INTRODUCTION
Holostephanus Szidat, 1936, belongs to the family Cyathocotylidae. Its ventral pouch exhibits a distinctive morphology and encloses the holdfast organ [
1]. Other unique features of this species include the gonad position (testes are diagonally arranged and well-developed) and the location of vitelline follicles which extend to the level of the ventral sucker or pharynx [
2]. Twelve species belong to the genus
Holostephanus; H.
luehei (type),
H. anhinga,
H. calvusi,
H. corvi,
H. curonensis,
H. dubius,
H. ibisi,
H. ictaluri,
H. lutzi,
H. metorchis,
H. nipponicus, and
H. phalacrocoraxus [
1].
Holostephanus species were originally described as arasites of birds, and adult flukes of
H. nipponicus and
H. metorchis were first recovered from the black kite,
Milvus migrans lineatus, in Japan [
3]. The first intermediate hosts are snails of the
Parafossarulus spp., including
P. spiridonovi, and the second hosts were proven to be freshwater fish,
Pseudorasbora parva [
3]. Adult worms of
H. ictaluri were discovered from the intestine of a catfish [
5],and Besprozvannykh et al. [
4] succeeded in propagating
H. nipponicus in chickens.
Infection status of freshwater fishes with digenetic trematode metacercariae has been extensively studied in the Republic of Korea. However, trematode fauna of wild birds in Korea has not been a subject of extensive investigation. As
Gymnophalloides seoi was isolated from a pancreatitis patient [
6], parasites of birds were recognized as a possible source of human infections. Hence, it is necessary to explore on digeneans infecting birds.
To our knowledge, there are 3 reports which studied on
Holostephanus spp. in Korea. The presence of
H. nipponicus was confirmed by the removal of 21 metacercariae from
P. parva [
7]. Nam et al. [
8] isolated 8 metacercariae from the pond smelt
Hypomesus olidus.
H. nipponicus adults were identified and reported in 2007 [
9]. In the present study, we collected metacercariae of
H. metorchis from
P. parva, and succeeded in rearing them into adult flukes in chicks, and identified to
H. metorchis.
DISCUSSION
The adult worms propagated in this study were identified as
H. metorchis. Morphologic differences are evident upon examination of
H. nipponicus and
H. metorchis adults [
3]. The anterior testis of
H. metorchis does not extend beyond the level of the ventral sucker and is adjacent to the cirrus pouch, whereas the testis of
H. nipponicus passes the ventral sucker and reaches the level of the pharynx. The distribution of vitellaria illustrates another difference between the 2 species. The vitelline follicles do not extend below the level of posterior testis in
H. nipponicus, but extend over the posterior testis to the posterior extremity in
H. metorchis [
3]. The worms of present study were 1,289 µm by 806 µm, fitting better to the measurements of
H. metorchis (960-1,400 × 650-900) than those of
H. nipponicus (1,000-1,100 × 650-800) [
3].
The characteristic organ of the genus
Holostephanus is the holdfast organ which occupies a large portion of the ventral concavity. The trematode,
Alaria mustelae, a member of the family Diplostomatidae, has a holdfast organ which is involved in extracorporeal digestion and absorption, not for attaching to the host mucosa [
10].
Phrixocephalus cincinnatus, a blood-feeding parasitic copepod, has a holdfast organ that functions for digestion of host erythrocyte as well as detoxification and storage of iron liberated from the catabolism of hemoglobin [
11]. The tribocytic organ of
Neodiplostomum seoulense plays a role in self-protection and host tissue lysis [
12]. Hence, the holdfast organ of
H. metorchis might play a role in digestion rather than adhesion, but its precise function remains to be elucidated.
Though metacercariae of this genus were sometimes observed in the Republic of Korea [
7,
8], there has been only one report on
Holostephanus adult flukes [
9]. It should result from the fact that
P. parva is the second intermediate host for
H. metorchis, which is also an important fish for
Clonorchis sinensis, the Chinese liver fluke. The members of
Holostephanus sp. generally have a small number of intrauterine eggs, and the adult
H. metorchis obtained in the present study had 1-2 eggs in the uterus. It is of note that the incidence of
H. ictaluri, a member of the family Cyathocotylidae, was extremely low in the host in comparison with other parasites [
5]. Since
H. metorchis are bird parasites, chicks were used as experimental hosts in the present study. However, the worm recovery rate was only 1.4%, lower than that of
H. nipponicus (9.6%) [
9]. Further experiments using more suitable experimental hosts are required.
Human infections with
H. metorchis have never been documented. This is not surprising since
P. parva is typically not consumed raw in the Republic of Korea. The adult fluke of
Mesostephanus indicum, a member of the Cyathocotylidae, prefers highly specific hosts and adults were recovered only from
Milvus migrans govinda, and other birds were refractory to the infection [
13]. However, the majority of cyathocotylid trematodes lack host specificity and
Mesostephanus longisaccus, was isolated from a naturally infected dog [
14]. Therefore, occurrence of human infections is likely if
H. metorchis metacercariae encyst in freshwater fish other than
P. parva. In fact, metacercariae of
H. nipponicus were detected from pond smelts,
Hypomesus olidus [
8], which are consumed raw by some Korean people. Screening for human trematode infections should be regularly performed in villages near river basins or ponds. Rigorous identification of intermediate hosts will also provide valuable information regarding the natural history of trematode infections in the Republic of Korea.