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Trematode Infection Follow-up

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

Further Outpatient Care

Further outpatient care includes health education. Thorough cooking of fish and aquatic vegetables, fruits, and plants is necessary to prevent ingestion of infective forms of the parasite.


Further Inpatient Care

Anemia may be treated with iron supplements and vitamins, which may be administered orally to facilitate iron absorption.



Transfer if access to specialized health care services is lacking.



Note the following practices for deterrence and prevention of trematode infection:

  • Elimination of molluscan hosts using molluscicides, such as metal salts, and organophosphates or carbamates to control infection with schistosomes and intestinal, liver, and lung flukes
  • Proper sanitary disposal of human and pig excreta to avoid water contamination
  • Chemotherapy of infected persons
  • Avoidance of consumption of contaminated water, water plants, fruits, fish, crab, and raw liver
  • Thorough cleaning and washing of raw vegetables and aquatic fruits to prevent infection with intestinal flukes
  • Thorough cooking of water-grown vegetables before eating
  • Cooking of crab and crayfish before eating to prevent infection with lung flukes


Schistosomiasis complications can include the following:

  • Cor pulmonale
  • Portal hypertension
  • Urinary bladder carcinoma
  • Neurological complications: Cerebral and cerebellar tumour–like neuroschistosomiasis can present with increased intracranial pressure, headache, nausea and vomiting, and seizures. Myelopathy (acute transverse myelitis and subacute myeloradiculopathy) is the most common neurological complication of S mansoni infection. Schistosomal myelopathy tends to occur early after infection and is more likely to be symptomatic than cerebral schistosomiasis. The conus medullaris and cauda equina are the most common sites of involvement. Severe schistosomal myelopathy can provoke a complete flaccid paraplegia with areflexia, sphincter dysfunction, and sensory disturbances.[18]

Lung fluke complications can include the following:

  • Lung abscess
  • Pleural effusion
  • Ectopic lesions in the brain

Liver fluke complications can include the following:

  • Intercurrent bacterial infections
  • Less commonly, pancreatitis in fascioliasis[43]
  • Anemia
  • Recurrent pyogenic cholangitis and cholangiocarcinoma in clonorchiasis

Intestinal fluke complications can include asthenia with ascites in fasciolopsiasis.



Prognosis is excellent in patients with mild-to-moderate trematode infection, with early disease, and/or without severe complications.

Patients with heavier worm infection are less likely to improve, and the outcome in such infections may be serious and fatal.


Patient Education

Avoid high-risk food habits. Inform patients about the danger of eating raw or undercooked vegetables and fish and the importance of cleaning, washing, and adequately cooking vegetables and fish or raw liver.[2]

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
VectorGeographical AreaType of Trematode
Biomphalaria glabrataBrazilS mansoni
Bulinus globosaNigeriaS haematobium
Bulinus truncateIranS haematobium
Oncomelania hupensis nosophoraJapanS japonicum
Thiara graniferaChinaP westermani; M yokogawai
Semisulcospira libertineChinaP westermani; M yokogawai
Polypylis hemisphaerulaChinaF buski
Parafossarulus manchouricusChinaC sinensis
Bithynia leachiGermanyO felineus
Pirenella conicaEgyptH heterophyes
Lymnaea truncatulaEnglandF hepatica
Table 2. List of Definitive and Intermediate Hosts and Sources of Infection of Major Trematodes
TrematodeDefinitive HostIntermediate Host

1st 2nd

Source of Infection
S haematobiumHumansFreshwater snails (genus Bulinus)AbsentContact with water contaminated by cercariae
S mansoniHumans, occasionally baboons and rodentsFreshwater snails (genus Biomphalaria)AbsentPenetration of skin by cercariae
S japonicumHumans, dogs, pigs, cattle, mice, mustelids, and monkeysAmphibian snails (Oncomelania species)AbsentPenetration of skin by cercariae
S mekongiHumans and dogsAquatic snails (Tricula aperta)AbsentPenetration of skin by cercariae
F hepaticaSheep, goats, cattle, and other herbivorous animalsAmphibian snails (family Lymnaeidae)Aquatic vegetations and watercressIngestion of aquatic plants and watercress infected with metacercariae
C sinensisHumans, dogs, pigs, cats, rats, and several species of wild animalsFreshwater 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 felineusHumans and other fish-eating mammalsAquatic snailsFreshwater fishEating fish infected with metacercariae
P westermaniHumans, wolves, foxes, tigers, leopards, lions, cats, dogs, and monkeysFreshwater snails (family Pleuroceridae and Thiaridae)Freshwater crab or crayfishIngestion of freshwater crabs or crayfish infected with metacercariae
F buskiPigs and humansPlanorbid snails of the genera Segmentina, Hippeutis, and PolypylisFreshwater plants such as water caltrops, water chestnut, water bamboo, water hyacinth, and lotusIngestion of freshwater aquatic plants that harbor metacercariae
Table 3. Comparative Features of Major Human Schistosoma Species
 S haematobiumS mansoniS japonicum
Body surface of maleFinely tuberculateGrossly tuberculateNontuberculate (smooth)
Testes4-6, in a cluster6-9, in a cluster7, in a linear series
Position of ovaryPosterior to middle of bodyAnterior to middle of bodyPosterior to middle of body
Number of eggs in uterus20-301-450-300
Size and shape110-170 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 glands2 pairs, oxyphilic2 pairs, basophilic4 pairs, oxyphilic
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