Schistosomiasis

Aka: Schistosomiasis, Schistosome Infection, Blood Fluke, Bilharziasis, Katayama Fever, Acute Schistosomiasis, Acute Toxemic Schistosomiasis, Schistosoma Mansoni, Schistosoma Japonica, Schistosoma Hematobium

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II. Epidemiology

  1. Prevalence: 140-240 Million worldwide (>85% in Africa)
  2. Affected regions: Sub-Saharan Africa and Southeast Asia

III. Pathophysiology

  1. Schistosomes are Trematodes (Flukes) in the phylum Platyhelminthes (Flatworms)
    1. Schistosomes are fresh-water Parasites
    2. Schistosomes infect freshwater snails as an intermediate host (all flukes have a water snail intermediary)
    3. As Flatworms, Schistosomes lack their own intestinal tract
      1. Schistosomes must instead obtain metabolized molecules from their environment (venous blood)
  2. Schistosmes (Blood Flukes) are divided into 3 species that infect humans
    1. Schistosoma japonicum (Asia)
      1. Lives in the hepatic and intestinal tract, and releases eggs into the stool
    2. Schistosoma Mansoni (South America, Caribbean, Africa)
      1. Lives in the hepatic and intestinal tract, and releases eggs into the stool
    3. Schistosoma haematobium (Africa)
      1. Lives in the genitourinary tract, and releases eggs into the urine
  3. Schistosme life cycle
    1. Schistosomes eggs hatch in fresh water, releasing larvae that infect freshwater snails
      1. Schistosome larvae mature within freshwater snails until mature
      2. Mature Schistosome larvae (cercariae) exit the snail, and are now infectious, awaiting a human host
    2. Mature Schistosome larvae (cercariae) invades human host via unbroken skin
      1. Occurs on exposuse to infested fresh water (boating or swimming in endemic regions)
    3. Mature schistosome larvae (cercariae) migrate to the intrahepatic portal venous system
      1. Larvae mature into adult worms and mate with other worms
      2. Adult worms may survive and reproduce for years within the venous system
      3. Schistosomes evade human host immune response via molecular mimicry
        1. Antigens on the surface are similar to human host molecules (e.g. ACTH)
        2. Schistosomes release other molecules that suppress PMN and Leukocyte response
    4. Female Schistosomes lay eggs in the venous system surrounding their target organs
      1. S. japonicum and S. mansoni lay eggs in the mesenteric veins surrounding the intestinal tract
        1. Eggs are excreted in stool, and hatch in freshwater to begin another cycle
      2. S. haematobium lays eggs in the perivesical veins surrounding the Bladder
        1. Eggs are excreted in urine, and hatch in freshwater to begin another cycle
  4. Illness via 3 sequential clinical manifestations
    1. Dermatitis
      1. Occurs on skin entry by mature schistosome larvae (cercariae)
    2. Katayama Fever
      1. Occurs after a 4-8 week Incubation Period following skin entry
      2. Katayama Fever symptoms occur when adult schistosomes lay eggs within the venous system
    3. Chronic fibrosis
      1. Chronic inflammation triggered by deposition of eggs within organs
      2. Eggs that are not excreted deposit in venous system and organ tissue (e.g. liver, lung, brain)
      3. Immune response to deposited eggs results in venous Granulomas, ulcerations
      4. Secondary venous obstruction follows with end organ complications (see findings below)

IV. Findings

  1. Initial skin invasion by mature schistosome larvae (cercariae)
    1. Transient intense Pruritus
    2. Rash
  2. Katayama Fever (4-8 weeks after skin invasion)
    1. Flu-like illness
      1. Fever
      2. Fatigue
      3. Malaise
      4. Arthralgias
      5. Lymphadenopathy
    2. Respiratory
      1. Nonproductive Cough
      2. Bronchospasm
    3. Gastrointestinal
      1. Abdominal Pain
      2. Diarrhea
      3. Weight loss
      4. Hepatosplenomegaly
    4. Skin
      1. Urticaria
  3. Chronic fibrosis (specific to venous distribution of deposited schistosoma eggs)
    1. Hepatic Fibrosis, Portal Hypertension and Splenomegaly
    2. Chronic Abdominal Pain and Diarrhea
    3. Chronic Hematuria
    4. Brain and spine complications
    5. Pulmonary Hypertension
    6. Female genital Schistosomiasis (cervical, uterine, tubo-ovarian involvement)

V. Differential Diagnosis: Acute Illness (Katayama Fever)

  1. See Fever in the Returning Traveler
  2. Malaria
  3. Typhoid

VI. Imaging: Acute Illness (Katayama Fever)

  1. Chest XRay with Pulmonary Infiltrates
    1. May be present 4 to 6 weeks after travel

VII. Labs:

  1. Complete Blood Count
    1. Eosinophilia during Katayama Fever
      1. May differentiate Schistosomiasis from other causes of Fever in the Returning Traveler
  2. Microscopic exam of stool and urine
    1. Schistosoma Eggs (uncommonly seen in light infections)
  3. Serology (ELISA)
    1. Highest Test Sensitivity

VIII. Complications

  1. Chronic infection affecting liver, Bladder, Uterus, Gastrointestinal Tract
  2. Severe neurologic sequelae (rare)
    1. CNS or spinal cord lesions (Schistosoma japonicum)

IX. Management

  1. Acute Toxemic Schistosomiasis (Katayama Fever)
    1. Prednisone for 3-6 days (until 48 hours after symptom resolution)
    2. Praziquantel 40-60 mg/kg orally for 2-3 doses on same day
      1. Higher dose range (60 mg/kg) is used for S. japonicum and S. mekongi
      2. Repeat Praziquantel in 4-6 weeks (with 2-3 days of Prednisone, after worms have matured)
      3. Repeat Praziquantel in 12 weeks (without Prednisone)
  2. Neuroschistosomiasis with myeloradiculopathy
    1. Pretreated with Prednisone for 2-3 days
    2. Praziquantel 40-60 mg/kg orally once (after steroid pretreatment)
    3. Prednisone is continued for up to 6 months

X. Prevention

  1. Praziquantel is NOT effective for Postexposure Prophylaxis
  2. Waste management and sanitation
  3. Elimination of intermediate host populations (specific freshwater snails)

XI. References

  1. (2024) Sanford Guide, Accessed on IOS 8/6/2025
  2. Gladwin, Trattler and Mahan (2014) Clinical Microbiology, Medmaster, Fl, p. 368-9
  3. Black, Martin, DeVos (2018) Crit Dec Emerg Med 32(8): 3-12

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