Schistosoma: Blood Flukes (Bilharzia)

Introduction to Schistosoma: Blood Flukes (Bilharzia)

  • Cause Schistosomiasis or bilharziasis
  • The three main species infecting humans are
    • Schistosoma haematobium
    • S. japonicum
    • S. mansoni
  • Two other species, more localized geographically, are S.mekongi (Thailand) and S. intercalatum (West central Africa)

Schistosomiasis: Geographical Distribution

  • S. haematobium in Africa and the Middle East
  • An isolated focus of endemicity was identified in India in Ratnagiri, south of Mumbai in 1952.
  • Schistosoma mansoni is found in parts of South America and the Caribbean, Africa, and the Middle East;
  • S. japonicum in the Far East (Japan)


  • Adult diecious (sexes are separate) —
    • Female 20 to 26 mm long , 0.25 mm wide
    • Male 10 to 15 mm long by Imm wide
  • Males have gynecophoric canal on ventral side where the female worm reposes
  • Lives in vesical and pelvic plexuses of veins
  • S.mansoni — smaller , lives in venules of inferior mesenteric plexus in sigmoidorectal area
  • S.japonicum —superior mesenteric vein draining ileocaecal region, also seen in intrahepatic portal venules and haemorrhoidal plexus of veins


Life Cycle

  • Complex
    • Human: Definitive host
  • Snails – First intermediate host
  • Second intermediate host – NONE

Life cycle in general

  • Eggs with spines, do not have operculum, passed in urine or faeces
  • First stage free swimming ciliated larvae : Miracidium hatches
  • Second Stage in mollusc k/a Sporocyst
    • Absent-Third stage Redia
    • Cercaria (kerkos=tail Greek ) Final larval stage in mollusc .
    • Free living larva escapes in water
    • Absent Metacercariae stage
  • Gravid fernale passes upto 300 eggs per day
  • Eggs pierce vesical wall and enter bladder lumen
  • Eggs passed in water
  • Ciliated Miracidium larva hatch out and swim
  • Penetrate tissues of intermediate host snail (Bulinus)
  • Miracidia lose cilia , in 4-8 weeks first and second generation sporocysts with large no. of cercariae by asexual reproduction.

Miracidium & Cercaria

Life cycle

  • Infective Stage: Cercaria (forked tail) swimming in fresh water
  • Mode of infection:
    • Cercaria penetrates human skin upon exposure to fresh water containing cercaria.
  • Enter peripheral venules
  • Migrate- vena cava-rt heart-pulm. Circulation-left heart-systemic circulation-liver-intrhepatic portal veins (grow and differentiate sexually)-inf. Mesenteric veins-vesical and pelvic plexus (mature, mate,lay eggs)
  • Eggs appear 10-12 weeks after infection
  • Adults may live for 20-30 years.
  • Humans only natural o host
  • No animal reservoir known

Differences from other flukes

  • Schistosoma has only one intermediate host (snail).
  • No reservoir.
  • Schistosoma eggs not operculated.
  • Forked tail cercaria.
  • Cylindrical worm, lacks muscular pharynx
  • It is the only trematode in which sex is separated (other trematodes are hermaphrodite)

Schistosoma haematobium

  • causes urinary schistosomiasis.
  • It occurs in Africa, India and the Middle East.
  • The intermediate host is the Bulinus snail.
  • They are exclusively parasites of man.
  • The mature worms live in copula mainly in the inferior mesenteric veins and the females deposit their eggs in the walls of the bladder and finally making their way into the urine.

S. mansoni

  • Intestinal schistosomiasis occurs in West and Central Africa, Egypt,, the Arabian Peninsula,
    Brazil, Surinam, Venezuela & West Indies.
  • The intermediate host is an aquatic snail of the genus Biomphatlaria.
  • Man —D. host, occasionally baboons and rats
  • The adult worms live in smaller branches of the inferior mesenteric vein draining sigmoidorectal region

Schistosoma japonicum

  • Found in China, Japan, Philippines, Indonesia.
  • The adults live in branches of the,Superior mesenteric vein draining ileocecal region
  • Disease of bowel; eggs passed in the feces.
  • Zoonosis – large number of mammals serve as reservoir hosts; cats, dogs and cattle playing major roles

Clinical Features

  • Many infections are asymptomatic.
  • Swimmer’s itch ( Cercarial Dermatitis) transient petechial lesions at site of entry of cercariae
  • Acute schistosomiasis (Katayama’s fever) may occur weeks after the initial infection, especialiy by S. mansoni and S. japonicum.
  • Manifestations include fever, cough, abdominal pain, diarrhea, hepatosplecnomegaly, and eosinophilia.
  • Symptoms due to egg deposition & extrusion
    • painless terminal haematuria,
    • frequency of micturition and burning,
    • (hyperplasia and inflammation of bladder mucogg)
  • Occasionally central nervous system lesions occur:
    • cerebral granulomatous disease may be caused by ectopic S. japonicum eggs in the brain, and granulomatous lesions around ectopic eggs in the spinal cord from S. mansoni and S. haematobium infections

Immuno-pathologic consequences

  • Chronic infection may cause granulomatous reactions and fibrosis in the affected organs, which may result in manifestations that include:
    • colonic polyposis with bloody diarrhea (S. mansoni mostly);
    • portal hypertension with hematemesis and Splenomegaly (S. mansoni ,S. japonicum.)
    • cystitis and ureteritis (S. haematobium) with hematuria, which can progress to bladder cancer;
    • pulmonary hypertension (S. mansoni ,S. japonicum .more rarely S. haematobium) :glomerulonephritis; and central nervous system lesions

Schistosomiasis: Diagnosis

  • History
  • Symptoms
  • Histology
  • Eggs in the urine or stool

Laboratory Diagnosis

  • Microscopic identification of eggs in stool or urine is the most practical method for diagnosis.
  • (Stool examination for infection with S. mansoni or S. japonicum and urine examination for S. haematobium)
  • Tissue biopsy (rectal biopsy for all species and biopsy of the bladder for S- haematobium) May demonstrate eggs when stool or urine examinations are negative
  • Ultrasonography

Other Microbiology Notes:



Dracunculus Medinensis

Ancylostoma duodenale (Hook worm)

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