Other parasites

Author: Haind Fadel, M.D. (see Authors page)

Revised: 10 January 2018, last major update December 2014

Copyright: (c) 2003-2018,, Inc.

PubMed Search: Schistosomiasis[TI] free full text[sb] pathology

Cite this page: Fadel, H. Schistosomiasis. website. Accessed March 20th, 2018.
Definition / general
  • Schistosomiasis or bilharziasis is among the most important parasitic diseases worldwide, afflicting 200 - 300 million individuals
  • Adult male and female blood flukes inhabit veins of the mesentery or bladder
  • Most important species infecting humans are Schistosoma mansoni, Schistosoma japonicum, Schistosoma mekongi, Schistosoma haematobium and Schistosoma intercalatum; other species infect humans less frequently
  • Schistosomiasis is an important cause of disease in many parts of the world, most commonly in places with poor sanitation
  • School age children who live in these areas are often most at risk because they tend to spend time swimming or bathing in water containing infectious cercariae
  • Risk factors: living in or travel to endemic areas and exposure to contaminated freshwater
  • S. mansoni - distributed throughout Africa:
    • In freshwater in southern and sub-Saharan Africa, including great lakes, rivers and smaller bodies of water
    • Also Nile River valley in Sudan and Egypt, South America (Brazil, Suriname, Venezuela), Caribbean (low risk, includes Dominican Republic, Guadeloupe, Martinique, Saint Lucia)
  • S. haematobium - distributed throughout Africa:
    • In freshwater in southern and sub-Saharan Africa, including great lakes, rivers and smaller bodies of water
    • Also Nile River valley in Egypt, Mahgreb region of North Africa
    • Found in areas of the Middle East
  • S. japonicum: Indonesia and parts of China and Southeast Asia
  • S. mekongi: Cambodia and Laos
  • S. intercalatum: parts of Central and West Africa
  • Eggs are deposited in smallest venule that can accommodate the female worm, where they elicit a strong granulomatous response that results in extrusion of the egg into the intestinal lumen or the bladder
  • Pathology is primarily related to sites of egg deposition, number of eggs deposited and host reaction to egg antigens
Diagrams / tables

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Life cycle

Clinical features
  • Symptoms of schistosomiasis result primarily from penetration of cercariae (cercarial dermatitis), from initiation of egg laying (acute schistosomiasis or Katayama fever) and as a late stage complication of tissue proliferation and repair (chronic schistosomiasis)
  • Hours after cercarial penetration, a papular rash may develop, associated with pruritus
  • This is a sensitization phenomenon resulting from prior exposure to cercarial antigens
  • Most severe form of dermatitis occurs in individuals who are repeatedly exposed to cercariae of nonhuman (primarily avian) schistosomes
  • Cercarial dermatitis or swimmer's itch occurs worldwide and is a well recognized entity in the United States (McPherson: Henry's Clinical Diagnosis and Management by Laboratory Methods, 22nd Edition, 2011 [Chapter 62])
  • Initiation of egg laying by mature worms 5 - 7 weeks after infection may result in acute schistosomiasis or Katayama fever, a serum sickness-like syndrome that occurs with heavy primary infection, especially that of S. japonicum
  • Antigenic challenge to the host is thought to result in immune complex formation (McPherson: Henry's Clinical Diagnosis and Management by Laboratory Methods, 22nd Edition, 2011 [Chapter 62])
  • Chronic infection results in continued deposition of eggs, many of which remain in the body
  • Granulomas produced around these eggs in the intestine and bladder are gradually replaced by collagen, resulting in fibrosis and scarring
  • Eggs trapped in the liver may induce pipe stem fibrosis with obstruction to portal blood flow
  • Occasionally, eggs are deposited in ectopic sites, such as the spinal cord, lungs or brain
Life cycle
  • Eggs are fully embryonated when passed and readily hatch when deposited in fresh water
  • Miracidia penetrate an appropriate species of snail host, where they undergo transformation and extensive asexual multiplication
  • After about 4 weeks, large numbers of fork tailed cercariae emerge from the mollusk
  • Cercariae swim actively about for hours and readily penetrate the skin of susceptible hosts, including humans
  • After penetration, the cercariae, now called schistosomules, enter the circulation and pass through the lungs before reaching the mesenteric portal vessels
  • Diagnosis is established by demonstrating eggs in feces or urine by direct wet mount or formalin ethyl acetate concentration methods
  • Zinc sulfate concentration is not satisfactory for recovery of heavy schistosome eggs
  • Eggs may be detected in biopsies of rectum, bladder or occasionally liver
  • Egg hatching methods may occasionally be requested to determine viability or less commonly, to detect a limited infection
    • Feces mixed in distilled water is placed in a flask, covered with foil (to keep out light), with neck or a sidearm exposed to bright light
    • Miracidia, if present, actively swim to the light and can be detected using a hand lens
  • Serologic tests may be useful for screening travelers to endemic areas, at risk patients with negative urine / stool examinations, monitoring response to therapy
  • Although not widely available, a limited number of reference laboratories and the CDC provide testing
  • CDC uses the Falcon assay screening test in a kinetic enzyme linked immunosorbent assay (FAST ELISA); sera that are positive by screening test are evaluated by immunoblot to improve specificity
  • Results vary based on antigens used and the test methods employed
Prognostic factors
  • Patients with coexisting HBV or HIV infections have worse prognoses
  • No vaccine is available
  • Recommendations:
    • Avoid swimming or wading in freshwater in endemic countries; ocean and chlorinated swimming pools are safe
    • Drink safe water
    • Apply vigorous towel drying after accidental, very brief water exposure to prevent penetration of skin by parasite; note: has limited effectiveness
Case reports
  • Praziquantel, taken for 1 - 2 days, is safe and effective for urinary and intestinal infections by all Schistosoma species
Microscopic (histologic) description
  • Focal ulcers, eggs may be calcified, surrounded by fibrosis or granuloma
  • S. haematobium: eggs are 110 - 170 by 40 - 70 microns, oval with terminal spine
  • S. japonicum: eggs are 70 - 100 by 55 - 65 microns, oval / round (more rounded than other types), minute subterminal or no spine
  • S. mansoni: eggs are 110 - 175 by 45 - 70 microns with thin transparent shell and definite lateral spine

  • Adult female schistosomes are slender, measuring up to 26 mm by 0.5 mm
  • Males, which are slightly shorter, enfold a female using the lateral margins of the body (the gynecophoral canal) to assist in sperm transfer
  • When examined in situ, schistosomes are often found in copula
Microscopic (histologic) images

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Images hosted on PathOut server:

Images contributed by Drs. Kiran Alam, Anshu Jain, Veena Maheshwari, Farhan A. Siddiqui and Ershadul Haq, J.N. Medical College (India):
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epithelium of
ureter with S.

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Schistosome eggs in submucosa

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Schistosome eggs in muscle layer

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Schistosome eggs in muscle layer - note terminal spine

Images contributed by Dr. Jennifer Stumph, Spectrum Health (USA):
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37 year old woman with S. mansoni eggs in colon biopsy

Images contributed by Dr. Lisa Cerilli:
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With colonic mucosa - type not identified

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CDC: images of all subspecies

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Granuloma surrounding egg - type not identified

Schistosoma haematobium - oval eggs with terminal spine:
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Anterior part

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Adult (female and male)

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Schistosoma haematobium eggs

Schistosoma japonicum - small eggs, more rounded, no / minute terminal spine:
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Schistosoma japonicum fluke

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Schistosoma mansoni - lateral spine:
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Schistosoma mansoni fluke

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Schistosoma mansoni eggs

Images contributed by Dr. Rico P. Lasaca, Divine Word Hospital (Phillipines):
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Type not identified