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Trematode Infection Workup

  • Author: Subhash Chandra Parija, MBBS, MD, PhD, FRCPath, DSc; Chief Editor: Mark R Wallace, MD, FACP, FIDSA  more...
Updated: Nov 23, 2015

Laboratory Studies


Diagnosis is made after microscopic demonstration of eggs in the stool (intestinal schistosomiasis; intestinal, liver, and lung fluke infections), sputum (pulmonary paragonimiasis), or urine (genitourinary schistosomiasis).

For improving the sensitivity of sputum examination for pulmonary paragonimiasis, serial samples (up to 6) should be examined.

A study by Slesak et al has reported the utility of performing Ziehl-Neelsen staining for demonstration of Paragonimus eggs. Ziehl-Neelsen staining was found to be much better than the conventional method of just performing a wet film direct examination of sputum in endemic areas.[22]  Paragonimus eggs measure around 100 µm long by 50 µm wide, are ovoid and thick-shelled, and can be detected in sputum and stool samples.

Less frequently, nonoperculate, terminal-spined eggs of S haematobium can be demonstrated in the rectal biopsy and aspiration findings obtained with proctoscopy or cystoscopy.

The flask-shaped eggs of C sinensis can also be demonstrated in the duodenal contents. Examination of fluid obtained from duodenal intubation is diagnostically more sensitive than examination of 2 stool specimens.

Formalin ether and/or ethyl acetate concentration is the most sensitive method for processing stool specimens for egg examination.

The Kato-Katz technique is a simple and sensitive quantitation technique used successfully in the field.[22] It is a commonly used semiquantitative method (measured in a defined quantity of stool - 5 mg) for counting eggs in persons with intestinal schistosomiasis and allows the degree of infection and treatment response to be assessed.

Schistosomal species can be differentiated based on the morphology of the eggs. Rectal or intestinal biopsy demonstrates a granulomatous reaction around the eggs. These biopsy examinations are more much sensitive than multiple stool examinations for detection of intestinal schistosomiasis.[23]

Urine, the specimen of choice for diagnosing urinary schistosomiasis, is collected between noon and 2 pm, the period when an increased number of eggs are excreted. The eggs in the urine are concentrated by centrifugation or membrane filtration.

The eggs of Fasciola and Fasciolopsis species are morphologically similar and indistinguishable. The eggs are oval and yellowish-brown, measuring around 130-150 µm long by 60-90 µm wide. Stool concentration techniques facilitate detection of these trematode eggs. Since the eggs are operculated, they do not float during a saturated salt flotation test. F buski is the largest intestinal trematode, measuring 2-8 cm long by 1-2 cm wide, while the smallest intestinal trematodes are H heterophyes and M yokogawai, measuring less than 0.5 mm.

Similarly, the eggs of Clonorchis, Heterophyes, Metagonimus, and Opisthorchis species are also morphologically similar and indistinguishable. Clonorchis eggs measure around 20 μm long by 15 μm wide. In rare cases, adult worms seen during upper GI endoscopy or when patients are started on antihelminthic therapy. Adult worms can be differentiated based on size; Clonorchis measures 10-25 mm long by 3-5 mm wide, O felineus measures 8-12 mm long by 2-3 mm wide, and O viverrini is narrower (1-2 mm wide).[24]

In Fasciola and Paragonimus species infections, the eggs cannot be demonstrated during the migratory phase of infection or in ectopic infections because no eggs are passed in the stool.

Coproantigen detection

Detection of antigen in the stool (coproantigen) is a nonmicroscopic method of diagnosis for intestinal trematodes. An enzyme-linked immunosorbent assay (ELISA) using a monoclonal antibody to an 89-kd antigen of O viverrini has been used to detect coproantigen in the stool of individuals with O pisthorchis infection. This test has been found to be highly sensitive and specific. Coproantigen detection tests are also available for the diagnosis of fascioliasis and capillariasis.[25, 26, 27]

Soluble egg antigen (SEA) detection

ELISA-based tests are available to assess urine or stool samples for SEA; this method provides an effective diagnosis of schistosomiasis and correlates well with quantitation egg count. Although these antigen detection tests have better efficacy than microscopy, they must be used in conjunction with microscopy to increase the probability of detection.[28, 29]


Several serologic tests, which can be used to detect either specific antibodies or antigens in the serum, are used in diagnosing trematode infections.

Various antibody-based serologic tests are used in the diagnosis of most trematode infections. These tests are used for diagnosis and for seroepidemiologic studies. Commonly used tests include indirect hemagglutination, indirect immunofluorescence, and ELISA. ELISA is most sensitive and practical.

These serologic tests are especially useful in the following situations:

  • Prepatent period and in chronic and ectopic cases of schistosomiasis, in which the eggs are difficult to demonstrate in the stool
  • Acute fascioliasis, because the eggs are not passed in the stool for as many as 4 months of infection
  • Cerebral and abdominal paragonimiasis, because the eggs are not passed in the sputum or stool

A major disadvantage of antibody-based serologic tests is the inability to differentiate between recent and past infections because antibodies remain in the serum even after parasitologic cure of the disease. Low sensitivity and cross-reactions between trematodes are other noted disadvantages.

Detection of specific antigen in serum and urine is particularly useful during acute and end-stage disease, when excretion of eggs is minimal. Knowing whether infection is recent or old is also useful because, in active or recent infection, the circulating antigen is present in the serum or urine but is absent in patients with older or treated infection.

Falcon assay screening test (FAST) ELISA is sensitive (95%) and specific (99%) for the diagnosis of urinary schistosomiasis. This test uses S hematobium adult worm microsomal antigen (HAMA) to reveal serum antibodies.

In schistosomiasis, antigen titers in serum and urine correlate well with the degree of infection, as demonstrated by the egg counts. ELISA is used for detection of proteoglycan gut-associated antigens such as circulating anodic antigen (CAA) and circulating cathodic antigen (CCA) in the urine and serum. The sensitivities of the urine CCA and serum CAA ELISA are substantially higher than those of a single egg count. The sensitivity of these assays increases with egg output. Both CAA and CCA can also be detected in sera and urine of egg-negative individuals. Recently, a novel dipstick test detecting the CCA of intestinal schistosomes in urine samples has been devised. The dipstick test is found to have a better sensitivity than the conventional stool microscopy for the diagnosis of intestinal schistosomiasis.[30]

For its convenience, ELISA has replaced the complement fixation test in the diagnosis of paragonimiasis. For serologic diagnosis, the criterion standard is a Western blot assay, which yields a sensitivity and specificity of nearly 99%. Newer techniques such as the dot immunogold filtration assay (DIGFA) are of supplementary value. Serological assays are particularly useful in the early phase of the disease (prior to egg excretion) and in extrapulmonary paragonimiasis, in which eggs are not expected in sputum or stool.

Immunoblot is a specific and sensitive test to detect schistosomiasis.

The circulating antigen has been detected in the sera of patients with C sinensis infection with the ELISA double-sandwich method. A dipstick ELISA can be used to assess urine samples for SEA; this method provides an effective diagnosis of schistosomiasis and correlates well with quantitation egg count. Diagnosis of fascioliasis by detection of circulating 28.5-kd tegumental antigen is also evaluated.[31] Fas2-ELISA is based on the detection of circulating immunoglobulin G (IgG) antibodies. Results show that Fas2-ELISA is a highly sensitive (92.4%) immunodiagnostic test for the detection of F hepatica infection in children living in human fascioliasis–endemic areas.[32] The CDC recommends the use of ELISA utilizing excretory secretory (ES) antigens followed by confirmation of positive results with immunoblot techniques.[33] Protein-banding patterns after isoelectric focusing have been used to differentiate F hepatica from F gigantica. This is useful for monitoring therapeutic studies. No cross-reaction with heterophyid flukes has been reported.

Skin test

Intradermal skin testing has been used for epidemiologic studies but cannot be used to differentiate past from current infection.

Skin testing using extracts of adult C sinensis or P westermani antigens has been used in Korea and China as an epidemiologic tool.

Molecular methods

Most molecular methods are still in the experimental stage. A polymerase chain reaction (PCR) using the primer named OV-6F/OV-6R has been developed for the detection of Oviverrini in experimentally infected hamsters. The method has been found to be 100% sensitive in hamsters.[34] Duplex PCR has been designed for the simultaneous detection and differentiation of the liver flukes F hepatica and F gigantica from stool samples.[35]

Parvathi et al have evaluated a nested PCR for the specific detection of Csinensis. The PCR assay was found not to show any amplification with closely related trematode, O viverrini.[36] Detection of Csinensis in stool samples has been attempted using a real-time PCR assay. The sensitivity of the assay was found to be 100%, and the PCR cycle threshold values showed significant correlation with egg counts.[37] Real-time PCR (targeting the internal-transcribed-spacer-2 sequence of the parasite) to detect Csinensis –specific DNA in fecal samples was found to correlate with the egg counts in the stool, thus also being useful for quantification.

Various PCR protocols have been evaluated for the detection of intestinal schistosomiasis among which the most widely used molecular targets are the 121-bp rDNA sequence and mitochondrial NADH1 gene for S mansoni and 230-bp sequence from retrotransposon SjR2 for S japonicum.[38] Recently, PCR targeting the 121-bp Dra1 gene fragments in urine is being used for the diagnosis of urinary schistosomiasis caused by S haematobium.[39]

PCR-based techniques have the advantage in that they can detect the presence of trematodes irrespective of the stage of their life cycle. A species-specific PCR assay using internal transcribed spacer (ITS2) sequences that can distinguish between common food-borne trematodes such as Paragonimus, Fasciolopsis, and Fasciola species has been evaluated in a study from Shillong, India, and has been found to be unaffected by the life-cycle stages of the trematode parasites.[40]

Other parameters

A complete blood cell count may reveal eosinophilia in patients with fasciolopsiasis, schistosomiasis, heterophyiasis, metagonimiasis, early stages of paragonimiasis, and acute Clonorchis species infection (disappears in chronic Clonorchis species infection). Anemia may be found in patients with schistosomiasis, fascioliasis, and paragonimiasis. Gross and microscopic hematuria may be found in individuals with schistosomiasis. Neutropenia may be found in patients with fasciolopsiasis. Elevation of cerebrospinal fluid (CSF) pressure and pleocytosis and eosinophilia in the CSF may occur in individuals with cerebral paragonimiasis.


Imaging Studies


Chest radiographs in patients with schistosomiasis may reveal cor pulmonale and pulmonary hypertension, if present.

Radiographs of the liver exhibit tractlike small abscesses and subcapsular lesions in patients with fascioliasis.

Patchy foci of fibrotic change with a characteristic "ring shadow" (ie, circular or oval thin-walled cyst with a crescent-shaped opacity along one side) is the characteristic finding on chest radiographs in patients with paragonimiasis.


Ultrasonography is useful in evaluating the gall bladder and biliary tract in individuals with fascioliasis. Adult worms may be visible on sonograms or may appear as curvilinear lucent areas in the contrast medium on cholangiograms.

This is a sensitive procedure used to demonstrate urinary obstruction and hepatosplenic disease in persons with schistosomiasis.

Portable ultrasonography can be used for determining the extent of pathological changes, particularly in the liver and bladder, and can be used to screen populations at the community level. In addition, it can be used to assess the effects of chemotherapy.

CT scanning and MRI

CT scanning is useful in the study of CNS manifestations of trematode infections.

In persons with cerebral paragonimiasis, longstanding cerebral infection forms and cystlike structures may calcify and may be seen as clusters similar in appearance to soap bubbles.

In recent years, CT scanning and MRI have also been found to be useful in spinal paragonimiasis in addition to cerebral paragonimiasis, with imaging features specific for spinal involvement.[21]

CT scanning helps detect parenchymal lesions in individuals with fascioliasis

MRI may be useful in the study of CNS manifestations of trematode infections. MRI can also reveal granuloma of the liver parenchyma in cases of fascioliasis.

Biliary and pancreatic imaging

Cholangiography in individuals with fascioliasis reveals the multiple cystic dilatations of the ducts. Large cystic dilatation, small cystic ectasias, and mulberrylike dilatation are considered diagnostic of fascioliasis.

Endoscopic retrograde cholangiopancreatography (ECRP) has been found to be helpful in the diagnosis and treatment of biliary fascioliasis.[41]



Colonic biopsy is a sensitive and specific procedure to aid in identifying parasite eggs in biopsy specimens for the diagnosis of intestinal schistosomiasis and intestinal trematode infections.

Biopsy of neural tissue can be performed for diagnosis of neuroschistosomiasis.[18]

Cystoscopy is useful to help identify schistosome eggs in mucosal biopsy specimens from the urinary bladder and to exclude other causes of hematuria.


Histologic Findings

Egg granuloma is the typical pathologic lesion in urinary schistosomiasis. These are found in the ureter and urinary bladder. The granuloma consists mainly of eosinophils, macrophages, and lymphocytes surrounding the egg at the center. In chronic infection, fibroblast proliferation and fibrosis are characteristic.

Finger-sized fibrosis in the portal areas is characteristic of S mansoni infection.

Periportal fibrosis, Symmers fibrosis, and perisinusoidal blockage are the typical findings in S japonicum infection.

Adult Paragonimus flukes elicit an acute inflammatory reaction with formation of eosinophilic granulomas and small multiple fibrous cysts in the liver. The eggs also elicit an acute inflammatory reaction consisting of eosinophils, formation of a fibrous capsule, rupture of cysts in bronchioles, eosinophilic empyema, and, finally, calcification. The cystic encapsulation of the eggs in the lung and, less frequently in the brain and in other abdominal organs, is the key pathologic feature in paragonimiasis.

During the acute stage of fascioliasis, the liver is enlarged and exhibits hemorrhagic necrotic tracts in the subcapsular areas infiltrated by eosinophils and other inflammatory cells. In chronic infection, the bile duct exhibits epithelial hyperplasia with minimal pericholangitis and proliferation of tissues.

The infection of the biliary tract by C sinensis, O viverrini, and Ofelineus demonstrates adenomatous hyperplasia, periductal inflammation, periductal fibrosis, and diffuse or localized dilatation of ducts and may be associated with cholangiocarcinoma in C sinensis.

Ulceration of gut epithelium and localized inflammation are the features of infection caused by F buski and other intestinal flukes.

Contributor Information and Disclosures

Subhash Chandra Parija, MBBS, MD, PhD, FRCPath, DSc Director-Professor of Microbiology, Head of Department of Microbiology, Jawaharlal Institute, Postgraduate Medical Education and Research, India

Subhash Chandra Parija, MBBS, MD, PhD, FRCPath, DSc is a member of the following medical societies: Royal College of Pathologists, Indian Society for Parasitology, Indian Medical Association, National Academy of Medical Sciences (India), Indian Association of Medical Microbiologists, Indian Association of Biomedical Scientists, Indian Association of Pathologists and Microbiologists, Indian Academy of Tropical Parasitology

Disclosure: Received salary from Jawaharlal Institute of Postgraduate Medical education & Research , Pondicherry , India for employment.


Thomas J Marrie, MD Dean of Faculty of Medicine, Dalhousie University Faculty of Medicine, Canada

Thomas J Marrie, MD is a member of the following medical societies: Alpha Omega Alpha, American College of Physicians, American Society for Microbiology, Association of Medical Microbiology and Infectious Disease Canada, Royal College of Physicians and Surgeons of Canada

Disclosure: Nothing to disclose.

Shekhar Koirala, MBBS Vice Chancellor, Department of Medicine, BP Koirala Institute of Health, Nepal

Disclosure: Nothing to disclose.

Specialty Editor Board

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Received salary from Medscape for employment. for: Medscape.

Chief Editor

Mark R Wallace, MD, FACP, FIDSA Clinical Professor of Medicine, Florida State University College of Medicine; Clinical Professor of Medicine, University of Central Florida College of Medicine

Mark R Wallace, MD, FACP, FIDSA is a member of the following medical societies: American College of Physicians, American Medical Association, American Society for Microbiology, Infectious Diseases Society of America, International AIDS Society, Florida Infectious Diseases Society

Disclosure: Nothing to disclose.

Additional Contributors

Larry I Lutwick, MD Professor of Medicine, State University of New York Downstate Medical School; Director, Infectious Diseases, Veterans Affairs New York Harbor Health Care System, Brooklyn Campus

Larry I Lutwick, MD is a member of the following medical societies: American College of Physicians, Infectious Diseases Society of America

Disclosure: Nothing to disclose.

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Adult worms in humans reside in the veins in various locations: Schistosoma mansoni in the inferior mesenteric veins, Schistosoma japonicum in the superior mesenteric veins, and Schistosoma haematobium in the vesical veins (these locations are not absolute). The females (size 7-20 mm; males slightly smaller) deposit eggs in the small venules of the portal and perivesical systems. The eggs are moved progressively toward the lumen of the intestine (S mansoni and S japonicum) and of the bladder and ureters (S haematobium), and they are eliminated with feces or urine, respectively. Under optimal conditions, the eggs hatch and release miracidia, which swim and penetrate specific snail intermediate hosts. The stages in the snail include 2 generations of sporocysts and the production of cercariae. Upon release from the snail, the infective cercariae swim, penetrate the skin of the human host, and migrate through several tissues and stages to their residence in the veins. Human contact with water is thus necessary for infection by schistosomes. Various animals serve as reservoirs for S japonicum and Schistosoma mekongi. Image courtesy of the US Centers for Disease Control and Prevention.
These are small operculated eggs. Size is 27-35 μm X 11-20 μm. The operculum, at the smaller end of the egg, is convex and rests on a visible "shoulder." At the opposite (larger, abopercular) end, a small knob or hooklike protrusion is often visible (as here). The miracidium is visible inside the egg. Image courtesy of the US Centers for Disease Control and Prevention.
Wet mounts with iodine. The eggs are ellipsoidal. They have a small, barely distinct operculum (upper end of the eggs in panel A). The operculum can be opened (egg in panel B), for example, when slight pressure is applied to the coverslip. The eggs have a thin shell that is slightly thicker at the abopercular end. They are passed unembryonated. Size range is 120-150 μm X 63-90 μm. Image courtesy of the US Centers for Disease Control and Prevention.
Adult flukes size range is 20-75 mm by 8-20 mm. Image courtesy of the US Centers for Disease Control and Prevention.
Eggs are excreted unembryonated in the sputum, or, alternately, they are swallowed and passed with stool (1). In the external environment, the eggs become embryonated (2), and miracidia hatch and seek the first intermediate host, a snail, and penetrate its soft tissues (3). Miracidia go through several developmental stages inside the snail (4): sporocysts (4a), rediae (4b), with the latter giving rise to many cercariae (4c), which emerge from the snail. The cercariae invade the second intermediate host, a crustacean such as a crab or crayfish, in which they encyst and become metacercariae. This is the infective stage for the mammalian host (5). Human infection with Paragonimus westermani occurs by eating inadequately cooked or pickled crab or crayfish that harbor metacercariae of the parasite (6). The metacercariae excyst in the duodenum (7), penetrate through the intestinal wall into the peritoneal cavity, and then through the abdominal wall and diaphragm into the lungs, where they become encapsulated and develop into adults (8) (7.5-12 mm X 4-6 mm). The worms can also reach other organs and tissues, such as the brain and striated muscles, respectively. However, when this occurs, completion of the life cycle is not achieved because the eggs laid cannot exit these sites. Time from infection to oviposition is 65-90 days. Infections may persist for 20 years in humans. Animals such as pigs, dogs, and a variety of feline species can also harbor P westermani. Image courtesy of the US Centers for Disease Control and Prevention.
The average egg size is 85 μm by 53 μm (range, 68-118 μm X 39-67 μm). They are yellow-brown, ovoidal or elongate, have a thick shell, and are often asymmetrical with one end slightly flattened. At the large end, the operculum is clearly visible. The opposite (abopercular) end is thickened. The eggs of P westermani are excreted unembryonated. Image courtesy of the US Centers for Disease Control and Prevention.
Table 1. Vectors and Geographical Areas Associated With Certain Trematode Types
Vector Geographical Area Type of Trematode
Biomphalaria glabrata Brazil S mansoni
Bulinus globosa Nigeria S haematobium
Bulinus truncate Iran S haematobium
Oncomelania hupensis nosophora Japan S japonicum
Thiara granifera China P westermani; M yokogawai
Semisulcospira libertine China P westermani; M yokogawai
Polypylis hemisphaerula China F buski
Parafossarulus manchouricus China C sinensis
Bithynia leachi Germany O felineus
Pirenella conica Egypt H heterophyes
Lymnaea truncatula England F hepatica
Table 2. List of Definitive and Intermediate Hosts and Sources of Infection of Major Trematodes
Trematode Definitive Host Intermediate Host

1st 2nd

Source of Infection
S haematobium Humans Freshwater snails (genus Bulinus) Absent Contact with water contaminated by cercariae
S mansoni Humans, occasionally baboons and rodents Freshwater snails (genus Biomphalaria) Absent Penetration of skin by cercariae
S japonicum Humans, dogs, pigs, cattle, mice, mustelids, and monkeys Amphibian snails (Oncomelania species) Absent Penetration of skin by cercariae
S mekongi Humans and dogs Aquatic snails (Tricula aperta) Absent Penetration of skin by cercariae
F hepatica Sheep, goats, cattle, and other herbivorous animals Amphibian snails (family Lymnaeidae) Aquatic vegetations and watercress Ingestion of aquatic plants and watercress infected with metacercariae
C sinensis Humans, dogs, pigs, cats, rats, and several species of wild animals Freshwater snails (family Bulinidae) Freshwater fish (family Cyprinidae) Eating raw or partially cooked freshwater fish or dried, salted, or pickled fish infected with encysted metacercariae
O felineus Humans and other fish-eating mammals Aquatic snails Freshwater fish Eating fish infected with metacercariae
P westermani Humans, wolves, foxes, tigers, leopards, lions, cats, dogs, and monkeys Freshwater snails (family Pleuroceridae and Thiaridae) Freshwater crab or crayfish Ingestion of freshwater crabs or crayfish infected with metacercariae
F buski Pigs and humans Planorbid snails of the genera Segmentina, Hippeutis, and Polypylis Freshwater plants such as water caltrops, water chestnut, water bamboo, water hyacinth, and lotus Ingestion of freshwater aquatic plants that harbor metacercariae
Table 3. Comparative Features of Major Human Schistosoma Species
  S haematobium S mansoni S japonicum
Body surface of male Finely tuberculate Grossly tuberculate Nontuberculate (smooth)
Testes 4-6, in a cluster 6-9, in a cluster 7, in a linear series
Position of ovary Posterior to middle of body Anterior to middle of body Posterior to middle of body
Number of eggs in uterus 20-30 1-4 50-300
Size and shape 110-170 μm long

40-70 μm wide

Terminal spine

114-175 μm long

45-68 μm wide

Lateral spine

70-100 μm long

50-65 μm wide

Central spine

Cephalic glands 2 pairs, oxyphilic 2 pairs, basophilic 4 pairs, oxyphilic
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