Morphological and molecular identification of two Paragonimus spp., of which metacercariae concurrently found in a land crab, Potamiscus tannanti, collected in Yenbai Province, Vietnam

Morphological and Molecular Characterization of Paragonimus skrjabini Complex from Yunnan, China: A Brief Report

Purpose

To perform environmental sampling and molecular identification of Paragonimus in endemic regions, which may help in minimizing transmission among humans.

Methods

Mountain crabs from the genus Potamiscus were collected and the encysted metacercariae were extracted and subjected to morphological identification, followed by animal inoculation in Sprague–Dawley (SD) rats. After 112 days of infection, animals were killed and adult worms were extracted from lungs and muscles. The morphology of adult worms was characterized by microscopy and molecular identification was done by polymerase chain reaction, followed by sequencing of cox1 and ITS2 genes. Phylogenetic analysis was done by maximum parsimony method.

Results

A total of 447 crabs were captured from the streams of Tongchang Town, Jinping County, Yunnan Province, China. The infection rate was found to be 41% (186 out of 447 crabs). The metacercariae of Paragonimus skrjabini was identified by the characteristics round or spherical encysted form measuring 410 to 460 × 400 to 460 µm. After animal infection in SD rats, adults were presumptively confirmed to be P. skrjabini, which was also confirmed by gene amplification and sequence analysis of cox1 and ITS2 regions. Paragonimus skrjabini clustered with previously reported P. skrjabini from Yunnan and Vietnam. The confidence values of their branches were > 95%. Phylogenetic analysis of the ITS2 region revealed two distinct clusters with distinct geographical grouping. Phylogenetic analysis with the combined data sets reiterated the geographical grouping with P. skrjabini from Yunnan clustering with strains from Vietnam.

Conclusion

Metacercariae of P. skrjabini was discovered in freshwater crabs in Yunnan province, China, and the strains were phylogenetically related to P. skrjabini from Vietnam.

Sporadic endemicity of zoonotic Paragonimus in raccoon dogs and Japanese badgers from Miyazaki Prefecture, Japan

Paragonimiasis is a zoonotic trematode infection caused by Paragonimus spp. To determine the recent status of Paragonimus infections in wild animals, this study investigated Paragonimus spp. in 39 raccoon dogs and 54 Japanese badgers from March 2019 to January 2021 in Miyazaki Prefecture, and examined metacercariae in freshwater crabs. Triploid P. westermani was found in one raccoon dog (2.6%), and metacercariae were recovered from Eriocheir japonica captured near the infected animal collected. One Japanese badger (1.9%) harbored P. skrjabini miyazakii; this prevalence was lower than the approximately 30% that was reported in the 1970s. Results indicated that zoonotic Paragonimus was sporadically prevalent in wild animals. Further investigation in various animals is awaited to elucidate current wildlife reservoirs for those Paragonimus.

Insights on foodborne zoonotic trematodes in freshwater snails in North and Central Vietnam

Foodborne zoonotic trematode (FZT) infections are common neglected tropical diseases in Southeast Asia. Their complicated life cycles involve freshwater snails as intermediate hosts. A cross-sectional study was conducted in Yen Bai and Thanh Hoa provinces in North and Central Vietnam, to investigate the diversity of cercariae of potential FZT and to construct the phylogenetic relationship of trematode cercariae based on the Internal Transcribed Spacer 2 (ITS2) region. Among 17 snail species collected from various habitats, 13 were infected by 10 cercarial groups among which parapleurolophocercous, pleurolophocercous, and echinostome cercariae were of zoonotic importance. The monophyletic tree separated cercarial sequences into different groups following the description of the cercariae families in which Haplorchidae, Opisthorchiidae, Echinochasmidae, and Echinostomatidae are important families of FZT. The overall prevalence was different among snail species and habitats and showed a seasonal trend. Parapleurolophocercous and echinostome cercariae emerged as the most common cercariae in snails in Yen Bai, while monostome, echinostome, and megalura cercariae were most common in Thanh Hoa. Using a molecular approach, we identified Parafossarulus striatulus as the first intermediate snail host of Clonorchis sinensis in Thac Ba Lake. Melanoides tuberculata and Bithynia fuchsiana were we identified preferred intermediate snail hosts of a diverse range of trematode species including intestinal flukes (i.e., Haplorchis pumilio and Echinochasmus japonicus) in Yen Bai and Thanh Hoa, respectively.

Molecular identification of morphologically similar microcercous cercariae of two trematode families, Paragonimidae and Troglotrematidae, concurrently found in the same snail species of the subfamily Triculinae

Microcercous cercariae possess a very short tail and are produced by digenean species of several families including medically important species, such as members of the genera Paragonimus Braun, 1899, Nanophyetus Chapin, 1927 and Troglotrema Odhner, 1914. During our survey of cercariae of Paragonimus spp. in Vietnam, we found microcercous cercariae from ten (0.29%) out of 3,400 snails of Triculinae gen. sp. 2. They were morphologically and molecularly analysed for species identification. The molecular analysis, based on ITS2 sequences, revealed two distinct species: four specimens were identical to Paragonimus proliferus Hsia et Chen, 1964 (Paragonimidae Dollfus, 1939), and the other six specimens were closest to members of the family Troglotrematidae Odhner, 1914 and were temporarily named Troglotrematidae gen. sp. Morphologically, cercariae of the two species found in this study are similar to each other in their gross characteristics but can be distinguished from one another by subtle morphological details. The cercaria of P. proliferus has an I-shaped excretory bladder and does not have mucous gland cells. In contrast, that of Troglotrematidae gen. sp. has a Y-shaped excretory bladder and mucous gland cells. Besides, the redia of P. proliferus is elongate with a short intestine and contains 5-6 cercariae whereas that of Troglotrematidae gen. sp. is more round with a longer intestine and harbours 3-4 cercariae. Our results have shown the importance of the shape of the excretory bladder and the presence/absence of mucous gland cells of the cercaria as well as the shape and size of the redia, and its intestinal length as valuable taxonomic characters of intramolluscan trematode larvae. In addition, the finding of similar microcercous cercariae of different species in the same snail species suggests that careful attention to morphological details is required in the differentiation of Paragonimus cercariae and those of closely related species.

Infection with Paragonimus westermani of boar-hunting dogs in Western Japan maintained via artificial feeding with wild boar meat by hunters

Infection of boar-hunting dogs with Paragonimus westermani was investigated in Western Japan. Blood and rectal feces were collected from 441 dogs in the three districts (205 in Kinki, 131 in Chugoku and 105 in Shikoku District). In a screening ELISA for serum antibody against P. westermani antigen, 195 dogs (44.2%) showed positive reaction. In the 195 dogs, 8 dogs were found excreting P. westermani eggs after molecular analysis of fecal eggs, and additional 7 were identified serologically for the parasite infection because of their stronger reactivity against P. westermani antigen than against antigens of other species of Paragonimus. A spatial analysis showed that all of the P. westermani infections were found in Kinki and Chugoku Districts. In this area, dogs’ experience of being fed with raw boar meat showed high odds ratio (3.35) to the sero-positivity in the screening ELISA, and the frequency of such experiences was significantly higher in sero-positive dogs. While clear relationship was not obtained between predation of boars by dogs during hunting and their sero-positivity. Therefore, it is suggested that human activity of feeding with wild boar meat is the risk factor for P. westermani infection in boar-hunting dogs. Considering that hunting dogs could play as a major definitive host and maintain the present distribution of P. westermani in Western Japan, control measures for the infection in hunting dogs, such as prohibition of raw meat feeding and regular deworming, should be undertaken.

Molecular and morphological variation of Paragonimus westermani in Vietnam with records of new second intermediate crab hosts and a new locality in a northern province

Paragonimus westermani is one of the most medically important lung flukes and is widely distributed in Asia. It exhibits considerable variation in morphological, genetic and biological features. In central provinces of Vietnam, a high prevalence of metacercariae of this species has been reported from the crab intermediate host, Vietopotamon aluoiense. In this study, we detected P. westermani metacercariae in two additional crab hosts, Donopotamon haii in Quang Tri Province, central Vietnam and Indochinamon tannanti in Yen Bai Province in the north. The latter is a new locality for P. westermani in a northern region of Vietnam where P. heterotremus is the only species currently known to cause human paragonimiasis. Paragonimus westermani metacercariae found in Vietnam showed considerable morphological variation but slight genetic variation based on DNA sequences from the nuclear ribosomal ITS2 region and the mitochondrial 16S gene. Co-infection of the same individual crabs with P. westermani and P. heterotremus and/or some other Paragonimus species was found frequently, suggesting potential for co-infection in humans. The findings of the present study emphasize the need for highly specific molecular and immunodiagnostic methods to differentially diagnose between P. westermani and P. heterotremus infections.

Molecular identification of the trematode Paragonimus in faecal samples from the wild cat Prionailurus bengalensis in the Da Krong Nature Reserve, Vietnam

Conventional identification of Paragonimus species and their natural definitive hosts is based on the morphological features of adult parasites isolated from the lungs of wild mammalian hosts. However, wild animals are protected by strict regulations and sampling is not always possible. Recently, molecular techniques have been developed to identify the internal transcribed spacer (ITS) sequences of Paragonimus eggs in faeces/sputum of human patients. Also, mammalian hosts can be identified using the D-loop sequence of mitochondrial DNA in faecal samples. In this study, we used molecular techniques on faeces from wild animals collected in Da Krong Nature Reserve, Quang Tri province, central Vietnam, where Paragonimus metacercariae are highly prevalent in mountain crabs, to identify Paragonimus species and their natural definitive hosts. The results indicated that wild cats, Prionailurus bengalensis, were infected with at least three different Paragonimus species, P. westermani, P. skrjabini and P. heterotremus. Because all of these species can infect humans in Asian countries, human paragonimiasis should be considered in this area.

Molecular characterization of Stictodora tridactyla (Trematoda: Heterophyidae) from Kuwait Bay using rDNA ITS and mtCO1

Stictodora tridactyla is an intestinal fluke in the family Heterophyidae that parasitizes shorebirds and mammals, including humans. Its metacercarial cyst stage was reported in the Arabian killifish, Aphanius dispar, at Kuwait Bay. In the present study, Cerithidea cingulata was found to serve as the first intermediate host of S. tridactyla. In order to establish the snail-fish link in the life cycle of S. tridactyla, complete sequences of ribosomal DNA internal transcribed spacer region 1 and 2 (rDNA ITS1 and ITS2) and partial sequence of cytochrome oxidase subunit 1 were obtained for metacercarial cysts isolated from the fish A. dispar and rediae isolated from the snail C. cingulata. Sequence alignment demonstrated that these larval stages belong to the same heterophyid species, S. tridactyla. Phylogenetic analysis based on rDNA ITS1, ITS2, and mtCO1 confirmed the position of S. tridactyla within the Heterophyidae and found it to cluster with Haplorchis spp. The present study represents the first molecular study correlating the larval stages of S. tridactyla using rDNA ITS1, ITS2, and mtCO1 and examining the phylogenetic relationships of S. tridactyla with different heterophyid species.

Evaluation of the MGL method to detect Paragonimus eggs and its improvement

Dog feces containing 500 Paragonimus westermani eggs per gram were examined by the Medical General Laboratory (MGL), the simple sedimentation (SS), and the Army Medical School III (AMS III) methods. The number of eggs per gram of feces (EPG) obtained by the MGL method was 17.2 and was significantly lower than those obtained by the SS method (324.0) and the AMS III method (505.6). When isolated P. westermani eggs were processed by the MGL method and four layers (ether, ether-fecal, formalin layers, and sediment) of the final centrifugation product were separately examined, almost 100% of eggs were found at the ether-fecal layer. Similarly, when fecal samples containing P. westermani, Paragonimus skrjabini miyazakii, Paragonimus ohirai, or Paragonimus harinasutai eggs were processed by the MGL method, more than 95% of the eggs were found in the supernatant layers. The formalin-ethyl acetate (FEA) method showed a similar tendency as the MGL method and over 90% of eggs remained in the supernatant layers. Contrary to Paragonimus eggs, 63 and 96% of Clonorchis and Metagonimus eggs were found in the sediment in the MGL method, respectively. When surfactant (Tween 80) was added to fecal solution, most of Paragonimus eggs spun down in the sediment in the MGL and FEA methods, suggesting that Paragonimus eggs have hydrophobic components on their surface. It is suggested that surfactant addition to the fecal solution should be considered when the MGL method is used for detection of Paragonimus eggs.

Paragonimus paishuihoensis metacercariae in freshwater crabs, Potamon lipkei, in Vientiane Province, Lao PDR

Among Paragonimus species, P. paishuihoensis is one of the most mysterious and poorly understood species. Metacercariae are characterized by having a unique dendritically branched excretory bladder. However, the morphology of the adult worm remains unknown. To date, metacercariae of this species have been reported only in China and Thailand. In this study, we first found P. paishuihoensis metacercariae in freshwater crabs, Potamon lipkei, in Hinheub District, Vientiane, Lao PDR, with a prevalence of 77.7% and the average intensity of 10.3 (range 1-28) metacercariae per crab. The molecular data based on ITS2 and CO1 markers indicated that P. paishuihoensis from Laos and Thailand were almost completely identical and were close to members of the Paragonimus bangkokensis/Paragonimus harinasutai complex. Attempts to infect experimental animals (cats, dogs, and rats) with P. paishuihoensis were unsuccessful, suggesting that these animals might be unsuitable definitive hosts for the species. Further studies are necessary to elucidate the taxonomic status and life cycle of P. paishuihoensis.

Molecular variation in the Paragonimus heterotremus complex in Thailand and Myanmar

Paragonimiasis is an important food-borne parasitic zoonosis caused by infection with lung flukes of the genus Paragonimus. Of the 7 members of the genus known in Thailand until recently, only P. heterotremus has been confirmed as causing human disease. An 8th species, P. pseudoheterotremus, has recently been proposed from Thailand, and has been found in humans. Molecular data place this species as a sister species to P. heterotremus, and it is likely that P. pseudoheterotremus is not specifically distinct from P. heterotremus. In this study, we collected metacercariae of both nominal species (identification based on metacercarial morphology) from freshwater crabs from Phetchabun Province in northern Thailand, Saraburi Province in central Thailand, and Surat Thani Province in southern Thailand. In addition, we purchased freshwater crabs imported from Myanmar at Myawaddy Province, western Thailand, close to the Myanmar-Thailand border. The DNAs extracted from excysted metacercariae were PCR-amplified and sequenced for ITS2 and cox1 genes. The ITS2 sequences were nearly identical among all samples (99-100%). Phylogenies inferred from all available partial cox1 sequences contained several clusters. Sequences from Indian P. heterotremus formed a sister group to sequences from P. pseudoheterotremus-type metacercariae. Sequences of P. heterotremus from Thailand, Vietnam, and China formed a separate distinct clade. One metacercaria from Phitsanulok Province was distinct from all others. There is clearly considerable genetic variation in the P. heterotremus complex in Thailand and the form referred to as P. pseudoheterotremus is widely distributed in Thailand and the Thai-Myanmar border region.

Metacercarial polymorphism and genetic variation of Paragonimus heterotremus (Digenea: Paragonimidae), and a re-appraisal of the taxonomic status of Paragonimus pseudoheterotremus

Paragonimus heterotremus, which is an important pathogen for human paragonimiasis in Asia, is recognized as having the smallest metacercariae (maximum diameter < 300 μm) of any previously reported Paragonimus species. Recently, P. pseudoheterotremus has been described from Thailand as a new species having metacercariae (about 200 μm) slightly smaller than those of Thai P. heterotremus. In fact, the small size of P. pseudoheterotremus metacercariae is compatible with those of P. heterotremus from India and China. In this study in Vietnam, we found variably sized small metacercariae which are expected to consist of both P. heterotremus and P. pseudoheterotremus. Contrary to expectation, the adult flukes obtained by separate infection of experimental cats with different sized metacercariae were all identified as P. heterotremus, using both morphological and molecular characteristics. The molecular analyses of an extensive collection of P. heterotremus/P. pseudoheterotremus isolates from Asian countries also indicated that genetic distances between different populations of P. heterotremus are even larger than that between P. pseudoheterotremus and P. heterotremus. The haplotype network showed that all P. heterotremus and P. pseudoheterotremus isolates formed a P. heterotremus complex consisting of three groups with strong geographical origins. In addition, the Indian P. heterotremus group is the root of the other P. heterotremus and P. pseudoheterotremus populations. Based on the observed metacercarial polymorphisms and genetic variation in P. heterotremus,P. pseudoheterotremus should be considered a geographically isolated population of the P. heterotremus complex.

Postmonorchis sp. inq. (Digenea: Monorchiidae) metacercariae infecting natural beds of wedge clam Donax trunculus in Italy

The wedge clam Donax trunculus Linnaeus, 1758 is one of the most common bivalve molluscs inhabiting the sandy shores of the Mediterranean Sea and is considered an important commercial resource. In this study, we report the first molecular, morphological and histopathological descriptions of metacercariae from a trematode belonging to the genus Postmonorchis (Digenea: Monorchiidae) that infects D. trunculus in natural beds of the Italian Tyrrhenian coast (Campania, Lazio and Tuscany). Morphological analysis of the parasite revealed a combination of features that exist in the 3 previously identified species of Postmonorchis, viz. P. donacis, P. variabilis and P. orthopristis, with the addition of new, distinctive morphological characteristics. The pathogen exhibited a predilection for the gill; however, it was also present in the labial palp and mantle in addition to the gut, kidney epithelium and foot. The inflammatory response was characterised by either a focal or diffuse haemocyte infiltration followed by the formation of multiple, large multi-layered capsules associated with tissue destruction. The prevalence of the pathogen ranged from 75 to 100%, while the infection intensity fluctuated among the study areas. Further studies regarding the life cycle of this parasite and the identification of other larval and adult stages and their respective hosts may confirm the identification of a new species of Postmonorchis that infects wedge clams in Mediterranean waters. The study of the parasite is completed by molecular analysis of the ITS1 and ITS2 rDNA sequences.

Detection of multiple species of human Paragonimus from Mexico using morphological data and molecular barcodes

Paragonimus mexicanus is the causal agent of human paragonimiasis in several countries of the Americas. It is considered to be the only species of the genus present in Mexico, where it is responsible for human infection. Through the investigation of P. mexicanus specimens from several places throughout Mexico, we provide morphological, molecular and geographical evidence that strongly suggests the presence of at least three species from this genus in Mexico. These results raise questions regarding the diagnosis, treatment, prophylaxis and control of human paragonimiasis in Mexico. We also provide a brief discussion regarding biodiversity inventories and the convenience of providing molecular and morphological information in biodiversity studies.

Molecular identification of Paragonimus species by DNA pyrosequencing technology

DNA pyrosequencing for PCR amplicons is an attractive strategy for the identification of microorganisms because of its short time performance for large number of samples. In this study, the primers targeting the fragment of ITS2 region of nuclear ribosomal RNA gene were newly developed for pyrosequencing-based identification of 6 Paragonimus species, Paragonimus bangkokensis, Paragonimus harinasutai, Paragonimus heterotremus, Paragonimus macrorchis, Paragonimus siamensis and Paragonimus westermani. Pyrosequencing determination of 39 nucleotides of partial ITS2 region could discriminate 6 Paragonimus species, and could also detect intra-species genetic variation of P. macrorchis. This DNA pyrosequencing-based identification can be a valuable tool to improve species-level identification of Paragonimus in the endemic areas.

Discovery of Paragonimus skrjabini in Vietnam and its phylogenetic status in the Paragonimus skrjabini complex

Two members of the Paragonimus skrjabini complex, P. skrjabini and P. miyazakii, are now considered as two sub-species, P. skrjabini skrjabini and P. skrjabini miyazakii. They are well known as important pathogens for human paragonimiasis in China and Japan. Recently, members of this species complex have been reported from India. Here we report the first discovery of P. skrjabini from freshwater crab hosts in Thanh Hoa province, Vietnam. For morphological and molecular phylogenetic studies, adult worms were obtained by experimental infection in cats and dogs. Molecular analyses of metacercariae and adults revealed that the P. skrjabini population from Thanh Hoa, Vietnam was almost completely identical with that from Yunnan province, China. Those populations from Thanh Hoa, Vietnam and Yunnan, China and those from Manipur, India were significantly different from P. skrjabini populations reported from other localities of China in cytochrome oxidase subunit 1 (CO1) gene sequences, indicating considerable genetic variation within the P. skrjabini complex. Moreover, low bootstrap values in the CO1 tree suggested that more variant genotypes belonging to P. skrjabini complex may be found in other Asian countries in between Vietnam and India, such as Myanmar, Laos and Thailand. Since P. skrjabini is known as a pathogen for humans, paragonimiasis cases caused by P. skrjabini might be found in Vietnam and other Asian countries.

Presence of three distinct genotypes within the Paragonimus westermani complex in northeastern India

The name Paragonimus westermani (Kerbert, 1878) is commonly applied to members of a species complex that includes the well-known Asian lung fluke of medical and veterinary importance. Unambiguous molecular and morphological evidence showing the presence of a member of the complex in India has recently been published. In the present study we report the occurrence of 2 more members of the P. westermani complex in northeastern (NE) India. Surveys of the freshwater crabs Maydelliatelphusa lugubris in NE India revealed 2 morphologically distinct types of lung fluke metacercariae. Phylogenetic analyses, using DNA sequences from ITS2, 28S and cox1 gene regions indicate that these lung metacercariae belong to P. westermani complex. Type 1 metacercariae have a more basal position within the complex whereas type 2 metacercariae are closely related to the relatively derived forms of P. westermani from NE Asia (Japan, Korea, China) and Vietnam. A third type of metacercaria (type 3), detected in another crab host, Sartoriana spinigera in Assam, was phylogenetically close to P. siamensis, also a member of the P. westermani group. Molecular evidence has demonstrated the existence of 3 genotypes of lung flukes within the Paragonimus westermani complex in NE India. Two of these were previously unknown.

Genetically variant populations of Paragonimus proliferus Hsia & Chen, 1964 from central Vietnam

Among about 50 nominal Paragonimus species, Paragonimus proliferus is rather a rare species, found only in Yunnan province, China, until our recent discovery of this species in Lai Chau province, northern Vietnam close to Yunnan, China. Here we add Quang Binh province, central Vietnam as a new endemic area of P. proliferus. Large excysted metacercariae found in mountainous crabs, Potamiscus tannanti, were morphologically identified as P. proliferus, which was confirmed further by molecular analyses. Second internal transcribed spacer (ITS2) sequences of the P. proliferus population in Quang Binh province were completely (100%) identical with those of P. proliferus populations in Lai Chau province, northern Vietnam and Yunnan province, China. However, cytochrome oxidase subunit 1 (CO1) gene sequences of Quang Binh population were significantly different (5.6%) from that of previously reported northern Vietnam and Yunnan, China populations. A phylogenetic tree revealed that all CO1 sequences of P. proliferus Quang Binh population formed a distinct group, which was clustered with northern Vietnam and Yunnan, China populations with the bootstrap value of 75%. This is the first record of the genetically variant population of P. proliferus, distribution of which is geographically remote from the previously reported endemic areas in the border between northern Vietnam and Yunnan, China, suggesting that P. proliferus may be much more widely distributed in the Indochina peninsula (or South-East Asia) than expected.

Sensitive and rapid detection of Paragonimus westermani infection in humans and animals by loop-mediated isothermal amplification (LAMP)

In the present study, a loop-mediated isothermal amplification (LAMP) assay was developed and validated for the detection of Paragonimus westermani adults, metacercariae, and eggs in human and animal samples. The LAMP amplification can be finished in 45 min under isothermal condition at 60°C by employing a set of four species-specific primer mixtures and the results can be checked by naked-eye visualization. No amplification products were detected with deoxyribunucleic acid (DNA) of related trematode species including Fasciola hepatica, Fasciola gigantica, Clonorchis sinensis, Opisthorchis viverrini, Schistosoma mansoni, and Schistosoma japonicum. The method was further validated by examining P. westermani DNA in intermediate hosts including freshwater crabs and crayfish, as well as in sputum and pleural fluid samples from patients of paragonimiasis. These results indicated that the LAMP assay was highly specific, sensitive, and rapid, and it was approximately 100 times more sensitive than conventional specific PCR. The LAMP assay established in this study provides a rapid and sensitive tool for the detection of P. westermani DNA in freshwater crabs, crayfish, sputum, and pleural fluid samples, which has important implications for effective control of human paragonimiasis.

Description of a new lung fluke species, Paragonimus vietnamensis sp. nov. (Trematoda, Paragonimidae), found in northern Vietnam

Based on morphology of metacercariae and adult worms together with molecular data from our previous study, we describe herein a new lung fluke species named Paragonimus vietnamensis sp. nov. Metacercariae of this new species is round and extremely large in size (nearly 800 mum in diameter) having a fragile outer cyst of variable thickness and a thin fragile inner cyst. There are little or no space between metacercaria and the inner cyst wall. These and other morphological features of metacercariae of P. vietnamensis sp. nov. are not completely identical with but have some similarities with those of P. microrchis, P. proliferus, or P. menglaensis. On the other hand, adult worms obtained by experimental infection of these large metacercariae are oval in shape, having a ventral sucker slightly larger than the oral one, and having singly arranged relatively short cuticular spines. These morphological features of adult worms are partly similar to but not identical with those of P. microrchis, P. skrjabini, P. yunnanenis, P. xiangshanensis, or P. harinasutai. Taken these morphological data and our previous molecular phylogenetic analyses of ITS2 and CO1 sequences of metacercariae and adult worms together, P. vietnamensis sp. nov. is different from any other known Paragonimus spp.

Paragonimus and paragonimiasis in Korea

The lung fluke, Paragonimus, and lung fluke disease, were reviewed, especially on the works performed in Korea. Among 43 species of Paragonimus recorded in the world literature, P. westermani, P. pulmonalis, and P. iloktsuenensis are known to distribute in Korea. Biological studies on P. westermani have revealed that its snail intermediate host is Semisulcospira spp. and major second intermediate hosts are crabs such as Eriocheir japonicus and E. sinensis or crayfish such as Cambaroides similis and C. dauricus. The final hosts other than man are dogs, cats, pigs, and wild animals. Paragonimiasis has been known to distribute widely over the south Korea, however, the prevalence among people and intermediate hosts is gradually decreasing in recent years. In Korean people, the lung fluke have caused pulmonary infections in a great many of cases but as well extrapulmonary ones including cerebral, spinal, subcutaneous, hepatic, splenic, abdominal, urinary, gynecologic, and other types. The definite diagnosis of paragonimiasis can be executed by the recovery of eggs from sputum or feces of patients, however, immunological methods such as intradermal test, complement-fixation test, precipitin reactions, immunofluorescent techniques, enzyme-linked immunosorbent assay (ELISA) are greatly helpful for egg-negative or extrapulmonary cases. Various drugs have been introduced for the treatment of paragonimiasis, and among them, bithionol, niclofolan and praziquantel have shown high therapeutic efficacy. The most recommendable one is considered praziquantel. For control of this disease, mass chemotherapy of patients seems to be the most efficient and feasible measure.