56 Other non-Fasciola trematode infections

A large variety of trematodes result in millions of fish- and other food-borne human infections annually. These are mostly in Asia but some more widespread. Man may be an important or occasional definitive host acquiring infection from eating fish, frogs, snails, crustaceans, ants, and water plants, and from drinking water. Consumption of these raw/undercooked produces human infection. Adult flukes occur in the biliary system, intestine, lungs, and occasionally ectopically.

Oriental liver flukes were first described in Western literature in the nineteenth century: Clonorchis sinensis from autopsies on Chinese workers in India and Mauritius when ‘putrid, half-raw’ fish, vegetables, or snails were suggested as the vehicle; Opisthorchis spp. in man were described later and Japanese workers transferred infection from snails to fish to cats and dogs. Intestinal Fasciolopsis buski was described in an Indian and the life cycle predicted in 1843. Paragonimus westermani and the reddish sputum were described in 1879/80 and, in 1915, snail and crab hosts determined (Grove 1990).

Many of the flukes and life cycle stages are very difficult to differentiate within and even between families. The structure of the flukes is described by Soulsby (1982) and Muller and Wakelin (2002). Molecular techniques to differentiate species now are useful for analyses of stages in intermediate and definitive hosts in epidemiological studies (Yang et al. 2000; Dzikowski et al. 2004; Intapan et al. 2004; Sugiyama et al. 2006).

Clonorchis sinensis Looss, 1907 is pinkish, transparent, spatulate, and up to 15 by 5 mm. Eggs (27–35 by 12–20 µm) are yellow-brown, with a thin shell, indistinct operculum fitting into a prominent rim, and knobbed, abopercular protuberance. The miracidium has asymmetrical internal structure. Chlonorchis is found in areas in south and north east China, Taiwan, northern Vietnam, Korea and eastern Russia. Definitive hosts include:

Opisthorchis viverrini Stiles and Hassall 1896 and Opisthorchis felineus Blanchard 1895 adults and eggs are very similar to C. sinensis. Opisthorchis viverrini is found particularly in north east Thailand and neighbouring Laos but other south east Asian countries. Allozyme analyses show at least two genetically distinct parasites using distinct snail species (Saijuntha et al. 2007). Opisthorchis felineus is found from Siberia through eastern, central and southern Europe. Opisthorchis novera and O. guayaquilensis are rare.

Definitive hosts: as C. sinensis.

Other Opisthorchiidae found occasionally in human bile ducts include Metorchis albidus and M. conjunctus in Alaska, Canada, Greenland, France, and Russia, acquired from cyprinid and suckerfish; Pseudamphistomum truncatum (Russia, though it occurs elsewhere) from fish such as sunbleach and topmouth gudgeon; and possibly P. aethiopicum (Ethiopia) described in intestinal cysts.

Dicrocoelium dendriticum on dry pastures in much of the world, D. hospes in Africa and Eurytrema pancreaticum in Asia and South America occur in land snails, ants or grasshoppers, and in ruminant bile or pancreatic ducts, respectively, and are reported rarely in man.

About 70 species, particularly echinostomes and heterophyids, infect man (Chai and Lee 2002; Fried et al. 2004).

Fasciolopsis buski Odhner, 1902 in the duodenum and jejunum is large, pinkish, reaches 10 by 3 cm, with a large acetabulum. Yellow-brown eggs are wider (130–140 by 80–85 µm) but similar to F. hepatica. Humans are a main host, pigs usually carry fewer worms. Intermediate hosts are small planorbid snails (listed by Mas-Coma et al. 2005) in stagnant ponds or slow-moving waters. Metacercariae encyst on plants and about 4% in water. Found in the Indian subcontinent, China, and south east Asia.

Gastrodiscoides hominis Leiper 1913 in the large intestine is red, about 5–14 mm by 5–8 mm, with a wide discoidal posterior, narrower conical/cylindrical anterior, and very large, posterior acetabulum. Eggs (130–160 by 65–75 µm) are colourless to pale green. Pigs are main definitive hosts and it occurs in wild boars, primates, muskrats, and is common in man in the Indian subcontinent, Kazakstan, Volga Delta in Russia, China, south east Asia, and the Philippines. Human infection has been reported in Nigeria and Zambia with no further information. Tiny Helicorbis snails in ponds and reservoirs produce cercariae and metacercariae are drunk in water or eaten on plants, frogs and crustaceans. Related Paramphistomatidae of ruminants described rarely in man are Watsonius watsoni (West Africa) and Fischoederius elongatus (China).

Heterophyes heterophyes Stiles and Hassall 1900, the Asian Heterophyes nocens, and Metagonimus yokogawai Katsurada 1913, are the most important heterophyids, but about 35 are known zoonotic. The minute, oval flukes are <2 mm long. Circum-oral spines, genital sucker, and reproductive organs are useful for identification, but differentiation of heterophyids is difficult. Descriptions of many are given by Pearson and Ow-Yang (1982) and Saito et al. (1997). Eggs are light brown (26–30 by 15–17 µm), slightly pointed at both ends, the operculum may fit into a slightly thickened rim, and the miracidium is bilaterally symmetrical.

Definitive hosts are piscivorous birds and mammals including dogs, cats, foxes, pigs, and sometimes man. Intermediate hosts for H. heterophyes and H. nocens are brackish water Pinella and Cerithidea snails, and mullet, goby, cichlid, etc., fish. Heterophyes dispar, H. katsuradai, Haplorchis yokogawai, H. pumilio, H. taichui, H. vanissimus, H. microchis, H. pleurolophocerca, Heterophyopsis continua, Pygidiopsis summa, Stellantchasmus falcatus, S. amplicaecalis, Diorchtrema spp., Stictodora fuscata and S. lari also are acquired from brackish water mullet, goby, shad, barbel, perch, in coastal areas. Metagonimus yokogawai develops in freshwater pleurocercid snails, i.e. Semisulcospira, and it, M. minutus, M. miyatai, and M. takahashii in cyprinid fish, i.e. sweetfish, minnow, carp, perch, trout, chub, dace, in large and small streams in the upper reaches of rivers. Human infection occurs particularly in China, Japan, south east Asia, and Russia, but also in Hawaii, North Africa, the Middle East, Greece, Turkey, Sardinia, Spain, France, the Balkans, Ukraine, and Brazil, and infection in animals/birds is much more widespread than these records. Centrocestus formosanus, C. armatus, C. caninus, C. cuspidatus in the same Asian areas are acquired from crustaceans and frogs as well as chub, carp, etc. Apophallus donicus in Canada, Cryptocotyle lingua in Canada and Greenland, Plagicola spp. in Brazil and the USA, and Procerovum calderoni and P. varium in Africa, China, and the Philippines are acquired from fish. Isoparorchis hypselobagri in China and India and Carneocephallus brevicaeca in the Philippines come from shrimp, crab and fish. Ascocotyle coleostoma is found in Egypt.

Echinostomes infect a broad spectrum of piscivorous birds but also mammals, including man, dog, and cat. Small, elongate adults (3–10 by 1–3 mm) have a strong ventral sucker and an oral sucker surrounded by a conspicuous ‘head-collar’ of large spines. Speciation is difficult. Eggs (80–120 by 60–70 µm) are yellow-brown, operculate, and unembryonated.

Human infections are recorded mainly in China and south east Asia but the parasites are cosmopolitan so occasionally elsewhere, e.g. India, Europe, the Americas. Species in man include Echinosoma revolutum (Looss 1899), E. malayanum, E. cinetorchis, E. hortense, E. jassyense. E. angustitestis, E. iliocanum, E. macrorchis, E. echinatum, Echinochasmus japonicus, E. juifoensis, E. perfoliatus, E. liliputanus, Echinoparyphium recurvatum, Himasthala muehlensi, Episthmium caninum, Hypoderaeum conoideum, Paryphostomum sufratyfex, Artifechinostomum mehrai, Acanthoparyphium typosenense, acquired from varied second hosts that are brackish or freshwater snails, bivalves, frogs, tadpoles, crustaceans, plus loach, salamanders, carp, perch, etc.

Structure and importance are described by Chai and Lee (2002), Fried et al. (2004) and Toledo et al. (2006).

Nanophyetus salmincola of carnivores is described in man in northwestern USA and eastern Russia from primarily salmonids. Brachylaima cribbi of house mice and poultry in southern Australia occurs in helicid and hygromiid snails eaten occasionally on fresh produce. Neodiplostomum seoulense is quite common in man and rodents in mountainous regions of Korea from tadpoles, frogs and paratenic host snakes. Gymnophalloides seoi has wading bird, i.e. oystercatcher, and mammal hosts, oysters being the most important source for man in Korea. Phanerosolus bonnei and Prosthodendrium molenkampi metacercariae occur in naiads of insects with monkey or bat definitive hosts and fish that eat flies infect man in Thailand, Indonesia and Laos. Rodents, birds and also man in Japan and south east Asia acquire four Plagiorchis spp. from chironomid larvae and freshwater fish. Spelotrema brevicaeca in the Philippines probably comes from shrimp and/or snails. Cotylurus japonicus encysted within freshwater snails infects humans in China. Prohemistomum vivax of carnivores is described rarely in Egypt.

The morphological similarity of Paragonimus species means identification and delineation of geographical distribution has been difficult, though recent analyses of cytochrome c oxidase 1 and internal spacer 2 are differentiating isolates.

Paragonimus westermani Braun 1899 is the main species infecting man in the Indian subcontinent, south east Russia, China, Japan, through south east Asia to Papua New Guinea. It is thick and reddish-brown (7.5–16 by 4–8 mm) (resembling a coffee bean) with large spines, some species more elongate. Diploid P. westermani occur throughout its range; triploids in northern areas; tetraploids in north east China (Blair 2000). Triploids have larger adults and eggs, few or no sperm, parthenogenetic reproduction, and are more pathogenic. North east Asian diploids use Pleurocercidae snails and dog, cat, fox hosts and are genetically different from those in the Philippines and Malaysia in Thiaridae snails and rat hosts (Iwagami et al. 2000). Eggs (75–118 by 34–71 µm) are yellowish to dark brown, unembryonated, with a flattened operculum fitted into a slight rim, and thickened opposite pole.

Other species infecting man are: P. heterotremus in China, Thailand, Laos, Japan, India; P. miyazakii and P. skrjabini in Japan and China, respectively; P. africanus and P. uterobilateralis in West Africa; P. mexicanus in Central and northern South America; P. kellicotti in North America; and P. bankokensis, P. caliensis, P. hueitungensis, P. ohira, P. phillipinensis, P. sadoensi, P. siamensis in Asia.

The varied hosts are listed by Blair et al. (1998). In Asia, P. westermani and Paragonimus heterotremus infect Semisulcospira and Oncomelania snails, respectively. Throughout their range a large variety of crabs and crayfish are second hosts, Definitive hosts include Canidae, Felidae, primates and civet cats and also mustelids and marsupials in the Americas. Some mammals, i.e. boar, rodents, that eat crustaceans act as paratenic hosts.

Achillurbainia and Poikilorchis spp. acquired from crabs/crayfish are described occasionally in Asia, Central and West Africa, and Latin America presenting as a retroauricular abscess, otitis media, and possibly mastoiditis. Aspirated eggs resemble Paragonimus (Schuster et al. 2007).

In other sites, Clinostomum complanatum of piscivorous birds acquired from freshwater fish attaches to the wall of the pharynx, larynx and eye in man in Japan and Thailand. Philophthalmus lacrymosus and P. palperbarum eye flukes of birds and pinnepeds are occasionally recorded in the conjunctival sac of man in Europe, Asia, North America, cercariae acquired swimming in salt or freshwater. Diplostomum spathaceum penetrates the skin of man to cause dermatitis but migrates to the eye lens, the metacercariae inducing cateracts. Rarely granulomata in the body are due to Alaria americana and A. marcianae mesocercariae (an intermediate unencysted stage between cercaria and metacercaria) acquired from snakes, birds and mammals.

Eggs may be unembryonated or embryonated and usually must reach water. Man is an important definitive host for C. sinensis, Opisthorchis spp., G. hominis, and F. buski, so lack of sanitation determines water contamination. Pigs are important reservoir hosts for C. sinensis, G. hominis, and F. buski. Human and pig faeces used to fertilize land might be washed into streams, but particularly are used to fertilize ponds and canals growing edible water plants for man, pigs and/or fish. Canidae and Felidae can maintain infections such as C. sinensis, O. felineus, and heterophyids such that C. sinensis infection occurred in fish (10%) in southern Khabarovsk Territory, Russia, yet human infection was rare as raw fish was rarely eaten. Wild piscivorous birds carry many fluke infections, e.g. heterophyids, echinostomes, to contaminate water, and wild animals drink and defaecate in small rivulets, creeks and streams where Paragonimus is found. Ruminants infect pastures with Dicrocoelium.

Where required, egg development takes 1–2 to several weeks depending on temperature. Eggs of some species are ingested by snails, the miracidia of others swim to find and penetrate snails. Snail habitat can be fresh, stagnant or brackish water in slow-flowing streams, rivers, canals, reservoirs, lakes, ponds, and coastal areas, and for Dicrocoelium land.

Flukes multiply as sporocysts then rediae in the snail over weeks to months depending on season to release cercariae that creep or swim to reach and encyst on plants or in/on fish, amphibian, crustacean or molluscan second hosts. Many small or fast growing fish, e.g. grass carp, are farmed in ponds or rice paddies, or are plentiful in streams, rivers and lakes.

A preference for ethnic dishes of fish, shellfish or snails eaten raw, marinated, smoked, fermented, or lightly salted, is a most important risk, and hand-mouth contamination may occur in food preparation. In the Philippines, raw fish and snails are dipped in salt and vinegar (‘kinilaw’) and coconut milk and lime (‘kuhol’, ‘kiamby-ay’). In northern Thailand, ‘koi pla’ is small/medium fish eaten soon after preparation; ‘pla som’ is fermented for days to weeks but some metacercariae are viable at 7 days. Sliced raw fish is common in Japan and southern China. A salted, putrefying mullet delicacy, ‘feseekh’, in Egypt favours infection with H. heterophyes. Salted or marinated fish can be important in infection with O. felineus.

Paragonimus is acquired when crabs or crayfish are eaten lightly cooked as soup or as ‘drunken crab’ marinated in brine, vinegar, soy, or wine. Juice extracted from shellfish is used as an oral medicant and in food dishes. Children in west Africa eat raw legs of crabs or shellfish is insufficiently roasted. Paragonimus can also be acquired from paratenic wild boar or pig eaten sliced and raw in Japan or undercooked.

Metacercariae of F. buski and G. hominis on water caltrops, chestnuts, tubers, bamboo, and leafy plants (sometimes crustaceans) grown in ponds or canals, are cracked with the teeth to peel, particularly by children, or eaten raw. Also, cercariae encysted in water may be stirred up and drunk in unfiltered water. About 11% of human F. buski infections were due to drinking water and water was a significant risk for E. liliputanus so the significance of water for other flukes needs re-examination.

Dicrocoelium dendriticum cercariae excreted from land snails in slime balls are eaten by ants that lock onto and are eaten with plants.

Excysted immature intestinal flukes remain in the intestine, liver flukes migrate through the common bile duct, and Paragonimus migrate across the peritoneal cavity, muscles, pleural cavity, and lungs to pair in cysts.

Infection occurs focally in relation to customs and residence near appropriate waters. Mixed species infections are common (Dung et al. 2007; Sayasone et al. 2009).

Recent national prevalence of C. sinensis in Korea was 2.1% reaching 31% in peoples living near a tributary of the Nakdong river (Lim et al. 2006); 0.4–0.8% prevalence in China reaching 2.4% in 27 endemic provinces, particularly Guangdong Province with pockets of >28; in north west Vietnam 5% were infected (Dung et al. 2007). As many as 13–85% of fish can be infected.

In northern Thailand, infection with O. viverrini fell from 34–64% in 1981 to 16–19% in 2001 with national reported cases falling from 1.8 in 2001 to 0.7/100,000 in 2006 (Kaewpitoon et al. 2008) but 86% infection has been reported in Laos (Sayasone et al. 2009). Opisthorchis felineus is widespread but participation of man declines from high in central Russia to rare in Western Europe. Prevalence is high in western Siberia, 1.6% infection in the north reached 38% in the in the Ob river basin and increasing in Kemerovo Region where 11 and 33% of residents in Tomsk city and a nearby village, respectively, were seropositive. Up to 50% of cyprinid fish were infected. The six outbreaks in Italy since 2003 were associated with tench and white fish, 83% of the former infected with metacercariae (Armignacco et al. 2008).

Recently, nationwide in China, echinostomes were detected in 0.015%, but 5% in endemic Fujian and Guangdong Provinces (Zhou et al. 2008); 0.2% in Korea with pockets of 22% in some inland areas (Chai and Lee 2002; Shin et al. 2008); and, in a tribal community in Bandipore, India, 10% were passing echinostome eggs. Two to 41% of fish carry metacercariae.

Nationwide in Korea 1.2% were infected with Metagonimus in 1981 but this has fallen to 0.3–0.5% although 20–70% were positive along large and small streams in eastern and southern coastal areas where sweetfish are available, and 5% positive along upper reaches of large rivers where minnow and carp are caught (Chai and Lee 2002; Shin et al. 2008). Brackish water heterophyids were less common, i.e. H. nocens 0.1% reaching 11% on islands, and G. seoi 0.1% reaching 4% on southwestern islands. In the Nghia Hung district, Vietnam, and several districts in Laos, heterophyids particularly Haplorchis spp., but also P. molenkampi, infected some 65% of adults with poly-trematode infection common (Dung et al. 2007; Chai et al. 2007; Sayasone et al. 2009). Heterophyids were in 5% of fish in areas in Vietnam and 42–100% of mullet and goby in some coastal areas of Korea.

Fasciolopsis buski occurs in many Asian countries though, in many, prevalence has declined, i.e. infection has virtually disappeared from central Thailand. Infection persists in other areas, i.e. 7% in a village in north east Thailand and 4% in children near Dongting Lake, China, and may be re-emerging in Uttar Pradesh, northern India. Prevalence is highest among peoples living near water caltrop plantations.

Prevalence of Paragonimus in eight Provinces in China was 1–7% with 30% of children infected in rural Shangluo (Zhou et al. 2008). Prevalence can be high, up to 50–100%, in crabs and crayfish. In Sin Ho district, Vietnam, 7.4% were infected. In north east India, particularly Arunachai Pradesh, infection is emerging with 21% prevalence of P. heterotremus in children ≤15 years (Devi et al. 2007). In endemic areas of Nigeria, Cameroon, and Liberia up to 10–17% may be infected. Infection occurs in small foci from Mexico to Brazil and Peru. In the USA infection is sporadic. In Japan infection is re-emerging from eating wild boar but also immigration, travel, and importation of infected crab/crayfish from China.

Liver flukes sucking blood produce ulceration, mechanical irritation, and desquamation in large and medium sized bile ducts, adenomatous hyperplasia of the epithelium, and goblet cell hyperplasia. Mucus rich bile, hyperplasic liver cells, and worms produce choleostasis and sludge in the gall bladder favouring secondary infection. Absorbed antigens and eggs lodged in walls induce granulomata, eosinophil and mononuclear cell infiltration with degranulation, cytokine production, and nitric oxide damage in the portal triad producing cholangitis, necrosis, and oedema. Periductal fibrosis becomes prominent around enlarged ducts. Eggs are a nidus for choleoliths. The extent of damage is related to intensity and chronicity of infection and multiple infections. Low numbers of flukes may be asymptomatic or induce abdominal discomfort and malaise. Higher levels (100–1,000 worms) have a more pronounced presentation.

Clonorchis sinensis and O. felineus, often asymptomatic, may present acutely 2–4 weeks after infection particularly in naive migrants, as fever, anorexia, right upper abdominal pain, weight loss, and sometimes allergic arthralgia, asthma, and urticaria. In the chronic phases hepatomegaly, portal hypertension, ascites, and jaundice may occur and sudden acute presentation may manifest due to obstruction from hyperplasia, periductal stenosis, worms, and choleoliths. Other severe complications include pyogenic cholangitis, pancreatitis, and cholangiocarcinoma (CHCA). Clonorchis sinensis is a Group 2A carcinogen (probably carcinogenic — International Agency for Research on Cancer). In Hong Kong and Korea, C. sinensis is considered responsible for 6% of liver cancers with risk also related to male sex, alcohol, and raw fish consumption.

Acute disease from O. viverrini is uncommon, infections mostly asymptomatic or benign. At any one time in a northeastern Thai village 5–10% of the population had some mild symptoms (malaise, flatulence, dyspepsia, pain) from gall bladder enlargement, dysfunction, and gallstones, with possibly hepatomegaly. Severe complications, although uncommon, are pyogenic cholangitis and importantly CHCA related to high intensity (>6,000 eggs/gm indicative of >120 worms) and duration of infection. Opisthorchis viverrini (Group 1 definite carcinogen) is associated with an increased incidence of bile duct CHCA (multicentric peripheral type), an important cause of death after 20–40 years of chronic, repeated infection (Kaewpitoon et al. 2008). Incidence of CHCA in endemic countries is high, i.e. 40–98/100,000 in Khon Kaen Province, Thailand, compared with 0.1–3 elsewhere in the world whereas hepatocellular carcinoma prevalences are comparable.

Experimental evidence reinforces the link between O. viverrini (and C. sinensis) and CHCA although mechanism(s) are not understood. Flukes may act as a promoter with hyperplastic epithelium susceptible to exogenous, endogenous or worm carcinogens. Excretions/secretions (ES) of O. viverrini induced cell proliferation in vitro. Tenascin, integral in epithelial cell interactions in tumourogenesis, is expressed in infected duct walls, surrounding tissues, and CHCA stroma. Opisthorchis viverrini cDNA has revealed potential mutagenic activity, i.e. progranulin, a cell-, including tumour cell-growth factor, and a kallikrein-like protease possibly associated with epithelial to mesenchymal cell progression (Laha et al. 2007). There may be synergism between the flukes and dietary N-nitro compounds that occur in commonly eaten ‘pla ra’, fermented fish sauce, etc. High incidence of CHCA occurred in fluke infected hamsters treated with N-nitrosodimethyamine but not hamsters with either alone. An important role for host produced NO and rOI in DNA damage has been suggested. Increased NO synthase and the vigorous production of NO in the bile ducts could represent an endogenous genotoxic substance. Host genetics will contribute.

Gastrodiscoides hominis seems well tolerated; in heavy infection inflammation of the large intestine causes diarrhoea. Fasciolopsis buski produces trauma of the anterior intestine, excess mucus production, ulceration, haemorrhage, and abscess formation, and possibly reduced vitamin B₁₂ absorption. Heavy infections in children (100 to >1,000 worms) produce a protein-losing enteropathy, profuse diarrhoea, pain, nausea, and occasionally obstruction. Ascites and facial oedema may be due to hypoalbuminaemia but allergy or ‘toxaemia’ has been suggested.

The large echinostomes seem less pathogenic than small heterophyids, N. seoulense and other species. These latter enclose and penetrate between villi into crypts producing inflammation, villous atrophy, necrosis, oedema, ulceration, and petechial haemorrhages. There may be mild digestive disturbances, intermittent or continuous usually mucoid, but occasionally bloody, perhaps profuse diarrhoea, anaemia, and anorexia.

Some heterophyids, particularly S. brevicaeca, H. heterophyes, M. yokogawai and their eggs, may penetrate the mucosa and embolize to various organs. Granulomatous lesions, particularly noticeable in brain and heart, have been fatal. Gymnophalloides seoi may produce signs of pancreatitis making it very important (Chai et al. 2003).

Paragonimus migrating in the pleural cavity induces pleural effusion, adhesions, pneumothorax, and focal haemorrhagic pneumonia with pain, cough, fever, fatigue, and marked eosinophilia. Greyish-white fibrous cysts, 1–3 cm in diameter, containing yellow-brown purulent fluid, open into a bronchus and produce chronic dry cough (97%), then rust-coloured sputum, haemoptysis, often with a ‘fish-taste’ (83–92%), chest and abdominal pain (42–70%), pleural effusion (26%), and eosinophilia (90%) (De et al. 2000; Devi et al. 2007).

Paragonimus skrjabini and P. miyazakii in particular migrate aberrantly to the chest, abdominal wall, liver, spleen, intestine, or extremities. The nodule or migrating mass often is painful inducing marked eosinophilia. Cerebral migration causes oedema, haemorrhage, and meningitis; severe headache, seizures, and hemiparesis manifest in 8% of patients.

Clinical signs and imaging are not pathognomonic from other liver diseases, many diarrhoeas and tuberculosis, all prevalent in endemic areas. Eating habits can be useful.

Faecal (sputum) samples examined by formalin-ether/ethyl acetate concentration, Stoll, or Kato thick smears can have sensitivity of 85–90% for O. viverrini and C. sinensis this reduced to 70% on a single examination of light infections (<20 worms). Duodenal aspiration may detect scarce eggs. Bile duct cannulation has been used. The eggs of many of the liver and minute intestinal flukes are very similar so skilful, time-consuming examinations are required. The operculum and egg length to width are useful but the considerable intraspecific variability usually divides eggs only into groups (Lee et al. 1984; Chai and Lee 2002). Recent advances include release of DNA from opisthorchid eggs and use of polymerase chain reaction (PCR) that detects the group (Müller et al. 2007), primers specific for different species also are now available. Eggs of echinostomes, F. buski and G. hominis are detected with detergent/sedimentation techniques and iodine or methylene green/blue stains. Worms can be expelled with praziquantel followed by a saline purgative (Chai et al. 2007). Pseudoparasitism, particularly by D. dendriticum, is ruled out by a controlled diet.

Cholangeography may outline liver flukes and damaged, dilated bile ducts showing irregular filling defects. M-mode sonogram may detect fluke movement. On heavy infection ultrasonography and CT scan show dilation of particularly the distal ducts, highly echogenic thickened bile duct walls, and sludge in the gallbladder (Choi and Hong 2007; Lim et al. 2008). CHCA shows as a large irregular mass with low density stippled areas or powder-like high-density areas (Rim 2005). Radiography for Paragonimus may show patchy consolidation of haemorrhage from migration and linear shadows from the pleural surface, presumably migratory tunnels filled with pleural fluid/air as obvious when accompanied by pleural effusion or pneumothorax. Radiography may be negative in about 20% of patients early in infection. Calcified/fibrous cysts are evident. CT shows cloudy infiltration of the lungs and poorly marginated nodules, frequently subpleural or subfissural possibly with a low attenuation central area. Focal pleural thickening was common (84%) and linear opacities leading to the nodule (tracks) characteristic (48%) (Kim et al. 2005).

Serodiagnosis, particularly ELISA, is increasingly used for liver and lung flukes. ES, superior to crude extracts, of Opisthorchis, Clonorchis and Paragonimus gives >80% sensitivity although cross-reactions remain among the Opisthorchiidae and Paragonimus spp. (Choi et al. 2003). A P. heterotremus ES antigen band on immunoblot was 98% sensitive and 100% specific when detected with IgG₄ antibodies (Wongkham et al. 2005). A variety of recombinant antigens now are becoming available and have shown >80–96% specificity in ELISA and western blot (Nagano et al. 2004; Ruangsittichai et al. 2006; Zhao et al. 2004; Hu et al. 2007; Ma et al. 2007; Lee et al. 2007). Tests for circulating antigens generally have lower sensitivity but high specificity.

Three 25 mg/kg doses of praziquantel given on 1 day, repeated on day 2 for heavy infections, has >90% efficacy against Clonorchis and Opisthorchis spp. Light infections may be treated with 40 mg/kg given once, producing fewer side-effects that are usually mild and transient (<24 hours), possibly severe if neurocysticercosis was present (Rim 2005).

A single dose of 10–15 up to 25 mg/kg praziquantel is highly effective against F. buski and other intestinal flukes. Triclabendazole has efficacy in pigs.

At least one course of praziquantel, 50–75 mg/kg as three divided doses for 3 days, had good efficacy (>87%) against pulmonary and ectopic Paragonimus, eggs disappearing in a few weeks, lesions clearing slowly. Triclabendazole, 10 mg/kg once or twice in a day or 5–10 mg/kg for 3 days, was >84% effective (Calvopinña et al. 2003).

In laboratory animals derivatives of artemisinin have shown promise against some liver and intestinal flukes and tribendimidine against liver flukes (Keiser and Utzinger 2007; Keiser et al. 2007).

The cost of treatment and lost wages from opisthorchosis and CHCA was estimated at US$120 million a year in Thailand. Control should involve:

Education on life cycles and preparation of fish, shellfish, water plants is important though it may be difficult to change cultural habits. In a southern Chinese village, 27–57% had no knowledge of C. sinensis’s life cycle/disease, ate raw fish >1–2 times a month, and/or fed fish with human/animal faeces (Zhang et al. 2007). In northern Thailand, even after >10 years of the control programme, 40% continued to eat raw fish and defaecate promiscuously. However, in Taiwan, an aggressive education programme controlled F. buski. Some information on killing metacercariae is available. Opisthorchis viverrini metacercariae were killed at 80°C for 5 min, 70°C for 30 min, vinegar treatment for 1.5 hours, salting at 33.6% for 24 hours, or freezing at –10°C for >5 days though neither freezing for 3 days nor the common Russian 3-day wet salting or 1.7% dry salting killed O. felineus. Irradiation could be adopted commercially. Metacercariae of F. buski could remain viable on plants for perhaps 2–3 months in damp, cool weather but are killed by desiccation and boiling.

Treatments targeting infected individuals after faecal examination should markedly reduce egg output as 74% of C. sinensis were aggregated in 10% of the population. Reducing intensity and duration of infection should reduce severe disease and CHCA risk. Data is required concerning rates of reinfection and the relevance of eggs produced by lightly infected individuals and particularly reservoir hosts. Even where man was the most important definitive host, treatment once a year was insufficient to control O. viverrini due to reinfection. Treatment plus education has success however. The Public Health Development Plan in northern Thailand combined yearly targeted treatment, community preparation, and health education (Jongduksuntigul and Imsomboom 2003; Kaewpitoon et al. 2008). Infection and disease incidence declined significantly with increased knowledge of Opisthorchis and reported decrease in ‘koi pla’ fish consumption. Occasional treatments could be targeted to season as infection may peak after monsoon flooding when fish abound in ponds and paddy fields or, in countries with a cold season, in late summer/autumn.

Increased socio-economic status usually decreases infection with improvements in standard of living, sanitation, hygiene, and education. Migration of rural poor to urban areas will remove them from the source of infection. An increase in the price of fish (through export or supply) may decrease levels of infection. Increased wealth though may increase consumption of raw, ethnic dishes in restaurants and homes. The re-emergence of opisthorchosis in Italy has been associated with this.

Changes in the environment and agriculture can impact on transmission. Increased water pollution from factories and insecticides was linked to the decline of C. sinensis in Japan. A decline in H. heterophyes along the Nile was attributed to changed water nutrients due to the Aswan Dam. A north east Thailand water development programme increased water flow reducing snails and O. viverrini. Conversely, construction of dams and/or migration of infected people into areas increased prevalence of O. felineus and it is important to consider changes in water flow in the whole network of water bodies, not just the immediate vicinity. Migration of labourers, with often asymptomatic and long-lived infections, from endemic areas to new lakes and irrigation areas and to countries where control has been successful, could introduce infection.

Modern pig farming using commercial foods will break the cycle of flukes in pigs. Water plants should be dried, fish viscera cooked before feeding to pigs. Heat treatment (even composting) of faeces used as fertilizer in fish ponds is useful. Predator snails/ducks/fish, etc., can reduce numbers of cercariae and snails or bivalves, i.e. clams.

Aquaculture is the fastest growing method of protein production increasing from thousands to many millions of tons of fish, particularly cyprinids, but also crustaceans, particularly in China, but also other south east Asian trematode endemic areas (Keiser and Utzinger 2005). In the same areas small scale fish production and marketing to nearby areas also has increased exponentially. In China, C. sinensis prevalence increased three-fold in the face of a decline of soil-transmitted helminths and S. japonicum, the increase related to aquaculture expansion. In Korea, soil-transmitted helminths have fallen further than fish-borne trematodes.

The increased global trade in chilled, potentially infected fish could result in use in ethnic dishes among immigrant populations and residents eating novel dishes. Increasing numbers of Paragonimus cases in Japan occur in Chinese inhabitants, with travel and consumption of local, ethnic dishes in endemic areas and import of shellfish contributing.