The herring gull, Larus argentatus, is one of the commonly observed migratory birds along the seashores of Korea during winter season. They are typically found in harbors, on rocky shores, or at river mouths and known to breed in North-East Siberia spanning Chuckchi peninsula and Taymyr peninsula. During the survey of parasites in migratory birds, we discovered adult or juvenile specimens of intestinal flukes, including Cryptocotyle lingua (Creplin, 1825) Fischoeder, 1903, Himasthla alincia Diez, 1909, Cardiocephaloides medioconiger (Dubois et Perez-Vigueras, 1949) Baer, 1969, and Diplostomum spathaceum (Rudolphi, 1819) Olsson, 1876, from the small intestine of a herring gull. In this paper, we described the morphology of each species with discussion on their possibility for human infections.

In November 2010, a herring gull in a critical condition was found on the shore of a small island (Yubu-do, Seocheon, Chungcheongnam-do Province) near the western coast of the Korean peninsula during the routine surveillance of parasitic infections in migratory birds. This island was chosen as one of the eco-tour zones along with several nearby areas since a lot of migratory birds drop by on their way to the breeding or wintering grounds. The herring gull was immediately transferred to our laboratory, and the small intestine was separated. Then, the intestinal segment was opened longitudinally in saline, and the intestinal contents were examined for the presence of parasites. Some trematode specimens were observed, and they were isolated, fixed in 10% neutral formalin, and stained with Semichon’s acetocarmine. Then, their species was identified under light microscopy.

Total 11 adult or juvenile trematode specimens, including 3 C. lingua (Heterophyidae), 1 H. alincia (Echinostomatidae), 5 C. medioconiger (Strigeidae), and 2 D. spathaceum (Diplostomidae), were recovered. Their measurements (Tables 1–4) and morphological characteristics were as follows.

Specimens (n=3) of C. lingua (Heterophyidae) were small, leaf-like or tongue-shaped, 1.53–2.03 (av. 1.81) mm in length and 0.58–0.63 (0.59) mm in width (Table 1; Fig. 1A). The size of uterine eggs (n=12) was 40–44 (42) μm long and 16–19 (18) μm wide. They were morphologically characterized by having a small ventral sucker imbedded into the large and complex ventrogenital apparatus, 2 obliquely tandem testes, and extensive distribution of vitellaria from the posterior extremity up to far anterior level of the ventrogenital apparatus.

Only 1 adult specimen of H. alincia (Echinostomatidae) was obtained, which was slender and elongated, 8.75 mm long and 0.93 mm wide (Table 2; Fig. 1B). The size of uterine eggs (n=10) was 98–110 (104) μm long and 65–70 (68) μm wide. It characteristically had a prominent head collar equipped with a crown of 31 collar spines (including 4 end group spines on each side). Two tandem testes were located near the terminal portion of the body, and vitellaria were distributed from the posterior extremity up to the posterior level of the cirrus sac.

One adult and 4 juvenile specimens of C. medioconiger (Strigeidae) were recovered. They were bisegmented, with a pyriform or cordiform forebody (1.25 mm long and 0.65 mm wide) and a cylindrical hindbody (3.50 mm long and 1.10 mm wide), 4.75 mm in total length (Table 3; Fig. 1C). The size of uterine eggs (n=10) was 110–118 (115) μm long and 70–78 (74) μm wide. Their forebody was longer than wide, from which freely emerged the lobes of the tribocytic organ. It characteristically had a well-developed voluminous copulatory bursa containing seminal vesicle and ejaculatory duct in the posterior part of the hindbody.

Two adult specimens of D. spathaceum (Diplostomide) were obtained. They were distinctively bisegmented, with a spoon-shaped forebody (0.68–0.85 mm long and 0.65–0.70 mm wide) equipped with prominent pseudosuckers, the anterior end of which was more or less trilobate, and a long cylindrical hindbody (1.08–1.10 mm long and 0.50–0.52 mm wide), 1.75–2.00 mm in total length (Table 4; Fig. 1D). The size of uterine eggs (n=13) was 108–125 (114) μm long and 65–85 (73) μm wide. The size of the ventral sucker was almost equal to that of the oral sucker. They had a sucker-like tribocytic organ, a ventral sucker near or close to the tribocytic organ, vitellaria distribution up to the anterior border of the tribocytic organ, and an ovary far from the junction of the fore- and hindbody.

Foreshore soil and environment along the western coast of the Korean peninsula have provided adequate conditions for migratory birds to drop by, and comprehensive surveys on parasites among these birds, including intestinal flukes, are required. In addition, it should be noted that some of the trematode species infected in migratory birds, for example, C. lingua is known to have a potential to infect humans also [1].

C. lingua was originally described in Europe in 1825 from the intestine of birds by Creplin under the name Distoma lingua [2]. In 1899, Looss created Tocotrema as a new genus and designated this species as Tocotrema lingua [3]. Subsequently, in 1903, this fluke was renamed as Cryptocotyle lingua by Fischoeder [4]. In 1915, Linton [5] found this fluke in USA but described it under the name Tocotrema lingua. However, in 1920, Ransom [4] synonymized the genus Tocotrema with Cryptocotyle, and in 1930, Stunkard [2] finally accepted C. lingua. It should be noted that human infection with this fluke was reported once in Greenland [6], and it is listed among the intestinal flukes potentially infecting humans [1]. The source of infection is various species of fish, including mullets [1]. In Korea, Cryptocotyle concava and Cryptocotyle sp. were reported several times from the small intestines of ducks [7] and feral cats [8–10]. However, no reports have been made on C. lingua, and this is a new fauna record in Korea. Major morphological differences between C. lingua and C. concava include the body shape (slightly elongated vs ovoid, respectively), ventrogenital apparatus or ventrogenital sac (large vs small), and testes (obliquely tandem vs almost side by side) [9].

H. alincia was originally described from the intestine of an avian species in Brazil [11] and is now well known to infect birds like gulls, herons, and oystercatchers in North and South Americas [12,13]. In Korea, the metacercariae of H. alincia were discovered in 2003 from a brackish water bivalve species (Mactra veneriformis) along the western coast [14]. Several other molluscan species were also found to harbor the metacercariae, and adult flukes were recovered from experimental chicks [15]. However, no natural infections of birds with H. alincia had been documented. Therefore, this is the first report of a natural infection of an avian host infected with H. alincia in Korea. This echinostome has never been reported from humans worldwide; however, it has a potential to infect humans considering that a related species, Himasthla muehlensi, was reported from a human infection [1,16]. It is also of note in Korea that the metacercariae of Himasthla kusasigi, another related species, were detected in a marine bivalve species, Meretrix lusoria [17], and its adult flukes were recovered from dunlin, Calidris alpina sakhalina [18]. The major differential points between H. alincia and H. kusasigi were the distribution of vitellaria, from the posterior extremity up to the posterior level of the cirrus sac (H. alincia) vs more anterior distribution beyond the cirrus sac (H. kusasigi). H. alincia also differs from a related species Himasthla limnodromi in the presence of small spines on the cirrus and larger body and egg sizes [14].

The genus Cardiocephaloides was created by Sudarikov in 1959 using C. brandesii as the type [19,20]. Prior to this, a related genus Cardiocephalus was erected by Szidat [21] in 1928–1929 and assigned C. longicollis (type), C. musculosus, C. hillii, and C. brandesii (n. sp.) in this genus. Another species, C. medioconiger, was described from birds (L. argentatus, Larus atricilla, and Larus delawarensis) in Pennsylvania, USA under the name Strigea bursigera [22]. Later, this species was assigned to Cardiocephalus with a new name C. medioconiger by Dubois and Pérez-Vigueras in 1949 [19,23]. However, it was found that the name Cardiocephalus was preoccupied as a taxon for an amphibian species by Broilli in 1904 [23]. Thus, Baer [24], Dubois [23], and Dubois and Macko [25] were of opinion that C. medioconiger should be assigned under the genus Cardiocephaloides but not Cardiocephalus. This opinion was accepted by Niewiadomska [26] in 2002 and the genus Cardiocephalus was synonymized with Cardiocephaloides. In our study, 5 specimens of C. medioconiger were recovered. Among them, 4 were immature, and only 1 was adult fluke having fully developed sexual organs. In Korea, this is the first report on the presence of this fluke. C. medioconiger is distinguished from other species of Cardiocephaloides in that it has a shorter and smaller body but has an enormous copulatory bursa containing the seminal vesicle and ejaculatory duct [23].

D. spathaceum was originally described from birds in Europe by Rudolphi in 1819 under the name Distoma spathaceum [20]. Later, in 1850, it was renamed as Hemistomum spathaceum by Diesing [27]. However, the name was changed into Diplostomum spathaceum by Olsson in 1876 [20]. Dubois [23] divided D. spathaceum into 4 subspecies (D. spathaceum spathaceum, D. spathaceum huronense, D. spathaceum indistinctum, and D. spathaceum murrayense). However, Yamaguti [20] elevated 3 of them (D. spathaceum, D. huronense, and D. murrayense) to distinct species and retained only D. spathaceum indistinctum as a subspecies. After Olsson, more than 50 species had been described in Diplostomum [20]. However, only 25 species were considered valid in a taxonomic revision of the genus by Shigin in 1993 [28]. The most common species in Europe and Asia were 6 in number, including D. spathaceum (Rudolphi, 1819), D. pseudospathaceum Niewiadomska, 1984, D. paracaudum (Iles, 1959), D. mergi Dubois, 1932, D. parviventosum Dubois, 1932, and D. baeri Dubois, 1937 [29]. Adult flukes of these 6 species can be morphologically discriminated depending on 8 kinds of characters, including the extent of body division, forebody shape, form of holdfast organ, hindbody/forebody ratio, acetabulum/oral sucker ratio, acetabulum to the hold fast organ distance, vitellaria distribution, and ovary position [29]. Both of our 2 specimens were exactly matched with the characteristics of D. spathaceum. This is the first report on the presence of D. spathaceum in an avian species in Korea. The second intermediate host of D. spathaceum is known to be freshwater, brackish water, or marine fish, and the metacercariae parasitize particularly in their eyes [20]. The existence of the metacercariae of Diplostomum sp. were documented several times in freshwater fish in Korea [30–32]. However, the species of these Diplostomum metacercariae in fish should be determined in the near future through animal experimental infection.

In conclusion, this study reported 4 species of intestinal flukes parasitic in a herring gull found in Korea, which included C. lingua, H. alincia, C. medioconiger, and D. spathaceum. Three of them (C. lingua, C. medioconiger, and D. spathaceum) are new trematode fauna in Korea. Considering the adequate conditions of Korean peninsula for migratory birds to drop by, further extensive studies on the infection status of birds with trematode parasites should be performed.