Acanthoparyphium shinanense n. sp. (Digenea: Echinostomatidae) from Experimental Chicks Infected with Metacercariae Encysted in Brackish Water Clams in the Republic of Korea

Acanthoparyphium shinanense n. sp. (Digenea: Echinostomatidae) is described from chicks experimentally infected with the metacercariae encysted in 2 brackish water clam species, Ruditapes philippinarum and Coecella chinensis, in the Republic of Korea. The metacercariae were round to oval, armed with 23 collar spines, and 0.216 (0.203–0.226) mm in diameter. From 5 chicks experimentally infected each with 200 metacercariae, 34 juvenile (5-day-old worms) and 104 adult flukes (7-day-old worms) were harvested from their small intestines, with the average worm recovery rate of 13.8%. The adult flukes were 3.18 (2.89–3.55) mm long and 0.68 (0.61–0.85) mm wide, with an elongated, posteriorly tapering body, and a prominent head collar armed with 23 collar spines arranged in a single uninterrupted row. The posterior testis of A. shinanense was longitudinally elongated, which is similar to Acanthoparyphium spinulosum Johnston, 1917 but unique from the other closely related species, including Acanthoparyphium tyosenense Yamaguti, 1939, Acanthoparyphium kurogamo Yamaguti, 1939, and Acanthoparyphium marilae Yamaguti, 1934. The eggs of A. shinanense were larger than those of A. spinulosum, and the anterior extent of 2 lateral groups of vitellaria was slightly more limited in A. shinanense than in A. spinulosum. Molecular analysis of nuclear and mitochondrial genes revealed low homology with A. spinulosum from USA (96.1% in 5.8S rRNA) and Ukraine (97.9% in 28S rRNA), Acanthoparyphium n. sp. from USA (98.0% in 28S rRNA), and Acanthoparyphium sp. from Australia, Kuwait, and New Zealand. Biological characteristics, including its first intermediate host and natural definitive hosts, as well as its zoonotic capability, should be elucidated.     

Surface ultrastructure of juvenile and adult Acanthoparyphium tyosenense (Digenea: Echinostomatidae)

The tegumental ultrastructure of juvenile and adult Acanthoparyphium tyosenense (Digenea: Echinostomatidae) was observed by scanning electron microscopy. One- to 3-day-old juveniles and 10-day-old adults were harvested from chicks experimentally fed metacercariae from a bivalve, Mactra veneriformis. The juvenile worms were minute, curved ventrally, and had 23 collar spines characteristically arranged in a single row. The lips of the oral sucker had 7 single aciliated sensory papillae and 4 grouped uniciliated sensory papillae. The ventral sucker had 25 aciliated round swellings on its lip. The anterolateral surface between the 2 suckers was densely packed with tongue-shaped tegumental spines, and the ventral surface just posterior to the ventral sucker was covered with peg-like spines. Retractile, peg-like spines were seen on the anterolateral surface, whereas scale-like spines with round tips and broad bases were sparsely distributed posterior to the ventral sucker. The cirrus was characteristically protruding and armed with minute spines. The surface ultrastructure of A. tyosenense was unique, especially in the number and arrangement of collar spines, shape, and distribution of tegumental spines and in distribution of sensory papillae.     

Echinostoma revolutum: metacercariae in Filopaludina snails from Nam Dinh Province, Vietnam, and adults from experimental hamsters

We detected metacercariae of Echinostoma revolutum in Filopaludina sp. snails purchased from a local market in Nam Dinh Province for the first time in Vietnam. Adult flukes were harvested from experimentally infected hamsters at days 14 and 17 post-infection. The metacercariae were round, 170-190 μm (n = 15) in diameter, with a cyst wall thickness of about 12 μm. A total of 37 collar spines were arranged around the head collar, and large excretory granules were seen in 2 canals of the excretory bladder. The 14-day old adult flukes were elongated, ventrally curved, and 5.0-7.2 × 0.8-1.3 mm (n = 20). The head collar had a total of 37 collar spines arranged in 2 alternating rows, including 5 corner spines on each side. The cirrus sac contained a saccular seminal vesicle, a prostatic gland, and an unarmed cirrus. Two tandem testes were smooth or slightly lobed. Eggs were ovoid to elliptical, 110-118 × 70-75 μm. These morphological characters were similar to those of E. revolutum and E. jurini. We tentatively identified it as E. revolutum because the validity of E. jurini remains to be elucidated. The taxonomic relationship of E. revolutum and E. jurini is discussed.     

Acanthoparyphium tyosenense (Digenea: Echinostomatidae): experimental confirmation of the cercaria and its complete life history in Korea

Cercaria yamagutii Ito, 1957, was found in the marine mesogastropods Lunatia fortuni and Glossaulax didyma from the tidelands of Simpo located at the estuary of the Mankyoung River, which runs to the western coast of Korea. Metacercariae were found in a marine bivalve Mactra veneriformis after being infected with C. yamagutii experimentally. When a sea gull, Larus crassiostris, was fed with the metacercariae collected from the infected M. veneriformis, adult worms were recovered 10 days later. It was confirmed that the parasites collected from L. crassiostris were Acanthoparyphium tyosenense Yamaguti, 1939. From the results of this life cycle study, it was determined that the first intermediate hosts of A. tyosenense are L. fortuni and G. didyma. The second intermediate and final hosts are M. veneriformis and L. crassiostris, respectively. Mactra veneriformis was experimentally infected with C. yamagutii isolated from L. fortuni and G. didyma by maintaining them in a water tank for 30 min at about 20 C. The cercariae entered M. veneriformis through their incurrent siphons. Five hours after infection, the cercariae tails began to separate from the bodies, and the cercariae formed cysts. Mature cysts were formed 340 hr (14 days) after infection and identified as the metacercariae of A. tyosenense. The prevalence of A. tyosenense metacercariae was 99.5% in naturally infected M. veneriformis. This is the first report of C. yamagutii as the cercaria of A. tyosenense, and the complete life cycle of A. tyosenense was established in Korea.     

First record and description of metacercariae of Curtuteria arguinae n. sp. (Digenea: Echinostomatidae), parasite of cockles Cerastoderma edule (Mollusca: Bivalvia) in Arcachon Bay, France

A new Himasthlinae species, Curtuteria arguinae, is described as metacercariae from the cockle Cerastoderma edule (L.), collected at Banc d'Arguin (southwestern France). These metacercariae encysted preferentially in the mantle and also in the foot of cockles. Encysted and chemically excysted metacercariae were studied by light microscopy and scanning electron microscopy, respectively. Excysted metacercariae were elongated and curved ventrally. They bore a 33-spine circumoral collar. Sensory papillae were arranged around the oral sucker and also symmetrically along the ventral surface body, from the collar to the acetabulum. The dorsal and ventral tegument surfaces were densely packed with similar pointed spines. The posterior end of the body was without any spines. Among the Curtuteria species described previously, only Curtuteria haematopodis Smogorjewskaja and Iskova, 1966 had the same number of circumoral collar spines. A 6-yr field survey showed that the cockle population at Banc d'Arguin was subjected to a summer infection of C. arguinae. Curtuteria arguinae phenology of infection is characterized by interannual variability and seasonality (beginning in July-August and maximum in autumn). The first intermediate and final hosts remain unknown.     

Tegumental ultrastructure of the juvenile and adult Himasthla alincia (Digenea: Echinostomatidae)

The tegumental ultrastructure of juvenile and adult Himasthla alincia (Digenea: Echinostomatidae) was observed by scanning electron microscopy. One-, 5- (juveniles) and 20-day-old worms (adults) were harvested from chicks experimentally fed metacercariae from a bivalve, Mactra veneriformis. The juvenile worms were elongated and curved ventrally. The head crown bore 31 collar spines, arranged in a single row. The lip of the oral sucker had 12 paired, and 3 single type I sensory papillae, and the ventral sucker had about 25 type II sensory papillae. The anterolateral surface between the two suckers was densely packed with tegumental spines with 4-7 pointed tips. The adult worms were more elongated and filamentous, and had severe transverse folds over the whole body surface. On the head crown and two suckers, type I and II sensory papillae were more densely distributed than in the juvenile worms. Retractile brush-like spines, with 8-10 digits, were seen on the anterolateral surface, whereas claw-shaped spines, with 2-5 digits, were sparsely distributed posteriorly to the ventral sucker. The cirrus characteristically protruded out, and was armed with small spines distally. The surface ultrastructure of H. alincia was shown to be unique among echinostomes, especially in the digitation of its tegumental spines, the distribution of sensory papillae and by severe folds of the tegument.     

Echinostoma macrorchis in Lao PDR: Metacercariae in Cipangopaludina Snails and Adults from Experimentally Infected Animals

The echinostome metacercariae encysted in Cipangopaludina sp. snails that were purchased from a market in Vientiane Municipality, Lao PDR, were identified as Echinostoma macrorchis (Digenea: Echinostomatidae) through recovery of adult flukes after experimental infection to rats and a cat. The metacercariae were round, 113-128 (121)×113-125 (120) µm, having a thick cyst wall, a head collar armed with collar spines, and excretory granules. The adult flukes recovered from the rats and cat at day 14 and 30 post-infection, respectively, were elongated, ventrally curved, and 3.9-6.3×0.7-1.1 mm in size. The head collar was distinct, bearing 43-45 collar spines with 5 angle spines on each side. Two testes were large (as the name implies), tandem, and slightly constricted at the middle, with irregular margins. Eggs were operculated, ovoid to elliptical, and 88-95×56-60 µm. In scanning electron microscopy, the head collar was prominent, with 43-45 collar spines. Scale-like tegumental spines were densely distributed on the ventral surface between the oral and ventral suckers. Sensory papillae were distributed mainly on the tegument around the 2 suckers. It is confirmed that E. macrorchis is distributed in Lao PDR using Cipangopaludina sp. snails as the second intermediate host.     

Postmetacercarial changes in Echinostoma caproni maintained in a defined medium plus calf serum

The present study examined postmetacercarial changes in the excysted metacercariae of Echinostoma caproni maintained in the defined medium Mixture 199 plus 20% calf serum for 7 days at 41 degrees C. The gas phase was atmospheric air. Each culture was inoculated with 25 excysted metacerariae. Cultures were maintained upright in closed 15 ml plastic centrifuge tubes each containing 10 ml of medium plus 200 units of penicillin/ml and 200 micrograms of streptomycin/ml. By 4 days in culture, most metacercariae had voided their excretory concretions. Organisms were clumped or solitary at the bottom of the cultures. Many organisms showed flaring of the oral collar and extension of both the collar and tegumentary spines. By 4 days in culture, posterior protuberances or bumps were noted on many of the organisms and some organisms showed abnormal vesicular growths or blebs at their posterior ends. Some mortality was noted in culture by day 5, but most organisms were still alive when the cultures were terminated on day 7.     

Acanthoparyphium tyosenense: the discovery of human infection and identification of its source

Acanthoparyphium tyosenense Yamaguti, 1939 (Digenea: Echinostomatidae), was originally reported as an avian intestinal parasite; here, its presence is reported in 10 humans in the Republic of Korea. The patients were 9 adults aged 35-66 yr (males and females) and a young girl aged 7 yr residing in 2 coastal villages in Puan-gun, Chollabuk-do. The worms were recovered after treatment with praziquantel and purgation with magnesium salts. A total of 158 specimens (1-107 specimens/individual) was collected, together with varying numbers of other intestinal flukes. The patients had eaten various kinds of brackish water mollusks caught in an estuary near their villages. Five bivalves and a gastropod species suspected as sources of human infection were collected and examined. Two bivalves (Mactra veneriformis and Solen grandis) and the gastropod (Neverita bicolor) were found to be infected with the metacercariae of A. tyosenense; adult flukes were confirmed after the experimental infection of chicks. The results show that A. tyosenense infects humans and that brackish water mollusks are the source of human infection.     

Echinostoma ilocanum infection in Oddar Meanchey Province, Cambodia

Fecal examinations using the Kato Katz technique were performed on a total of 1,287 villagers (945 students and 342 general inhabitants) of Oddar Meanchey Province, Cambodia in May 2007 and November 2009. The overall intestinal helminth egg positive rate was 23.9%, and the most prevalent helminth species was hookworms (21.6%). Other helminth eggs detected included echinostomes (1.0%), Enterobius vermicularis (0.8%), small trematode eggs (0.7%), which may include Opisthorchis viverrini and Haplorchis spp., and Hymenolepis nana (0.4%). In order to recover adult echinostomes, we treated 2 patients with 10-15 mg/kg praziquantel and purged. Total 14 adult echinostomes, 1 and 13 worms from each patient, were collected. The echinostomes characteristically had 49-51 collar spines and 2 round or slightly lobated testes. They were identified as Echinostoma ilocanum (Garrison, 1908) Odhner, 1911. So far as literature are concerned, this is the first record on the discovery of human E. ilocanum infection in Cambodia.     

A new cercaria and metacercaria of Acanthoparyphium (Echinostomatidae) found in an intertidal snail Zeacumantus subcarinatus (Batillaridae) from New Zealand

A new 23-collar-spined cercaria and metacercaria are described from intertidal molluscs of the coast of New Zealand. The new cercaria found emerging from the mud snails Zeacumantus subcarinatus (Sowerby) (Prosobranchia: Batillariidae) is characterized mainly by the number and arrangement of the cephalic glands, the size of the suckers, and the size and number of the collar spines. The cercaria encysts in the cockle Austrovenus stutchburyi (Wood) (Bivalvia: Veneridae) which lives in sympatry with the first intermediate host throughout New Zealand. Laboratory infections of cockles by cercariae from naturally infected snails resulted in metacercariae identical to those found in naturally infected cockles. The main features of the cercaria and metacercaria are the presence of a reniform collar with 23 spines, two pairs of small cephalic glands at the oral sucker level, another two pairs of much longer ones posterior to the pharynx, and the excretory vesicle Y-shaped with the main collecting canals extending to the anterior level of the ventral sucker. Each of the main collecting canals had 10-11 pairs of bilateral diverticula between the anterior edge of the ventral sucker and anterior body end. A brief discussion of its possible life cycle and ecology is also provided.     

Three Echinostome Species from Wild Birds in the Republic of Korea

Three echinostome species, i.e., Patagifer bilobus, Petasiger neocomense, and Saakotrema metatestis, are newly recorded in the trematode fauna of the Republic of Korea. They were recovered from 3 species of migratory birds (Platalea minor, Podiceps cristatus, and Egretta garzetta), which were donated by the Wildlife Center of Chungbuk (WCC) and the Conservation Genome Resource Bank for Korean Wildlife (CGRB). Only 1 P. bilobus specimen was recovered from the intestine of a black-faced spoonbill (P. minor), and characterized by the bilobed head crown with a deep dorsal incision and 54 collar spines. Twenty P. neocomense were recovered from the intestine of a great crested grebe (P. cristatus), and they had a well-developed head crown with 19 spines and 2 testes obliquely located at the posterior middle of the body. Total 70 S. metatestis were collected from the bursa of Fabricius of 1 little egret (E. garzetta). It is characterized by stout tegumental spines covered in the entire leaf-shaped body, posterior extension of the uterus, presence of the uroproct and a well-developed head crown with 12 pairs of collar spines on each side. By the present study, these 3 echinostome species are newly added to the trematode fauna in Korea.     

Echinostoma mekongi: Discovery of Its Metacercarial Stage in Snails,Filopaludina martensi cambodjensis,in Pursat Province,Cambodia

Echinostoma mekongi was reported as a new species in 2020 based on specimens collected from humans in Kratie and Takeo Province, Cambodia. In the present study, its metacercarial stage has been discovered in Filopaludina martensi cambodjensis snails purchased from a local market nearby the Tonle Sap Lake, Pursat Province, Cambodia. The metacercariae were fed orally to an experimental hamster, and adult flukes were recovered at day 20 post-infection. They were morphologically examined using light and scanning electron microscopes and molecularly analyzed by sequencing of their mitochondrial cox1 and nad1 genes. A total of 115 metacercariae (1–8 per snail) were detected in 60 (60.0%) out of 100 Filopaludina snails examined. The metacercariae were round, 174 μm in average diameter (163–190 μm in range), having a thin cyst wall, a head collar armed with 37 collar spines, and characteristic excretory granules. The adult flukes were elongated, ventrally curved, 7.3 (6.4–8.2)×1.4 (1.1–1.7) mm in size, and equipped with 37 collar spines on the head collar (dorsal spines in 2 alternating rows), being consistent with E. mekongi. In phylogenetic analyses, the adult flukes showed 99.0–100% homology based on cox1 sequences and 98.9–99.7% homology based on nad1 sequences with E. mekongi. The results evidenced that F. martensi cambodjensis snails act as the second intermediate host of E. mekongi, and hamsters can be used as a suitable experimental definitive host. As local people favor to eat undercooked snails, these snails seem to be an important source of human infection with E. mekongi in Cambodia.     

Echinostoma hortense and Heterophyid Metacercariae Encysted in Yellowfin Goby, Acanthogobius flavimanus, from Shinan-gun and Muan-gun (Jeollanam-do), Korea

Fishborne trematode (FBT) metacercariae were investigated in yellowfin goby, Acanthogobius flavimanus, collected from Shinan-gun and Muan-gun, Jeollanam-do (province), Korea. All collected fishes were examined using the artificial digestion method. In all of 15 gobies from Aphae-myeon in Shinan-gun, metacercariae of Stictodora spp. (334 metacercariae/fish), Heterophyes nocens (153/fish), and Heterophyopsis continua (20/fish) were detected. In 2 of 14 gobies from Jido-myeon in Shinan-gun, 8 Echinostoma hortense metacercariae in total were detected. In 15 gobies from Haeje-myeon in Muan-gun, the metacercariae of H. continua were found in 100%, Stictodora spp. in 86.7%, and H. nocens in 6.7% of fish examined. The average numbers of metacercariae per infected fish were 23.3 (H. continua), 416.0 (Stictodora spp.), and 2.0 (H. nocens), respectively. The metacercariae of E. hortense found in gobies were elliptical, with 150 × 138 µm in average size, and had 27 collar spines on the head crown. The above results suggest that yellowfin gobies from 2 localities may be the potential infection sources of FBT. Moreover, it is proved for the first time that the yellowfin goby, A. flavimanus, acts as a second intermediate host for E. hortense.     

Fishborne Trematode Metacercariae in Freshwater Fish from Guangxi Zhuang Autonomous Region, China

A survey was performed to investigate the infection status of fishborne trematode (FBT) metacercariae in freshwater fish from Guangxi Zhuang Autonomous Region, China. A total of 307 freshwater fish of 31 species were collected from 5 administrative regions of Guangxi Zhuang Autonomous Region. They were examined by artificial digestion method from July 2003 to August 2004. No metacercariae were detected in fish from Fusui-xian. In fish from Mashan-xian and a market in Nanning, 3 species of metacercariae, Haplorchis taichui, Haplorchis pumilio, and Centrocestus formosanus, were mainly detected. Metacercariae (8 in number) of Clonorchis sinensis were found in 1 Chanodichthys dabryi purchased from a market in Nanning. In fish from Yangshuo, Metagonimus yokogawai metacercariae were detected from all 18 fish species examined. Total 13 C. sinensis metacercariae were found in 3 out of 10 Hemibarbus maculatus from Yangshuo. All 7 Zacco platypus from Yangshuo were infected with 8-112 Echinochasmus perfoliatus metacercariae. In fish from Binyang-xian, H. pumilo metacercariae were mainly detected in all 5 fish species examined, and only 1 metacercaria of C. sinensis was found in a Hemiculter leucisculus. From the above results, it was confirmed that some species of freshwater fish play a role of second intermediate hosts for FBT in Guangxi Zhuang Autonomous Region, China. In particular, 4 species of intestinal flukes, M. yokogawai, H. taichui, H. pumilio, and C. formosanus, were prevalent in fish hosts, whereas C. sinensis metacercariae were detected only in 3 fish species.     

Surface ultrastructure of Metagonimus miyatai metacercariae and adults

A scanning electron microscopic study was performed to observe surface ultrastructures of excysted metacercariae and adults of Metagonimus miyatai. Metacercariae were collected from the scale of the pale chub (Zacco platypus), and adult flukes were harvested 1-4 weeks after infection to rats. In excysted metacercariae, the oral sucker was devoid of tegumental spines and had type I and type II sensory papillae. Anteriorly to the ventral sucker, spines were dense and digitated into 5-7 points, whereas near the posterior end of the body spines were sparse and digitated into 2-3 points. In one-week adults, 7 type II sensory papillae were arranged around the lip of the oral sucker, and at inner side of the lip one pair of small and two pairs of large type 1 sensory papillae were seen on each side. The distribution of tegumental spines was similar to that of metacercariae, but they were more differentiated with 9-11 pointed tips. In two- to four-week old adults, the surface ultrastructure was nearly the same as in one-week old adults, however, sperms were frequently seen entering into the Laurer''s canal. Conclusively, the surface ultrastructure of M. miyatai was generally similar to that of M. yokogawai, however, differentiation of tegumental spines and distribution of sensory papillae around the oral sucker were different between the two species, which may be of taxonomic significance.     

The Jeju weasel, Mustela sibilica quelpartis, a new definitive host for Gnathostoma nipponicum Yamaguti, 1941

Adult gnathostomes were discovered in the stomach of the Jeju weasel, Mustela sibilica quelpartis, road-killed in Jeju-do (Province). Their morphological characters were examined to identify the species. Total 50 gnathostome adults were collected from 6 out of 10 weasels examined. In infected weasels, 4-6 worms were grouped and embedded in each granulomatous gastric tumor, except 1 weasel. Male worms were 25.0×1.4 mm in average size, and had a tail with pedunculate papillae, a spicule, and minute tegumental spines. Females were 40.0×2.5 mm in average size, and had a tail without tegumental spines. Pointed and posteriorly curved hooklets were arranged in 8-10 rows on the head bulb. Tegumental spines were distributed from behind the head bulb to the middle portion of the body. The spines were different in size and shape by the distribution level of the body surface. Fertilized eggs were 65.5×38.9 μm in average size, and had a mucoid plug at 1 pole. These gnathostomes from Jeju weasels were identified as Gnathostoma nipponicum Yamaguti, 1941. By the present study, it was confirmed for the first time that G. nipponicum is distributed in Jeju-do, the Republic of Korea, and the Jeju weasel, M. sibilica quelpartis, plays a crucial role for its definitive host.     

Surface ultrastructure of the adult stage of Acanthotrema felis (Trematoda: Heterophyidae)

The surface ultrastructure of Acanthotrema felis (Trematoda: Heterophyidae) adults, recovered from a kitten experimentally infected with the metacercariae, was observed using a scanning electron microscope. The worm was leaf-like, ventrally concave and covered with scale-like multi-pointed tegumental spines. The spines on the anterior surface were short but broad, and had 10-12 pointed tips. The cytoplasmic processes protruded around the spines, like pockets for the spines. The ventrogenital opening was crescent, or kidney-shaped, and had protuberances with minute spines on its surrounding tegument. The spines on the posterior surface were long, but narrow, with 6-8 pointed tips. The cytoplasmic processes on this tegument were ridge-like, and elevated along the row of the spines. The surface ultrastructure of A. felis is generally similar to that of other heterophyid flukes, but some features are characteristic, and may be of taxonomic and bio-ecological significance.     

Centrocestus formosanus (Digenea: Heterophyidae) encysted in the freshwater fish, Puntius brevis, from Lao PDR

The metacercariae of Centrocestus formosanus, a minute intestinal trematode of mammals and birds, were detected in the freshwater fish, Puntius brevis, from Vientiane Municipality, Lao PDR. The metacercariae were experimentally fed to mice, and adult flukes were recovered in their small intestines 7 days later. The adult flukes were morphologically characterized by having 32 (rarely 34) circumoral spines arranged in 2 alternative rows, a large bipartite seminal vesicle, an oval-shaped ovary, and an X-shaped excretory bladder. Based on these characters, the adults were identified as Centrocestus formosanus (Nishigori, 1924). The taxonomic significance of C. formosanus, in relation to a closely related species, C. caninus (Leiper, 1913), is briefly discussed. It has been first verified by adult worm recovery that C. formosanus is prevalent in Vientiane areas of Lao PDR, taking the freshwater fish, P. brevis, as a second intermediate host.     

Fishborne Trematode Metacercariae in Luang Prabang,Khammouane, and Saravane Province,Lao PDR

Fishborne trematode (FBT) metacercariae were investigated in fish from 3 Provinces of Lao PDR. Total 242 freshwater fish of 40 species were collected in local markets of Luang Prabang (59 fish of 16 species), Khammouane (81 fish of 19 species), and Saravane (97 fish of 14 species), and each of them was examined by artificial digestion method. Four species of metacercariae (Opisthorchis viverrini, Haplorchis taichui, Haplorchis yokogawai, and Centrocestus formosanus) were detected. O. viverrini was detected in 35 fish (14.5%), and their density was 252 per infected fish (Luang Prabang, 88 metacercariae in 5 fish; Khammouane, 187 in 6 fish; Saravane, 303 in 24 fish). H. taichui was found in 102 fish (42.1%), and their density was 485 per infected fish (Luang Prabang, 260 metacercariae in 38 fish; Khammouane, 1,084 in 23 fish; Saravane, 359 in 41 fish). H. yokogawai was detected in 92 fish (38.0%), and their density was 222 per infected fish (Luang Prabang, 362 metacercariae in 17 fish; Khammouane, 126 in 20 fish; Saravane, 214 in 55 fish). Metacercariae of C. formosanus were found in 8 fish (3.3%), and their density was 3 per infected fish. In the present study, it has been confirmed that FBT metacercariae, in particular, H. taichui, H. yokogawai, and O. viverrini, are highly prevalent in fish from Luang Prabang, Khammouane, and Saravane Province, Lao PDR.