Neodiplostomum seoulense (Digenea: Neodiplostomidae)
was described first from the small intestine of house
rats (Seo et al., 1964a) and later in a total of 26 docu-
mented human infections (Seo et al., 1982; Hong et al.,
1984, 1986). Several species of frogs and tadpoles were
found to be the second intermediate hosts, and
snakes, Rhabdophis tigrina, take the role of a paratenic
host and the source of human infections (Hong, 1982;
Seo et al., 1988). A species of freshwater snail,
Hippeutis cantori, is known to be the first intermediate
host (Seo et al., 1988; Chai and Lee, 2002).
With regard to the natural definitive host for N.
seoulense, limited information is available. Several sur-
veys on intestinal parasites of rodents in the Republic
of Korea (South Korea) suggested that N. seoulense
was a species having one natural definitive host, R.
norvegicus (Seo et al., 1964a, b, 1981; Yong et al., 1991),
� Brief Communication �
Apodemus agrarius as a new definitive host for
Jong-Yil CHAI1)*, Jae-Hwan PARK1), Sang-Mee GUK1), Jae-Lip KIM1), Hyo-Jin KIM1), Won-Hee KIM1),
Eun-Hee SHIN1), Terry A. KLEIN2), Heung-Chul KIM3), Sung-Tae CHONG3),
Jin-Won SONG4)and Luck-Ju BAEK4)
1)Department of Parasitology and Tropical Medicine, Seoul National University College of Medicine, and
Institute of Endemic Diseases, Seoul National University Medical Research Center, Seoul 110-799,
2)Regional Emerging Infectious Disease Consultant, 18thMedical Command, APO AP 96205-5281,
3)5thMedical Detachment, 168thMedical Battalion (AS), 18thMedical Command, APO AP 96205-5247,
4)Department of Microbiology, College of Medicine, Korea University, Seoul 136-701, Korea
Abstract: A total of 1,496 rodents and insectivores were live-trapped at Yeoncheon-gun (n = 351), Paju-shi (804),
and Pocheon-gun (343), Gyeonggi-do (Province), and examined for intestinal helminths, including Neodiplostomum
seoulense, seasonally from December 2004 to September 2005. Six species of rodents, including Apodemus agrarius
(1,366), Mus musculus (32), Micronytus fortis (28), Eothenomys regulus (9), Micronys minutus (6), and Cricetulus tri-
ton (3), and 1 species of insectivores Crocidura lasiura (54) were collected. A total of 321 adult N. seoulense were
collected from 19 (1.4%) A. agrarius. The worm burden ranged from 1 to 101 per A. agrarius (mean; 16.9). No N.
seoulense was observed in other rodent or insectivore species examined. The infection rate during autumn (4.5%)
was higher than those during spring (0.8%), summer (0.8%), and winter (0.5%). The average number of N. seoulense
in infected A. agrarius was the highest in spring (66.0 specimens), followed by autumn (15.2), winter (4.5), and sum-
mer (3.3). This study first confirms that A. agrarius is a natural definitive host for N. seoulense, and demonstrates that
the infection rates and intensities vary seasonally and geographically.
Key words: Neodiplostomum seoulense, wild rodent, Apodemus agrarius, prevalence, worm burden, Gyeonggi-do
Korean Journal of Parasitology
Vol. 45, No. 2: 157-161, June 2007
�Received 23 January 2007, accepted after revision 16
�Funding for portions of this work was provided by the
U.S. Department of Defense, Global Emerging Infections
Surveillance and Response System, Silver Spring, Maryland,
and the Armed Forces Medical Intelligence Center, Ft.
Detrick, Maryland, USA.
*Corresponding author (e-mail: firstname.lastname@example.org)
and their distributions were limited to certain parts of
South Korea, i.e., Seoul, Namyangju, Yongin, Young-
wol and Jungwon (Seo et al., 1964a, b, 1981). No N.
seoulense was found in other rodent species, including
Apodemus agrarius, Micronys minutus, Micronytus fortis
and Mus musculus, or an insectivore, Crocidura russula,
which were collected from Koyang-gun, Gyeonggi-do,
and Iri-si and Iksan-gun, Jeollabuk-do (Yong et al.,
1991). However, in mountainous and non-urban areas
(agricultural land and military training sites), com-
mensal R. norvegicus is not commonly collected. This
strongly suggests that field rodents, including A.
agrarius, and other small mammals may be involved
in the maintenance of N. seoulense life cycle in sylvatic
Along the northern boundary of Gyeonggi-do near
the demilitarized zone (DMZ), where civilians are
prohibited entry, military training sites are located.
The natural plant and animal ecology along the train-
ing sites perimeter and hills, where military activities
are limited, is well preserved. The present study was
undertaken to determine the infection status of small
mammals, including rodents and insectivores in areas
near the DMZ, with N. seoulense, as a part of a com-
prehensive rodent surveillance program in 3 localities
of northern Gyeonggi-do.
A total of 1,496 small mammals were caught using
Sherman traps (3 x 3.5 x 9” folding traps; H.B.
Sherman, Tallahassee, Florida, USA) as described by
Chai et al. (2007) from 3 areas in Gyeonggi-do, i.e.,
Yeoncheon-gun (n = 351), Paju-shi (804) and Pocheon-
gun (343), at military field training sites near the
DMZ. Animals were trapped seasonally from
December 2004 through September 2005. Total 7
species, including 6 species of rodents, i.e., A. agrarius
(1,366), Cricetulus triton (3), M. musculus (32), M. fortis
(28), Eothenomys regulus (9) and M. minutus (6), and 1
species of insectivores, i.e., Crocidura lasiura (54), were
collected. Animals were euthanized in accordance
with an approved animal use protocol under biosafety
level 3 (BSL-3) laboratory conditions.
Gastrointestinal organs, including the stomach, the
small intestine, and the large intestine (from the stom-
ach to the end of the rectum) were removed and pre-
served in 70% alcohol until examination. Gastro-
intestinal tracts were opened and examined for intesti-
nal helminths under a stereomicroscope (Chai et al.,
2007). Trematodes collected were placed on a micro-
scope slide with a coverslip, stained with Semichon’s
acetocarmine, and identified using a research micro-
scope. Data on nematodes, cestodes, and trematodes
other than Plagiorchis muris (Chai et al., 2007) and N.
seoulense (this paper) will be published separately.
A total of 321 adult N. seoulense were collected from
19 (1.4%) of 1,366 A. agrarius (Table 1). Worm burdens
in each infected A. agrarius ranged 1 to 101 (mean;
16.9). N. seoulense was not observed in other rodent
and insectivore species captured. Seasonal variation in
infection rates and intensity was observed. The infec-
tion rate was higher in A. agrarius captured in autumn
(4.5%) than those in spring (0.8%), summer (0.8%) and
winter (0.5%) trapping periods (Fig. 1). The mean
Korean J. Parasitol. Vol. 45, No. 2: 157-161, June 2007
Table 1. Infection rates of Neodiplostomum seoulense among small mammals captured from northern Gyeonggi-do
(Province), near the demilitarized zone (DMZ)
Number of positive/number of animals examined (%)
- 0/18 (0)
Total 19/1,366 (1.4) 0/54 (0) 0/3 (0)0/9 (0) 0/28 (0)0/6 (0) 0/32 (0)19/1,498 (1.3)
number of worms (= worm burden) in N. seoulense-
infected A. agrarius was the highest in spring (66.0),
followed by autumn (15.2), winter (4.5), and summer
(3.3) (Fig. 1). In spring, infected A. agrarius was found
only in Yeoncheon-gun, in winter, only in Paju-shi,
and in summer, in Yeoncheon-gun and Pocheon-gun
(Fig. 2). In autumn, infected A. agrarius was found in
all surveyed areas (Fig. 2).
In the present study, A. agrarius was identified as a
new definitive host for N. seoulense in northern dis-
tricts of Gyeonggi-do. Infection rates in A. agrarius
were relatively low, ranging from 0.5% to 4.5% sea-
sonally. The overall mean worm burden was also rela-
tively low, i.e., 16.9 per mouse, with heavy infections
of over 50 specimens (101, 70, and 61) only in 3 mice.
Adult N. seoulense reside in the duodenum, and if
the worm burden is high, the location of worms
Chai et al.: Apodemus agrarius, a new definitive host for N. seoulense
Spring Summer AutumnWinter
Infection rate (%)
No. of worm
Infection rate (%)
Fig. 1. Seasonal changes in infection rates and mean worm
burdens in A. agrarius. Spring = March (n = 255); Summer
= June (495); Fall = September (247); and Winter =
Fig. 2. Seasonal and geographical distribution of Apodemus agrarius infected with Neodiplostomum seoulense. Surveyed
areas include Paju-shi, Yeoncheon-gun, and Pocheon-gun, Gyeonggi-do (Province). A: spring, B: summer, C: autumn,
and D: winter. DMZ: demilitarized zone (heavy black line). Closed circle means 1 infected A. agrarius.
extends to the jejunum and ileum (Hong, 1982). They
evoke mechanical damages in the mucosa, and cause
villous atrophy, with blunting and fusion of villi, and
crypt hyperplasia in experimental mice (Lee et al.,
1985). In the first documented human case, the patient
suffered from acute epigastric discomfort and pain
with diarrhea and fever, and admitted to the Seoul
National University Hospital through the emergency
room (Seo et al., 1982).
It has been shown that mice heavily infected with
N. seoulense can be fatal within a month (Huh et al.,
1988; Kook et al., 1998, Chai et al., 2000). When several
strains of laboratory mice, including mast cell-defi-
cient W/Wv, their normal littermates +/+, C57BL/6,
C3H/HeJ, and a hybrid (BALB/cA x C3H/HeJ) F1
mice, were infected with 200 metacercariae, all mice
died by day 23 post-infection (Kook et al, 1998). Even
a small infection dose of 25 metacercariae was highly
lethal to C3H/HeJ mice (Kook et al., 1998). Consider-
ing this high pathogenicity of N. seoulense in laborato-
ry mice, it is suggested that high worm burdens in the
field mouse A. agrarius may also be harmful and lethal
to the host and only mice with low to moderate infec-
tions may survive in the field. However, lethality of
mice may be enhanced by other zoonotic infections,
e.g., hantavirus infection, scrub typhus, and lep-
tospirosis, that are relatively common in A. agrarius in
the surveyed areas (JW Song, personal communica-
A nationwide survey of frogs and tadpoles in South
Korea demonstrated their high infection rates and
intensities with N. seoulense metacercariae (Hong et
al., 1985). Therefore, tadpoles and frogs are suggested
to be the main source of N. seoulense infection in A.
agrarius (Lee et al., 1986; Seo et al., 1988). Snakes, in
particular, the grass snake R. tigrina, were also report-
ed to be infected with N. seoulense metacercariae
(Hong et al., 1985). However, there is little possibility
that snakes serve as the source of N. seoulense infection
in A. agrarius, since snakes are a predator, rather than
a prey, of A. agrarius.
The infection rates and worm burdens of N. seoul-
ense in A. agrarius varied seasonally. The mean worm
burden was the highest in spring, whereas the infec-
tion rate was the highest during the autumn sampling
period. The ecology of the second intermediate hosts,
i.e., tadpoles and frogs, may give an impact on these
observed differences. During the summer months,
many tadpoles emerge and develop into adult frogs.
With the increased number of tadpoles and frogs dur-
ing the summer months, rodents that prey on them
may have an increased potential to become infected
with N. seoulense, resulting in higher infection rates
during the autumn season. If tadpoles or frogs contin-
ue to be in touch with water contaminated with N.
seoulense cercariae for extended periods of time, they
have greater opportunities to be infected with N.
seoulense. Prior to hibernation, frogs will be frequently
exposed to water and cercariae ingested would accu-
mulate in their muscle tissues becoming metacercariae
(Hong et al., 1985). Frogs emerging from hibernation
that harbor metacercariae are subject to be preyed
upon by wild animals, including rodents. The worm
burden would be relatively high for wild animals that
consume frogs heavily infected with N. seoulense
metacercariae in spring, even though their infection
rates were relatively low.
The geographical distribution of N. seoulense-infect-
ed A. agrarius increased over time from spring to
autumn and was limited to Paju-shi during the winter
season. As there is no evidence of A. agrarius migrat-
ing due to changes in environmental temperatures or
other factors, this is most likely due to very low infec-
tions associated with high mortality rates of infected
mice. N. seoulense was not found in the other 5 species
of small mammals examined. Also in previous stud-
ies, N. seoulense was not collected from these species
(Seo et al., 1964a; Yong et al., 1991). Hence, it may be
possible that small mammal hosts, i.e., M. musculus,
M. minutus, C. triton, M. fortis, E. regulus, and C.
lasiura, are less suitable for N. seoulense infection than
A. agrarius or R. norvegicus.
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Chai et al.: Apodemus agrarius, a new definitive host for N. seoulense