—209—

The Korean Journal of Parasitology Vol. 39, No. 3, 209-221, September 2001

Mini-Review

Trematodes of the family : a minireview

Sandie KING1),2) and Tomá˘s SCHOLZ1)*

Institute of Parasitology1), Academy of Sciences of the Czech Republic, Brani˘sovská 31, 37005 Ceské˘ Bud˘ejovice, Czech Republic, and Institute of Biomedical and Life Sciences2), University of Glasgow, Glasgow, Scotland, UK, G12 8QQ

Abstract: Examination of the original descriptions of genera placed within the family Opisthorchiidae has revealed that only 33 of the original 43 genera are valid members of this family. Further study of these descriptions should also reveal that many of the subfamilies are also invalid. With reference to the original descriptions of these genera, and subsequent literature, a brief survey of the family has been compiled. Information on the spectrum of definitive hosts that these helminths parasitize is provided, as well as information on the life cycles, geographic distribution, and socioeconomic impacts of the family. More in-depth information is given on those that are of particular medical importance; namely, , viverrini, and O. felineus. The final aims of this review are to provide information on the entire genera of the family Opisthorchiidae, which will aid understanding of the phylogenetic relationships not only within the family, but also within the Class . Key words: Opisthorchiidae, liver flukes, classification, survey of genera, spectrum, distribution

humans (Yamaguti, 1971). INTRODUCTION The family Opisthorchiidae has been studied fairly exhaustively in a number of monographs The liver flukes () of the genera or comprehensive volumes: Skrjabin and Clonorchis Looss, 1907 and Opisthorchis Petrov (1950), Yamaguti (1958, 1971, and Blanchard, 1895 are notorious as causative 1975). In these works, the family was thought agents of human transmitted by to include approximately 50 genera, . The disease clonorchiasis, and the closely encompassing several hundred species; related opisthorchiasis, annually infect however, the lack of more recent research has millions of people in regions where raw or brought the taxonomic status and validity of undercooked fish harbouring opisthorchiid these genera into question. Also, as no new metacercariae are consumed, particularly in data have been compiled recently as a specific Southeast and circumpolar parts of condensed volume, very little is known about Eurasia and . However, the the family as a whole. The species compos- family Opisthorchiidae also includes numer- ition, geographical distribution, life cycles and ous genera whose members rarely infect phylogenetic relationships within the family have all been insufficiently examined. It is the purpose of this review, therefore, to ·Received 14 August 2001, accepted after give a brief summary of the family as a whole revision 27 August 2001. and to provide information concerning the *Corresponding author (e-mail: tscholz@paru. cas.cz) of the family in an attempt to —210— eliminate invalidities and synonyms amongst fication used by Brooks et al. (1985) are not genera. Information will also be provided on reliable enough to base the phylogenetics of the host spectrum, life cycles, geographic the digeaneans on. After his analysis of the distribution, and socioeconomic factors of the characters used by Brooks et al. (1985), he family. states that 97 of the 212 characters used in the cladistic classification of the Digenea were CLASSIFICATION AND TAXONOMY invalidly used, resulting in the homoplasy increasing by 18% to 37%, with an accom- The main problem with classifying any panying decrease in resolution from 82% to group of digenetic trematodes is the fact that 32%. they have complex life cycles involving 2-4 Cribb et al. (2001) have attempted to further hosts and many different larval stages. Thus, unravel some of the mysteries surrounding there are many morphologically different forms digenean evolution and expand on such on which taxonomy can be based. studies of Brooks et al. (1985) and Pearson Yamaguti (1971) refers to his previous clas- (1992), among others, in order to resolve some sifications and, indeed, those of others, when of the many inconsistencies that can be found he maintains that the adult morphology of in this area of study. They have attempted to digeneans is the best method of classification. do this by combining many data sets, However, he also states: “We must be content including those containing morphological and for the present with an artificial classification molecular characters, to provide a truer based chiefly on adult morphology.” It is with representation of the relationships between this statement in mind that another school of digenean families. The use of combined data thought has arisen, which many believe is a sets can be controversial in itself, as outlined much more accurate means of classification. It by de Queiroz et al. (1995) who have reviewed involves the examination of cercariae, as these the arguments both for and against the use of are believed to be recapitulations of ancestral combined data sets in phylogenetic analyses. forms, and thus enable the production of a Although the studies mentioned above have more phylogenetically accurate classification. used extensive phylogenetic analysis in order It is considered that the use of cercarial forms to try and resolve the relationships between all also eliminates the possibility of misidenti- the families within the Digenea, such work is fication through convergent evolution (Brooks important when individual families are re- et al. 1985). examined in order to validate subfamilies and Brooks et al. (1985) went some way towards genera. In essence, when classifying addressing the issues concerned with the trematodes within a particular family, in this classification of digeneans and have carried case the Opisthorchiidae, it is the species that out an extensive study on the phylogenetic are closely related enough to be included relationships of the entire Digenea. Using both within the same family, subfamily, and adult morphology and larval forms, they have that are being identified. The taxonomic state attempted to provide a starting point for future of the entire class or subclass being studied phylogenetic studies on this diverse group, plays an important role in such a re- which, they hope, will grow into a compre- examination as closely related families can hensive and increasingly accurate method of provide information on the composition of the classification. However, the use of evidence family of interest. from both adult and cercarial forms seems to Perhaps still considered as the most further outline the prevalence of conflicting extensive attempt to form a complete theories concerning the classification of this classification of digenetic trematodes was that group. Until such inconsistencies can be of Yamaguti (1971) (with the life cycles treated overcome to a greater extent, the Digenea and separately in 1975). Although three decades methods used to classify them seem destined old, the material in these volumes is still to remain under scrutiny. Indeed, Pearson referred to as one of the most comprehensive (1992) argues that the methods of classi- studies of digeneans, and for this review, these —211— volumes were a starting point from which to The two genera found not to belong to the source the original descriptions of members of Opisthorchiidae are thought to be synonymous the Opisthorchiidae. Subsequently, it was with one another, and, as indicated by their these original descriptions which formed the morphology, belong to the family Acantho- basis of the reclassification reported in this stomidae (subfamily Acanthostominae of the study. Cryptogominidae according to some authors) Firstly, by referring to papers containing the as originally reported. Thus, with all of the original descriptions of each genus, 11 genera above genera omitted from the family, it is can be omitted from the family on the basis of proposed that the family Opisthorchiidae being invalid or not belonging to the contain only 33 genera, as listed below: Opisthorchiidae at all: Valid genera of Opisthorchiidae Invalid genera Agrawalotrema Sahay & Sahay, 1988 Allometorchis Baer, 1943 Allogomtiotrema Yamaguti, 1958 Synonym of Metametorchis Morozov, 1939 — Amphimerus Barker, 1911 see Skrjabin & Petrov (1950) Cladocystis Poche, 1926 Assamia Gupta, 1955 Clonorchis Looss, 1907 Preoccupied by Assamia Sorensen, 1884 Cyclorchis Luhe, 1908 (Arachnida) and Assamia Buckton, 1896 Delphinicola Yamaguti, 1933 (Insecta) — Lakshminarayana & Hafeezullah Diasiella Travassos, 1949 (1974) Erschoviorchis Skrjabin, 1945 Satyapalia nom. nov. proposed by Euamphimerus Yamaguti, 1941 Lakshminarayana & Hafeezullah (1974) Evranorchis Skrjabin, 1944 Brachymetra Stossich, 1904 Gomtia Thapar, 1930 Preoccupied by Brachymetra Mayr, 1865 Hepatiarius Fedzullaev, 1961 (Insecta) — Poche (1926) Holometra Looss, 1899 Synonym of Ratzia Poche, 1926 Metametorchis Morozov, 1939 Diasia Travassos, 1922 Metorchis Looss, 1899 Preoccupied by Diasia Travassos, 1923 — Microtrema Kobayashi, 1915 Yamaguti (1958) Nigerina Baugh, 1958 Synonym of Diasiella Travassos, 1949 Oesophagicola Yamaguti, 1933 Gomtiotrema Gupta, 1955 Opisthorchis Blanchard, 1895 Preoccupied (Yamaguti 1958, 1971) Pachytrema Looss, 1907 Allogomtiotrema nom. nov. Proposed by Parametorchis Skrjabin, 1913 Yamaguti (1958) Paropisthorchis Stephens, 1912 Minuthorchis Linton, 1928 Plotnikovia Skrjabin, 1945 Synonym of Pachytrema Looss, 1907 — Pseudamphimerus Gower, 1940 Yamaguti (1958, 1971) Pseudamphistomum Luhe, 1908 Multivitellaria Phadke & Gulati, 1929 Pseudogomtiotrema Gupta & Jain, 1991 Synonym of Pachytrema Looss, 1907 — Ratzia Poche, 1926 Yamaguti (1958, 1971) Satyapalia Lakshminarayana & Hafeezullah, Notaulus Skrjabin, 1913 1974 Synonym of Opisthorchis Blanchard, 1895 — Thaparotrema Gupta, 1955 Yamaguti (1958, 1971) Trionychotrema Chin & Zhang, 1981 Tubangorchis Skrjabin, 1944 Genera not belonging to Opisthorchi- Witenbergia Vaz, 1932 idae Neotropicotrema Caballero & Caballero, 1975 Secondly, on the basis that characteristics of (Family Acanthostomidae) a less refined nature can be common to one or Perezitrema Baru˘s & Moravec, 1967 more genera, each genus can be allocated to a (Family Acanthostomidae) specific subfamily. Under Yamaguti’s (1971) —212— classification, the above genera would be present; primary excretory pore in cercariae accommodated by no less than 13 subfamilies, extending short distance into tail; dorsoventral many of which are represented by only one or fin fold present on cercarial tail; small, two genera (number of genera in parentheses): generally less than 40 µm long; eggs ingested and hatch in molluscan host; no cirrus sac; no Subfamilies of Opisthorchiidae cirrus. Allogomtiotrematinae (Gupta, 1955) Yamaguti, Order Opisthorchiiformes also includes 1958 (2) family Ward, 1917 and Aphallinae Yamaguti, 1958 (1) family Odhner, 1914. Delphinicolinae Yamaguti, 1933 (1) Family Opisthorchiidae Looss, 1899 Diasiellinae Yamaguti, 1958 (1) Diagnosis: as above. Parasitic in pulmonate Metorchiinae Luhe, 1909 (4) and prosobranch gastropods, in fish, and in Oesophagicolinae Yamaguti, 1933 (1) liver, gall bladder, bile duct, and rarely Opisthorchiinae Yamaguti, 1899 (14) intestine of fish, amphibians, reptiles, birds Pachytrematinae (Railliet, 1919) Ejsmont, and mammals. Cosmopolitan. 1931 (1) Clearly, Brooks et al. (1985) have primarily Plotnikoviinae (Skrjabin, 1945) Skrjabin et based their classification on the examination Petrov, 1950 (1) of cercarial stages, whereas Yamaguti (1971) Pseudamphimerinae Skrjabin et Petrov, 1950 relied chiefly on adult morphology. As (3) mentioned in the previous section on Pseudamphistominae Yamaguti, 1958 (2) classification and taxonomy, much controversy Ratziinae (Dollfus, 1929) Price, 1940 (1) still surrounds which of these methods is more Tubangorchiinae Yamaguti, 1958 (1) reliable, and whether or not the use of The number of subfamilies supposedly combined data sets would provide a truer housed within the Opisthorchiidae is in- representation. Indeed, Wykoff et al. (1965) consistent with the number of genera they reported that they were unable to distinguish contain, especially when over half of the above between and O. felineus mentioned subfamilies contain only one genus. using adult or morphology alone, and that Although not having been re-examined for this the only means of separating the two species study, we suggest that future revisions of the was to use the flame patterns of cercaria family will result in the subfamilies being or metacercaria. reduced considerably in number to leave a truer representation. SURVEY OF GENERA Thirdly, the overall classification of the Opisthorchiidae family, as well as being On the basis of the revision of the pertinent covered in Yamaguti’s volumes, was ex- literature of family Opisthorchiidae, we tensively covered by Brooks et al. (1985), with propose that the following genera (housed in specific regard to the phylogeny and adaptive the subfamilies suggested by Yamaguti (1971)) radiation of Platyhelminthes as a constitute a true representation of the family whole. with all invalidities and synonyms removed. Position of Family Opisthorchiidae according Clearly, the family has been greatly reduced in to Brooks et al. (1985): size, now compromising of only 33 genera; Order Opisthorchiiformes La Rue, 1957 however, it is still a cosmopolitan family with Diagnosis: Digenea with oculate cercariae; the species inhabiting various geographical secondary excretory pore in cercariae terminal; areas and having many different hosts. It in adults midventral; spinose should also be noted that this reduction adult body; rediae without appendages; represents the first step in a critical re- cercariae encyst in next host; mesostomate examination of the validity of opisthorchiid cercarial excretory system; cercarial tail not genera, which should be expanded upon to furcate; cercarial excretory bladder lined with include the examination of specimens. epithelium; seminal receptacle —213—

Allogomtiotrematinae Falconidae) Allogomtiotrema Yamaguti, 1958 Type locality: East Prussia (now Poland) and (Synonym Gomtiotrema Gupta, 1955) Europe Type species: Allogomtiotrema attu (Gupta, Metametorchis Morozov, 1939 1955) (syn. Gomtiotrema attu Gupta, 1955) (Synonym Allometorchis Baer, 1943) Type host: Wallagonia attu (: Type species: Metametorchis skrjabini Siluriformes: Siluridae) (Morozov, 1939) (syn. Parametorchis (Meta- Type locality: Gomti River, Lucknow, metorchis) skrjabini Morozov, 1939) (Asia) Type host: Putorius putorius (Mammalia: Satyapalia Lakshminarayana & Hafeezullah, Carnivora: Mustelidae) 1974 Type locality: Gorkov Region, Russia (Synonym Assamia Gupta, 1955) Metorchis Looss, 1899 Type species: Satyapalia gauhatiensis Type species: Metorchis albidus (Braun, (Gupta, 1955) (syn. Assamia gauhatiensis 1893) (syn. Distomum albidum Braun, 1893) Gupta, 1955) Type host: Felis catus domesticus (Mam- Type host: rita (Actinopterygii: malia: Carnivora: Felidae) Siluriformes: ) Type locality: Europe, Palestine, Alaska, Type locality: Brahmaputra River at France, and Volga delta Gauhati, Assam, India Parametorchis Skrjabin, 1913 Type species: Parametorchis complexus Aphallinae (Stiles et Hassall, 1894) Skrjabin, 1913 (syn. Witenbergia Vaz, 1932 Distoma (Dicrocoelium) complexus Stiles & Type species: Witenbergia witenbergi Vaz, Hassall, 1894) 1932 Type host: cats Type host: Pseudoplatystoma tigrinum Type locality: not designated (New York, (Actinopterygii: Siluriformes: Pimelodidae) Maryland and D.C.) Type locality: Rio Piracicaba, Estado de Sao Paolo, Brazil Oesophagicolinae Oesophagicola Yamaguti, 1933 Delphinicolinae Type species: Oesophagicola laticaudae Delphinicola Yamaguti, 1933 Yamaguti, 1933 Type species: Delphinicola tenuis Yamaguti, Type host: Laticauda laticaudata (Reptilia: 1933 Squamata: Elapidae) Type host: Delphinus dusumieri (Mammalia: Type locality: Isigaki-zima, Okinawa, Japan Cetacea: Delphinidae) Type locality: Pacific coast of Japan Opisthorchiinae Agrawalotrema Sahay & Sahay, 1988 Diasiellinae Type species: Agrawalotrema ritai Sahay & Diasiella Travassos, 1949 Sahay, 1988 (Synonym Diasia Travassos, 1922) Type host: (Actinopterygii: Type species: Diasiella diasi (Travassos, Siluriformes: Bagridae). 1922) Type locality: Patna, Bihar (India) Type host: Plotus anhinga (syn. of Anhinga Amphimerus Barker, 1911 anhinga) (Aves: Pelecaniformes: Anhingidae) Type species: Amphimerus ovalis Barker, Type locality: Brazil 1911 Type host: soft-shell turtles Trionyx spinifer Metorchiinae Lesson or T. muticus Lesueur (Reptilia: Holometra Looss, 1899 Testudines: Trionychidae) Type species: Holometra exigua (Muhling, Type locality: Mississippi River in Minnesota 1898) (syn. Distomum exiguum Muhling, 1898) and Iowa, North America Type host: Circus rufus (Aves: Falconiformes: —214—

Clonorchis Looss, 1907 Opisthorchis Blanchard, 1895 Type species: Clonorchis sinensis (Cobbold, (Synonym Notaulus Skrjabin, 1913) 1875), syn. Distoma sinense Cobbold, 1875 Type species: (Rivolta, Type host: man, dog, rat, pig, rabbit, hare, 1884) (syn. Distomum felineum Rivolta, 1884) Mustela (Mammalia: Carnivora: Mustelidae), Type host: not designated Vulpes (Mammalia: Carnivora: Canidae) Type locality: not designated Type locality: Japan, Korea, , , Paropisthorchis Stephens, 1912 Indochina, and India Type species: Paropisthorchis canius (Barker, Cladocystis Poche, 1926 1911) (syn. Opisthorchis caninus Barker, 1911) Type species: Cladocystis trifolium (Braun, Type host: dog 1901) (syn. Distomum trifolium Braun, 1901) Type locality: India Type host: Ardea cocoi (Aves: Ciconiiformes: Pseudogomtiotrema Gupta & Jain, 1991 Ardeidae), Salminus maxillosus (Actinopterygii: Type species: Pseudogomtiotrema caranxi Characiformes: Characidae) Gupta & Jain, 1991 Type locality: Brazil Type host: Caranx malabaricus (Cuvier & Cyclorchis Lühe, 1908 Valenciennes) (Actinopterygii: Perciformes: Type species: Cyclorchis amphileucus (Looss, Carangidae) 1896) (syn. Distomum amphileucum Looss, Type locality: Puri, Orissa, Bay of Bengal, 1896) India Type host: Naja haje (Reptilia: Squamata: Thaparotrema Gupta, 1955 Elapidae) Type species: Thaparotrema vittalani Gupta, Type locality: Alexandria () 1955 Evranorchis Skrjabin, 1944 Type-host: Rita rita (Hamilton) (Actino- Type species: Evranorchis ophidiarum pterygii: Siluriformes: Bagridae) (Tugangui & Masilungan, 1935) (syn. Type locality: Brahmaputra River at Opisthorchis ophidiarum Tubangui & Gauhati, Assam, India Masilungan, 1935) Trionychotrema Chin & Zhang, 1981 Type host: Lapemis hardwickii (Hardwick’s Type species: Trionychotrema taenioidea Jin Sea Snake) Gray, 1836 (Reptilia: Squamata: & Zhang, 1981 Hydrophiidae) Type host: Chinese turtle, Trionyx sinensis Type locality: Paombong, Bulacan, Luzon, (Wiegmann) (Reptilia: Testudines: Triony- the chidae) Gomtia Thapar, 1930 Type locality: Hubei Province, China Type species: Gomtia piscicola Thapar, 1930 Type host: Siluroid fish Pachytrematinae Type locality: River Gomti, Lucknow, India Pachytrema Looss, 1907 Hepatiarius Feizullaev, 1961 (Synonym Minuthorchis Linton, 1928 and Type species: Hepatiarius longissimus (syn. Multivitellaria Phadke & Gulati, 1929) Distomum longissimus Linstow, 1883) Type species: Pachytrema calculus Looss, Feizullaev, 1961 1907 Type host: Ardea stellaris (Aves: Type host: Larus ridibundus (Aves: Ciconiiformes: Ardeidae) Lariformes: Laridae) Type locality: Turkestan, Asia Type locality: Trieste, Italy Nigerina Baugh, 1958 Type species: Nigerina hardoiensis Baugh, Plotnikoviinae 1958 Plotnikovia Skrjabin, 1945 Type host: Phalacrocorax niger (Aves: Type species: Plotnikovia podilymbae (Olsen, Ciconiiformes: Phalacrocoracidae) 1938) (syn. Diasia podilymbae Olsen, 1938) Type locality: Hardoi District, Uttar Pradesh, (Original specific name podilymbae corrected India to podilymbi Olsen, 1939 (Yamaguti, 1971)) Type host: Podiceps podiceps (Aves: —215—

Ciconiiformes: Podicipedidae), Gavia immer Type locality: Italy (Europe) (Aves: Ciconiiformes: Gaviidae) Type locality: Minnesota, USA Tubangorchiinae Tubangorchis Skrjabin, 1944 Pseudamphimerinae Type species: Tubangorchis caintaensis Erschoviorchis Skrjabin, 1945 (Tubangui, 1928) (syn. Metorchis caintanensis Type species: Erschoviorchis lintoni (Gower, Tubangui, 1928) 1939) Skrjabin, 1945 (syn. Amphimerus lintoni Type host: Hypotaenidia philippensis Pelzeln, Gower, 1939; Erschoviorchis lintoni Skrjabin, 1873 (syn. Rallus owstoni (Rothschild, 1895)) 1945) (Aves: Gruiformes: Rallidae) Type host: Gavia immer (Aves: Gaviiformes: Type locality: Cainta, Rizal Province, Luzon, Gaviidae) the Philippines Type locality: Woods Hole, Massachussets, USA DEFINITIVE HOSTS Euamphimerus Yamaguti, 1941 Type species: Euamphimerus nipponicus It is the definitive host of members of this Yamaguti, 1941 family that provides the site for development of Type host: Luscinia calliope calliope (Aves: opisthorchiid metacercariae into the adult Passeriformes: Muscicapidae) forms, and thus the means to reproduce. Adult Type locality: Siriyazaki, Aomori Prefecture, opisthorchiid worms exploit all major Japan groups as definitive hosts, with Pseudamphimerus Gower, 1940 birds being the most common exclusive host Type species: Pseudamphimerus sterni for adult flukes. However, like humans and Gower, 1940 other mammals, this is probably due to the Type host: common tern (Sterna hirundo fact that fish are a major dietary component Linnaeus) (Aves: Ciconiiformes: Laridae) (genera in bold are specific to the vertebrate Type locality: Michigan, USA class):

Pseudamphistominae Fish Microtrema Kobayashi, 1915 Agrawalotrema Sahay & Sahay, 1988 Type species: Microtrema truncatum Allogomtiotrema Yamaguti, 1958 Kobayashi, 1915 Cladocystis Poche, 1926 Type host: Pig (Sus scrofa) (Mammalia: Gomtia Thapar, 1930 Artiodactyla: Suidae). Opisthorchis Blanchard, 1895 Type locality: not designated, Japan Pseudamphistomum Lühe, 1908 Pseudamphistomum Lühe, 1908 Pseudogomtiotrema Gupta & Jain, 1991 Type species: Pseudamphistomum truncatum Satyapalia Lakshminarayana & (Rudolphi, 1819) (syn. Distomum truncatum Hafeezullah, 1974 Rudolphi, 1819) Thaparotrema Gupta, 1955 Type host: Phoca vitulina (Mammalia: Witenbergia Vaz, 1932 Canivora: Phocidae) Type locality: Europe Amphibians Ratzia Poche, 1926 Ratziinae Ratzia Poche, 1926 Reptiles (Synonym Brachymetra Stossich, 1904) Amphimerus Barker, 1911 Type species: Ratzia parva (Stossich, 1904) Cyclorchis Lühe, 1908 (syn. Brachymetra parva Stossich, 1904) Evranorchis Skrjabin, 1944 Type host: Rana esculenta Linnaeus Oesophagicola Yamaguti, 1933 (Amphibia: Anura: Ranidae) (progenetic Ratzia Poche, 1926 metacercariae) Trionychotrema Chin & Zhang, 1981 —216—

Birds reproduction, as well as being the first stage of Amphimerus Barker, 1911 in the fluke’s life cycle. Cladocystis Poche, 1926 Adult worms of Clonorchis sinensis are capable Clonorchis Looss, 1907 of producing up to 4000 eggs per day (Roberts Diasiella Travassos, 1949 and Janovy, 1996) and these are passed from Erschoviorchis Skrjabin, 1945 the definitive host in the faeces. Based on the Euamphimerus Yamaguti, 1941 original descriptions, eggs of species of the Hepatiarius Fedzullaev, 1961 family Opisthorchiidae have measurements in Holometra Looss, 1899 the range 21-100 µm in length and 10-120 µm Metametorchis Morozov, 1939 in breadth, they are usually operculate and Metorchis Looss, 1899 yellow/brown in colour. Opisthorchiid eggs Nigerina Baugh, 1958 closely resemble those of heterophyid flukes Opisthorchis Blanchard, 1895 and this causes problems in identification, Pachytrema Looss, 1907 which is important in cases of human Plotnikovia Skrjabin, 1945 (Ditrich et al. 1990). Indeed, Ditrich Pseudamphimerus Gower, 1940 et al. (1992) also showed that not only were Tubangorchis Skrjabin, 1944 opisthorchiid and heterophyid eggs similar, but that great variability was present among Mammals eggs from the same species. Amphimerus Barker, 1911 Clonorchis Looss, 1907 Miracidia Delphinicola Yamaguti, 1933 The miracidia are already contained within Metametorchis Morozov, 1939 the eggs and are well developed, with their Metorchis Looss, 1899 internal organization being somewhat Microtrema Kobayashi, 1915 asymmetrical (Roberts and Janovy, 1996). The Opisthorchis Blanchard, 1895 eggs are passed from the definitive host in the Pachytrema Looss, 1907 faeces and the miracidia will only hatch upon Parametorchis Skrjabin, 1913 the eggs being ingested by the first Paropisthorchis Barker, 1912 intermediate host. This host is usually a Pseudamphistomum Luhe, 1908 gastropod (Prosobranchia) mollusc of the families , Hydrobiidae, or LIFE CYCLES Thiaridae; for example, the most common species ingesting Clonorchis sinensis eggs are: The life cycles of Opisthorchiidae flukes are Parafossarulus manchouricus, Bulimus complex in that they have various larval fuchsianus (= fuchsiana), and Alocima stages, three different hosts (two intermediate longicornis (= B. longicornis) (Smyth, 1994). As and one definitive), and both asexual and the diet of aquatic snails such as these sexual stages of reproduction. Although consists mainly of vertebrate faeces, the eggs opisthorchiid flukes can have several are passively transferred to the first generations within different larval stages, there intermediate host where they hatch and begin are five main larval stages before the sixth and subsequent stages of development (La Rue, final adult form emerges. Each stage of the 1951). Following hatching from the egg, the fluke’s life cycle is morphologically distinct miracidia migrate along the digestive tract to from the others, making each stage uniquely the snail’s gut where the next stage of suited to its environment and role in the development can begin. development of the adult form. Sporocyst Egg The second developmental stage within the The egg or, more specifically, the first intermediate host is that of the sporocyst, encapsulated of opisthorchiid worms which develops directly from the miracidium. is the result of the only sexual stage of Sporocysts have no , but absorb —217— nutrients directly from the host tissue, in this case the snail’s gut wall. In the case of Metacercaria Clonorchis sinensis, sporocysts develop from The final larval stage, and that which takes the miracidia within 4 hours of the eggs being place in the second intermediate host, is the ingested, and then to develop into redia within development of the metacercaria. Although the 17 days (Roberts and Janovy, 1996). metacercarial stage is not unique to the Opisthorchiidae, it is not found in all Redia trematodes either; for example, the cercaria of The penultimate stage of intra-molluscan other species can directly penetrate the skin development is that of the redia, which develop of, or be eaten by, the definitive host (Smyth, from sporocysts. As reported by La Rue (1951), 1973). Metacercaria of Clonorchis sinensis can daughter generations of rediae are morpho- also be found in certain ; namely, logically indistinct from one another and so it Caridina, Macrobrachium, and Palaemonetes, is difficult to accurately determine how many and were found to be experimental infective to different generations will have developed guinea pigs (Roberts and Jonovy, 1996). within the molluscan host. Adam et al. (1995), Encysted in the muscle of the fish host, the however, have gone some way to further define metacercariae excyst upon consumption of the the larval stages of Opisthorchis viverrini with infected tissue by the definitive host and the their study on the ultrastructure of redia and young flukes migrate to the liver where they pre-emergent cercaria of this species. mature to adults. La Rue (1951) reported that the juvenile flukes can either migrate to the Cercaria liver through chemotaxis, or by following the The cercariae of opisthorchiid flukes are the actual physical track of the bile duct. larval forms that enter the second intermediate Behaviour of the metacercaria is also covered host. Although Yamaguti (1975) reported that by Cable (1972), along with a review of the the cercaria of opisthorchiid trematodes was of behaviour of the other larval stages of a variety of types, the classification of Brooks digenetic trematodes. et al. (1985) is much more succinct and states, among other things, that opisthorchiid IMPORTANCE TO HUMAN HEALTH cercariae are oculate with dorsoventral fin folds present on the tail (pleurolophocercous). Far and away the most important members From other literature it seems that this is an of family Opisthorchiidae, with regards to accepted general description of opisthorchiid human health, are Clonorchis sinensis, cercariae (Smyth, 1994; Roberts and Janovy, Opisthorchis viverrini and O. felineus. These 1996). The morphology of opisthorchiid species are responsible for the diseases of cercaria from Laos has also been examined by clonorchiasis and opisthorchiosis, respectively, Ditrich et al. (1992). Upon leaving the snail and collectively it is thought that these species host, the cercaria must actively find their are responsible for infecting over 30 million second intermediate (fish) host and this is people worldwide (Roberts and Janovy, 1996) achieved by hanging upside down in the water with a further 290 million people at risk (Rim and slowly sinking to the bottom. When any et al., 1994). It should be noted, however, that object comes into contact with the cercaria, different levels of infection are consistently such as a swimming fish, or even the currents quoted in literature, so these figures can be in they create, they rapidly swim upwards, before no way absolute. In addition to the health beginning to sink again. This adaptation problems (outlined further, below) associated favours contact with the fish host and on with theses helminth species, comes a huge touching the epithelium of the fish (usually a economic burden for countries where infection species of the Cyprinidae), the cercaria with these species is endemic; for example, attaches by its oral , loses its tail and infections with O. viverrini alone, can cost forms a cyst underneath the scales or in the Northeast Thailand $120 million per year muscle. (Murrell et al., 1991). —218—

Clinically, clonorchiasis and opisthorchiasis raw or only slightly cooked, the metacercarial are indistinguishable from one another as they cysts can survive to excyst on consumption both produce symptoms of chills, diarrhoea, and mature to adults in their definitive host, fever, lower abdominal pain, jaundice, swelling in this case humans. Species of cyprinid fish of the liver, and malnutrition. Morphologically are cultivated for human consumption and the the species are very similar, evident by the fact increased breeding of the grass that some authors refer to the first species as Ctenopharyngodon idellus for this purpose Opisthorchis sinensis (see Kassai, 1999), and creates a wealth of second intermediate hosts are only distinguishable by the branched for opisthorchiid cercariae, which can increase (dendritic) testes of Clonorchis in comparison the prevalence rates of clonorchiasis (Bisseru to the lobed testes of Opisthorchis. For the and Chong, 1969). In Russia, frozen, raw, purpose of convenience and ease of pickled or smoked fish are eaten and a study explanation, therefore, all human infections by Fan (1998) showed that metacercariae from caused by any of these species shall be Clonorchis sinensis can actually remain viable, referred to as clonorchiasis, unless otherwise even after the fish has been frozen or salted for stated. several days. Other piscivorous mammals The flukes causing clonorchiasis inhabit the such as cats, dogs, and farm livestock can also liver of their host, which may make them act as definitive hosts such a wide range of somewhat less immediately life threatening definitive hosts ensures that large numbers of than the flukes that cause schistosomiasis, for eggs are washed into water reservoirs with example. Nevertheless, this disease can have faecal matter and so the life cycle can begin devastating effects on entire communities in again. addition to the aforementioned symptoms, in The living conditions to which people are cases of prolonged and heavy infections where exposed also contribute to a high incidence of the worm load is in excess of 200, cirrhosis, this disease in Asian countries. As the main pancreatitis, and cholangiocarcinoma can also occupation in many areas is fishing or occur. Indeed, cholangiocarcinoma, rare in agriculture, many villages are built on the other areas of the , has a high prevalence edges of lakes or reservoirs. Such villages in countries where clonorchiasis is endemic rarely have adequate plumbing and (Haswell-Elkins et al., 1992; Lee et al., 1994). consistently lack sanitation facilities, thus Clonorchiasis caused by Clonorchis sinensis people usually defecate either close to or and Opisthorchis viverrini is endemic actually in the water, which allows the throughout Southeast Asia in countries such opisthorchid eggs to begin the aquatic stages as China, Korea, Japan, Taiwan, , of their life cycle (Ditrich et al., 1990). Laos, Thailand, and Cambodia (Tinga et al., Due to the medical significance of this 1999). Infection with Opisthorchis felineus is disease, many studies have been carried out to prevalent in Russia, particularly Siberia, and investigate the impact that it has on human also in Kazakhstan and the Ukraine (Mas- populations. Such studies provide information Coma and Bargues, 1997). The disease can on, among other things, the prevalence and also be found in Europe and North America, spread of the disease in endemic regions, although to a much lesser extent, and which drugs are best at combating infection, probably only due to the increasing import of and how public health programmes can help fish from countries where the disease is to curb the spread of the disease (see Rim et widespread. Perhaps the most prominent al., 1994 for review; Yu, 1996). One such reason for such a distribution of the disease study by Sornami et al. (1973) highlighted the lies in the traditional eating habits of the problems caused by the construction of the inhabitants of the aforesaid Asian and Russian Ubolratana dam in Northeast Thailand, with countries. Many traditional Asian dishes such regards to the health status of individuals as yue-shan chuk (a Cantonese delicacy) living in the newly built villages around the consist of raw fish with rice, vegetables, and banks of the man-made lake. The villagers soup. Since the fish in these dishes is eaten were traditionally farmers living in the area —219— flooded by the building of the dam; it was a opisthorchiid trematodes are of low veterinary natural progression, therefore, that they significance, these helminths can still have moved to the banks of the lake and replaced adverse affects on domestic and wild . farming with fishing. The study was carried Since humans and other animals are out over a period of several months during the inextricably linked in many Asian early rainy season of the area, and was communities, where farming and fishing are primarily concerned with the gathering of the main sources of employment, such information to investigate the general health diseases as clonorchiasis can be so prevalent status of the villagers. The results clearly that they become established within entire showed that the living conditions of the communities. Obviously, the health risks to villagers greatly increased their chances of farm animals can be debilitating for the being the definitive host for the opisthorchiid community as the disease weakens the worms as well as other helminth parasites. animals, thus lowering the quality of the The traditional dishes of raw or undercooked produce derived from them, or renders them fish, coupled with the poor sanitation facilities too weak to be used for manual work. The (only one house from the three villages had a situation is worsened by the fact that such latrine) provided a near perfect environment animals act as reservoir hosts for the for the spread of infections (Sornmani et al., opisthorchiid worms and so their close contact 1973). with humans can exacerbate the problems of Another study, carried out by Joo et al. infection and control within human (1997), reported on the changing patterns of populations. clonorchiasis in Korea over a period of two Similarly, wild animals can act as reservoir years. This study also provides information on hosts for many helminths, and this allows the the prevalence of metacercariae in species of parasites to have a continual presence in the fresh-water common to the area, and environment. As discussed by Shimalov et al. highlights the need for a continued and (2000), otters (Lutra lutra Linnaeus, 1758) in amplified treatment programme. Russia are a host to many species of Just as there is a wealth of information on helminths, including three species from the the rates of infection of clonorchiasis, there is family Opisthorchiidae; namely, Opisthorchis also a multitude of studies as to what is the felineus, Pseudamphistomum truncatum, and best form of treatment for this disease. Many Metorchis bilis. Introduced species in studies (e.g., see Rim et al., 1981; Chen et al., Russia are also hosts to a wide range of 1983; Radomyos et al., 1998) have con- helminths. Ivanov and Semenova (2000) centrated on the use of praziquantel as the studied introduced animal species, including most effective drug with which to treat cases of American muskrats (Ondatra zibethicus), clonorchiasis, and outline extensively the best American mink (Mustela vison), and Siberian dosage and frequency of administration. raccoon dogs (Nyctereutes procyonoides), and However, other studies have questioned the found that they had acquired parasitic fauna effectiveness of this drug in the treatment of of the area, which includes M. bilis and P. clonorchiasis; namely, Hong et al. (1998) and truncatum. Tinga et al. (1999) who showed that certain dosages of praziquantel were ineffective as a CONCLUSIONS means of treating clonorchiasis. However, due to the fact that praziquantel targets more than It is clear that even with invalid genera one helminth species means that it will removed from the family, the Opisthorchiidae probably remain the drug of choice for are still an interesting and cosmopolitan group diseases such as clonorchiasis. of digeneans, and although much work has been done on the medically important species VETERINARY IMPORTANCE of family, relatively little is known about the other genera. This review is intended to be the Although Kassai (1999) states that first step in a critical re-examination of the —220—

Opisthorchiidae, the subsequent steps of Separate versus combined analysis of which should be based on the examination of phylogenetic evidence. Annu Rev Ecol Sys 26: specimens. It is hoped that this review has 657-681. ˘ gone some way to providing a truer Ditrich O, Giboda M, Strba J (1990) Species classification of the family as a whole, at determination of eggs of opisthorchiid and genera-level at least, and that this information heterophyid flukes using scanning electron microscopy. Angew Parasitol 31: 3-9. may be expanded upon in future studies to Ditrich O, Scholz T, Giboda M (1990) Occurrence further resolve inconsistencies and invalidities of some medically important flukes within the family, particularly with regards to (Trematoda: Opisthorchiidae & Heterophy- subfamily classification. idae) in Nam Ngum water reservoir, Laos. Southeast Asian J Trop Med Public Health 21: ACKNOWLEDGEMENTS 482-488. Ditrich O, Giboda M, Scholz T, Beer SA (1992) The authors wish to thank Dr. David A. Comparative morphology of eggs of the Gibson, Dr. Rodney A. Bray, and Mrs. Eileen Haplorchiinae (Trematoda: Heterophyidae) Harris of The Natural History Museum, and some other medically important London for their help. A stay at The Natural heterophyid and opisthorchiid flukes. Folia History Museum, London by T. Scholz was Parasitol 39: 123-132. supported by the Sys-Resource Programme. Ditrich O, Na˘sincova V, Scholz T, Giboda M This study was undertaken during the Work (1992) Larval stages of medically important Placement of S. King at the Institute of flukes (Trematoda) from Vientiane Province, Parasitology, Ceské˘ Budéjovice. Laos. Part II. Cercariae. Ann Parasit Hum Comp 67: 75-81. Fan PC (1998) Viability of metacercaria of REFERENCES Clonorchis sinensis in frozen or salted freshwater fish. Int J Parasitol 28: 603-605. Adam R, Arnold H, Hinz E, Storch V (1995) Haswell-Elkins MR, Sithithaworn P, Elkins D Morphology and ultrastructure of the redia (1992) Opisthorchis viverrini and cholan- and pre-emergent cercaria of Opisthorchis giocarcinoma in Northeastern Thailand. viverrini (Trematoda: Digenea) in the Parasitol Today 8: 86-89. intermediate host Bithynia siamensis Hong ST, Yoon K, Lee M, et al. (1998) Control of goniophalus (Prosobranchia: Bithyniidae). clonorchiasis by repeated praziquantel Appl Parasitol 36: 136-154. treatment and low diagnostic efficacy of Bisseru B, Chong LK (1969) Opisthorchis viverrini sonography. Korean J Parasitol 36: 249-254. (Poirer, 1886) a trematode parasite of man in Ivanov VM, Semenova NN (2000) Parasitological west Malaysia. Trop Geogr Med 21: 138-146. consequences of animal introduction. Russ J Brooks DR, O’Grady RT, Glen DR (1985) Ecol 31: 281-283. Phylogenetic analysis of the Digenea Joo CY, Chung MS, Kim SJ, Kang CM (1997) (Platyhelminthes: Cercomeria) with comments Changing patterns of Clonorchis sinensis on their adaptive radiation. Can J Zool 63: infections in Kyongbuk, Korea. Korean J 411-443. Parasitol 35: 155-164. Cribb TH, Bray RA, Littlewood TJ, Pichelin SP, Kassai T (1999) Veterinary Helminthology. Herniou EA (2001) The Digenea. Inter- Butterworth-Heinemann, UK. relationships of the Platyhelminthes. Taylor & La Rue GR (1951) Host-parasite relations among Frances, UK. the digenetic trematodes. J Parasitol 37: 333- Cable RM (1972) Behaviour of digenetic 342 Trematodes. Behavioural aspects of parasite Lee JH, Yang HM, Bak UB, Rim HJ (1994) transmission. London: Academic Press Inc., Promoting role of Clonorchis sinensis infection UK. on induction of cholangiocarcinoma during Chen M, Hua X, Wan Z, et al. (1983) Praziquantel two-step carcinogenesis. Korean J Parasitol in 237 cases of clonorchiasis sinensis. China 32: 13-18 Med J 96: 935-940. Mas-Coma S, Bargues MD (1997) Human liver de Queiroz A, Donoghue MJ, Kim J (1995) flukes: a review. Rev Iber Parasitol 57: 145- —221—

218. Intarakhao C, Harinasuta C (1973) A study Murrell KD, Cross JH, Harinasuta C (1991) A on the pattern of socioeconomic and health landmark in food-borne parasitic zoonoses. status in relation to parasitic diseases in the Parasitol Today 7: 68-69. inhabitants around Ubolratana Dam in Pearson JC (1992) On the position of the Northeast Thailand. Southeast Asian J Trop digenean family Heronimidae: an inquiry into Med Public Health 4: 421-434. a cladistic classification of the Digenea. Syst Smyth JD (1973) The Physiology of Trematodes. Parasitol 21: 81-166. University Reviews in Biology, UK. Radomyos B, Wongsaroj T, Wilairatama P, et al. Smyth JD (1994) Introduction to Animal (1998) Opisthorchiasis and intestinal fluke Parasitology (3rd Edition). Cambridge infections in northern Thailand. Southeast University Press, UK. Asian J Tropl Med Public Health 29: 123-127. Tinga N, Van De N, Viet Vien H, et al. (1999) Little Rim HJ, Farag HF, Sornmani S, Cross JH (1994) effect of praziquantel or arteminisinin on Food-borne trematodes: Ignored or emerging. clonorchiasis in northern Vietnam. A pilot Parasitol Today 10: 207-209. study. Trop Med Int Health 4: 814-818. Rim HJ, Lyu KS, Lee JS, Joo KH (1981) Clinical Yamaguti S (1958) Systema Helminthum Vol I evaluation of the theraputic efficacy of Digenetic Trematodes. Interscience Publi- praziquantel (Embay 8440) against Clonorchis shers Inc., New York. sinensis infection in man. Annal Trop Med Yamaguti S (1971) Synopsis of Digenetic Parasitol 75: 27-33. Trematodes of Vol. I, II. Keigaku Roberts LS, Janovy J (Jr) (1996) Foundations of Publishing Co., Japan. Parasitology (5th Edition). Wm C Brown Yamaguti S (1975) A Synoptical Review of Life Publishers, USA. Histories of Digenetic Trematodes of Shimalov VV, Shimalov VT, Shimalov AV (2000) Vertebrates. Keigaku Publishing Co., Japan. Helminth fauna of otter (Lutra lutra Linnaeus, Yu S-H (1996) Control of parasitic diseases in 1758) in Belorussian Polesie. Parasitol Res China: current status and prospects. China 86: 528. Med J 109: 259-265. Skrjabin KI, Petrov AM (1950) Superfamily Wykoff DE, Harinasuta C, Juttijudata P, Winn Opisthorchoidea Faust, 1929. Trematodes of MM (1965) Opisthorchis viverrini in Thailand- man and animals. Osnovy trematodologii, The life cycle and comparison with O. Vol. 4. Moscow: Nauka. felineus. J Parasitol 51: 207-214. Sornmani S, Vivantanaseth P, Bunnag T,