INTRODUCTION
Flukes of the genus Plagiorchis have been found in the small intestine of mammals, including humans, birds, reptiles, amphibians, and fish (Radomyos et al., 1989). The second intermediate hosts of the flukes are aquatic insects, such as mosquito larvae, insect naiads, freshwater snails, and freshwater fishes (Tanabe, 1922; Asada et al., 1962; Komiya, 1965; Hong et al., 1996).
In Korea, of the genus Plagiorchis, P. koreanus, P. orientalis, P. corpulentus, P. magnacotylus, P. vespertilionis, P. rhinolophi, and P. kyushuensis were reported from bats (Park, 1939a, 1939b; Sogandares-Bernal, 1956; Kifune et al., 1983). Plagiorchis muris was collected from wild or house rats captured in Yongin-gun and Pochon-gun, Kyonggi-do, and in Chorwon-gun and Yangyang-gun, Kangwon-do, and from house rats in Hadong-gun, Kyongsangnam-do, and in Seoul (Seo et al., 1964, 1981; Lee et al., 1990). Moreover, human infection by P. muris was reported in Hamyang-gun, Kyongsangnam-do (Hong et al., 1996). From these reports, the distribution of P. muris is considered to be nationwide in Korea. The second intermediate hosts, such as dragonflies and freshwater fishes were found to be infected by the metacercariae of P. muris (Hong et al., 1998). In albino rats, the metacercaria of P. muris grew rapidly to an ovigerous adult on day 4 post-infection (Hong et al., 1998).
The dragonfly is one of the most predominant insects found in Korea during summer. Dragonflies, naiads, and chironomids, all with the metacercariae of P. muris, are easy targets of prey for fish, birds, and other mammals (Tanabe, 1922; Komiya, 1965). In this context, it is likely that dragonflies play a role in the life cycle of P. muris in Korea. There have not been any studies regarding the second intermediate host of the genus Plagiorchis. This survey was, therefore, conducted to elucidate the infection status of the dragonflies with P. muris metacercariae in Korea.
MATERIALS AND METHODS
Dragonflies were caught near ricepaddies, canals, streams, and/or villages in 20 local areas in Korea from May 1993 to August 1994 (Table 1). Captured dragonflies were identified according to Shin's method (1993) and examined individually or as a whole for the metacercariae of P. muris by artificial digestion method. As a way of estimating the metacercarial burden in the dragonflies in a given area, dragonflies of the same species were digested as a whole. On the other hand, in order to get an information on the infection rates of the dragonflies with P. muris metacercariae, a dominant species was subjected to the individual examination. The dragonflies were digested for 1 hr in artificial digestion juice (pepsin 5-6 g (Sigma, St. Louis, USA), 10 ml HCl in 1 L H2O) at 37℃. The digest was washed and sedimented in physiological saline several times to clarity. The metacercariae of P. muris were collected from the sediment of digest under the dissecting microscope and identified under the light microscope (Hong et al., 1998). The data of individual examination was not included to those of the whole examination. The metacercariae collected were grouped into 50 to 100 in number and fed to albino rats with a gastric needle. The flukes were recovered from the small intestine of albino rats up to 28 days after the metacercarial infection. All of the flukes recovered were stained with Semichon's acetocarmine and identified microscopically to P. muris (Hong et al., 1998).
RESULTS
The metacercaria
The metacercaria of P. muris, 0.169 (0.165-0.174) mm by 0.158 (0.148-0.170) mm, was elliptical to spherical in shape with a hyaline thin wall (Fig. 1). The oral sucker was subterminal and armed with a short flame-shape stylet. The acetabulum was round, smaller than the oral sucker and was located in the middle of the body. The excretory bladder was Y-shape and filled with refractile granules. No primordium of genital organs was found in the metacercaria.
Infection status of the dragonflies
A total of 4,172 dragonflies was caught from 20 different areas through this survey. The species of captured dragonflies consisted of Sympetrum darwinianum, S. eroticum, S. pedomontanum, S. infuscatum, Pantala flavoscens, Calopteryx atrata, and Orthetrum albistylum speciosum. Of the species, S. darwinianum caught from 18 local areas was the most predominating species followed by S. eroticum, S. pedomontanum, and P. flavoscens in descending order (Table 1).
When estimated as a whole, the burden of the dragonflies with P. muris metacercariae was the highest in S. eroticum by 0.8 per head followed by C. atrata, S. darwinianum, S. pedomontanum, and S. infuscatum. Geographically, the metacercarial burden in the dragonflies was very heavy in two areas, Hamyang-gun and Kosong-gun, Kyongsangnam-do; moderate in seven areas, Yongin-gun, Taebaek-shi, Inje-gun, Muju-gun, Yonggwang-gun, Koryong-gun and Mun-gyong-gun; and very low in the rest of the surveyed areas (Table 1).
The infection rate with the metacercaria was 44.0% and the mean metacercarial burden was 3.2 for 90 individually examined S. darwinianum. The highest value of the metacercarial infection rate was found from those collected in Hamyang-up, Hamyang-gun, Kyongsangnam-do (Table 2). The metacercaria, however, was not found in S. darwinianum caught from six areas.
Sympetrum eroticum, the second frequent species, was caught from 13 areas and was the dominant species in four areas. Through the individual examination, the highest infection rate was found in S. eroticum from Yongo-myon, Kosong-gun, but the the heaviest metacercarial burden was detected from Yongin-up, Yongin-gun (Table 2). On the other hand, through the whole examination, the metacercarial burden of the species was the heaviest from Yongo-myon, Kosong-gun (Table 1). The metacercaria was not detected from S. eroticum caught from four areas.
Sympetrum pedomontanum and S. infuscatum were caught from six areas, and their metacercarial burden was 0.3 each. Sympetrum pedomontanum was more abund-ant in numbers than S. infuscatum in the surveyed areas. The metacercaria of P. muris was found only from 2 out of 235 P. flavoscens dragonflies caught from six areas (Table 1).
Calopteryx atrata were caught along the canals and/or streams in three areas. The infection rate of C. atrata was 30.0%. The mean metacercarial burden was 1.5 for C. atrata captured from Yongo-myon, Kosong-gun (Table 2). The dragonfly, O. albistylum speciosum, was very rare and was captured from only three areas. The metacercaria was not detected in this dragonfly (Table 1).
DISCUSSION
The dragonflies, Sympetrum spp., were caught at ricepaddies where they emerge from pupae and mature into adults. It was easy to catch dragonflies in the early morning or on rainy days. Calopteryx atrata were abundant along canals and streams, and it seemed that they preferred such places. Pantala flavoscens were caught from a swarm of dragonflies flying in the air (Shin, 1993).
The size and shape of the cyst and the features of somatic organs of the metacercaria agreed well with the characters of P. muris metacercaria (Komiya, 1965; Hong et al., 1998).
The adult P. muris was collected from field rats captured from Chorwon-gun and Pochon-gun (Seo et al., 1964), and from house rats found in Yongin-gun and Hadong-gun (Seo et al., 1981). The metacercarial burden in the dragonflies in these areas appeared not to be closely related to the occurrence of the adult flukes in the rat, even though there was a big difference between the two surveys. In Yongin-gun, it was suggested that the occurrence of adult P. muris in house rats could be related to the metacercariae in dragonflies, because field rats could feed on dead dragonflies.
Seven species of Plagiorchis have been recorded from Korean bats (Park, 1939a, 1939b; Sogandares-Bernal, 1956; Kifune et al., 1983). Bats are nocturnal animals with a precision ultrasonar system. It is likely that bats feed on the insects such as dragonflies, mosquitos, and chironomids which are known to be the second intermediate host of P. muris (Tanabe, 1922; Komiya, 1965).
The aquatic insect larvae have been suggested to be a source of human infection of P. muris in Japan (Asada et al., 1962). The surface water of springs, wells, ponds, streams and rivers contain a wide variety of aquatic insects and/or insect larvae which might be infected with P. muris metacercariae as shown in the present survey. By drinking the surface water without a proper treatment, human could be infected by P. muris in Korea as was in Japan. In this respect, it is likely that the larvae, unlikely the adults, of dragonflies are a possible source of human plagiorchiasis.
As far as the metacercarial burden in the dragonflies is considered, the insectivorous diurnal birds can be treated as the final hosts of P. muris in Korea. To broaden our knowledge on P. muris, further researches must be done on the metacercarial infection rate in mosquitos and chironomids. Furthermore, adult flukes found in the birds deserve further investigation.
