coreanus), and some species of amphibians, frogs (Rana nigromaculata, Rana rugosa, Rana catesbiana), salamanders (Hynobius naevius, Hynobius nebulosus), and toad (Bufo vulgaris japonicus) have been reported in Japan, Korea, and China [8-15].

Human infections with E. hortense have been reported in Japan, Korea, and China [16-18]. Especially in the Republic of Korea, a 22.4% (59/263) egg-positive rate was reported in a survey of residents in Cheongsong-gun, Gyeongsangbuk-do [13], and recently, clinical cases have been diagnosed occasionally by the gastroduodenal endoscopy [19-23].

Endemic foci of heterophyid flukes are scattered around the southwestern coastal areas and islands, especially in Shinan-gun and Muan-gun (Jeollanam-do), Korea. Some species of estuarine fish, including Lateolabrax japonicus, Clupanodon punctatus, Mugil cephalus, and A. flavimanus, have been reported as sources of human infection in these endemic areas [2]. Among these fish hosts, A. flavimanus is sometimes heavily infected with heterophyid metacercariae, and are popularly eaten raw by many residents in southwestern coastal areas [3-7].

Several species of heterophyid fluke metacercariae, i.e. H. nocens, H. continua, P. summa, S. fuscata, S. lari, and Acanthotrema felis, were found in the yellowfin gobies in the Republic of Korea [3-7,24]. In the present study, metacercariae of H. nocens, H. continua, Stictodora spp., and P. summa were detected, and especially, high infection rates and intensities were revealed in gobies from Aphae-myeon in Shinan-gun. Metacercariae of H. continua, Stictodora spp., and H. nocens were detected in 15 gobies from Haeje-myeon in Muan-gun. However, except for E. hortense, no other metacercariae were found in 14 gobies from Jido-myeon in Shinan-gun. These findings together suggest that those who consume raw yellowfin gobies, A. flavimanus, from Shinan-gun and Muan-gun are at a high risk of infection with heterophyid flukes and E. hortense.