Echinostome Infections in the Striped-Field Mouse, Apodemus agrarius, and the Ussuri White-Toothed Shrew, Crocidura lasiura, Caught Near the Demilitarized Zone, Gyeonggi-do (Province), Republic of Korea

Trematodes of the family Echinostomatidae, i.e., echinostomes, are intestinal parasites of animals, which can infect humans, are an important group of food-borne zoonotic parasites around the world [1]. A total of 20 species belonging to 9 genera are known to cause human infections, and among them, 7 species of Echinostoma and 5 species of Echinochasmus are the most common and important groups affecting humans and animals [1]. Natural definitive hosts for these echinostomes are known to be, in most cases, birds and mammals including rodents [1].

In the Republic of Korea (ROK), several studies have been performed on echinostome infections in rodents [2-6], cats [7,8], and dogs [9]. Among rodent surveys, Echinostoma hortense, Echinostoma cinetorchis, and Euparyphium murinum infections were reported from Rattus norvegicus (Norway rat), Rattus rattus alexandrines (roof rat), A. agrarius, Mus musculus yamashinai (common field mouse), and Microtus fortis pellceus (reed vole), collected in Seoul and other localities in Korea [2-4]. In addition, Echinostoma revolutum and Echinoparyphium recurvatum were recovered from rats (probably R. norvegicus) captured in Yangyang-gun, Gangwon-do (province) [5], and E. japonicus from A. agrarius from Goyang-gun, Gyeonggi-do and Iksan-gun, Jeollabuk-do [6].

The demilitarized zone (DMZ) of Gyeonggi-do and Gangwon-do, ROK, is highly conserved and ecologically unique [10]. Many military field training sites are located near the DMZ, where civilian entry is prohibited. Echinostome infections had never been reported from this area. A comprehensive rodent surveillance program was conducted in 3 localities of northern Gyeonggi-do near the DMZ [11-13]. The present paper focused on the status of echinostome infections in field mice, A. agrarius and insectivores, C. lasiura, captured as a part of the rodent surveillance program.

A total of 1,498 small mammals (rodents and insectivores) were live-captured using Sherman traps (3 × 3.5 × 9 folding traps; H.B. Sherman, Tallahassee, Florida, USA) from military training sites at 3 areas in Gyeonggi-do, i.e., Paju-si (city), Pocheon-gun (township), and Yeoncheon-gun (township), located near the DMZ from December 2004 to September 2005. The captured mammals included the striped field mouse, Apodemus agrarius (n = 1,366), the Ussuri white-toothed shrew, Crocidura lasiura (54), Mus musculus (32), Micronytus fortis (28), Myodes (=Eothenomys) regulus (9), Micronys minutes (6), and Tscherckia (=Cricetulus) triton (3). Seasonal trapping was conducted in December 2004, March 2005, June 2005, and September 2005. Captured specimens were euthanized in accordance with the approved animal use protocol under biosafety level 3 (BSL-3) laboratory conditions.

The stomach, small intestine, and large intestine to the end of the rectum were removed and preserved in 70% alcohol until examination. The gastrointestinal content was examined for intestinal helminths under a stereomicroscope [11,12]. Trematodes were removed and placed on a microscope slide with a coverslip, fixed with 70% alcohol, stained with Semichon's acetocarmine, and identified using a research microscope. Data on Plagiorchis muris [11] and Neodiplostomum seoulense infections [12] were already published, and those on nematodes and cestodes will be published separately.

A total of 15/1,420 (1.1%) A. agrarius and C. lasiura were found to be infected with several species of echinostomes (Table 1; Fig. 1). The echinostome species included E. hortense (Fig. 1A), E. cinetorchis (Fig. 1B), E. murinum (Fig. 1C), and E. japonicus (Fig. 1D). The infection rate of A. agrarius for E. hortense was 0.5%, (7/1,366 mice examined), and total 16 E. hortense specimens were recovered (Table 1). The infection rate was higher in mice (1.8%; 6/325) captured in Yeoncheon-gun than the mice captured in Pocheon-gun (0.3%; 1/288) and Paju-si (0%; 0/753). A total of 5 A. agrarius were infected with E. cinetorchis, and a total of 9 specimens were recovered (Table 1). E. cinetorchis infections were found exclusively in mice captured in Paju-si (0.7%; 5/753). Three specimens of E. murinum were recovered from 2 A. agrarius captured in Paju-si (0.3%; 2/753). E. japonicus (3 specimens) were found only in 1 C. lasiura captured in Paju-si (8.3%; 1/12).

Table 1.

Infection status of echinostomes in Apodemus agrarius and Crocidura lasiura collected near the DMZ of Gyeonggi-do (province), Republic of Korea

Fig. 1

(A) Echinostoma hortense adult recovered from a striped-field mouse, Apodemus agrarius, caught at Yeoncheon-gun, Gyeonggi-do. The characteristic features of this species include the presence of 27-28 collar spines on its head collar and a submedially located ovary. Scale bar = 1 mm. (B) Echinostoma cinetorchis adult recovered from A. agrarius caught at Paju-si, Gyeonggi-do. The characteristic features of this species include the presence of 37-38 collar spines on its head collar, abnormal location or disappearance of 1 or both testes, and a medially located ovary. Scale bar = 1 mm. (C) Euparyphium murinum adult recovered from A. agrarius caught at Paju-si, Gyeonggi-do. The characteristic features of this species include the presence of 45 collar spines on its head collar and a short uterine tubule. Scale bar = 0.5 mm. (D) Echinochasmus japonicus adult recovered from an insectivore, the Ussuri white-toothed shrew, Crocidura lasiura, caught at Paju-si, Gyeonggi-do. The characteristic features of this species include a short, plump body with the presence of 24 dorsally interrupted collar spines and 2 tandem arranged testes. Scale bar = 0.025 mm.

Small mammals, in particular, A. agrarius and C. lasiura, inhabiting near the DMZ of Gyeonggi-do have been found to serve as the natural definitive hosts for 4 species of echinostomes. Three of the 4 species, E. hortense, E. cinetorchis, and E. japonicus, are zoonotic parasites capable of infecting humans, as human infections with these echinostome species have been reported in the ROK [14-17]. Infections of A. agrarius with E. cinetorchis and of C. lasiura with E. japonicus are reported for the first time in the ROK.

There was no difficulty in identifying the specimens of E. hortense, E. cinetorchis, and E. japonicus. However, there was difficulty in identifying E. murinum [18], because of the presence of several closely related species having 45 (43-47) collar spines. Such species included Echinostoma macrorchis [19], Echinostoma gotoi [19], Euparyphium paramurinum [20], and Euparyphium albuferensis [21]. The possibility of E. macrorchis for our specimens was excluded by the smaller body size, shorter and less coiled uterine tubule, and smaller egg size in our specimens. E. gotoi could be excluded because our specimens had globular testes, smaller sized eggs, and short uterus whereas E. gotoi has elongated elliptical or S-shaped testes, bigger sized eggs, and longer and more coiled uterus [19]. Two species of Euparyphium (E. paramurinum and E. albuferensis) were morphologically more similar to our specimens. However, the differential points between E. murinum and E. paramurinum given were minor [18,20], and these 2 species should be synonymous, so we excluded E. paramurinum for consideration. E. albuferensis was the most recently reared species of Euparyphium, which is similar to E. murinum [21]. However, the former differed from the latter in its larger body, larger suckers, more extensive distribution of vitelline follicles, and in the morphology of collar spines [21]. Our specimens were most similar to E. murinum in the body size and the distribution of vitelline follicles.

Infection rates of mice and insectivores with E. cinetorchis and E. japonicus, respectively, were higher in those captured at Paju-si compared with those captured at 2 other locations. Meanwhile, most of E. hortense infections were found in mice caught at Yeoncheon-gun. Whether this difference has any significant meaning is unclear at present. However, since all these 3 zoonotic echinostomes can infect humans through eating improperly cooked freshwater fish [1], attention should be paid to the possible presence of human infections with these echinostomes in these areas