Schistosomiasis

Schistosomiasis is a parasitic disease caused by schistosomes, a type of trematode. Infection occurs when skin comes into contact with parasite-infested water. Clinical manifestations vary depending on the stage of the infection and the type of schistosome. The initial skin penetration may cause a pruritic maculopapular rash known as swimmer's itch. During parasite migration through the bloodstream, acute schistosomiasis syndrome (also known as Katayama fever) may manifest with fever, cough, and angioedema. Chronic infection by schistosomes causes a granulomatous inflammatory response to schistosome eggs with symptoms varying based on disease severity and location. Genitourinary schistosomiasis may manifest with hematuria and dysuria. Long-standing infection increases the risk of bladder cancer. Intestinal schistosomiasis may manifest with diarrhea and abdominal pain, whereas hepatosplenic schistosomiasis can lead to hepatosplenomegaly and/or portal hypertension. Diagnosis is confirmed by identifying eggs on microscopic examination of stool or urine. Acute schistosomiasis syndrome is treated symptomatically with glucocorticoids. The mainstay of treatment for parasite eradication is praziquantel.

• Frequency: over 200 million people infected annually worldwide ^[1]
• Occurrence: mainly rural areas with freshwater sources and poor sanitation

Epidemiological data refers to the US, unless otherwise specified.

• Pathogen: schistosomes (parasitic trematodes of the genus Schistosoma)
  • Schistosoma mansoni: Africa, South America, and the Caribbean
  • Schistosoma haematobium: Africa and the Middle East
  • Schistosoma japonicum: China and Southeast Asia
• Lifecycle
  • Infected humans (definitive host) excrete schistosome eggs in urine or feces.
  • Eggs hatch in water and release miracidia.
  • Miracidia infect specific freshwater snails (intermediate hosts) where they develop into cercaria, which are released back into the water.
  • When humans come in contact with contaminated water (e.g., while swimming), cercaria can penetrate the skin and enter the circulation.
  • Maturation into adult schistosomes and migration to the veins of the target organs
  • Females lay eggs, leading to capillary closure and chronic inflammation in the affected organs.
  • Penetration of eggs in lumen of the intestine or bladder (depending on the species).

Clinical features depend on the stage, schistosome type, and infected organs.

1. Local reaction (swimmer's itch or cercarial dermatitis): pruritic maculopapular rash at the point of entry of cercaria into human skin
2. Acute schistosomiasis syndrome (Katayama fever)
   • Serum sickness-like disease: immune complex formation of antigens released from eggs and/or adult worms with host antibodies
   • Incubation period: 3–8 weeks ^[2]
   • Clinical features: fever, fatigue, cough, myalgia, angioedema
   • Usually self-resolves after 2–10 weeks. ^[3]
3. Chronic schistosomiasis: Deposition of eggs leads to chronic inflammation and granuloma formation.

Suspect schistosomiasis in all individuals who have been exposed to freshwater in endemic areas, even if they are asymptomatic. Consider consulting infectious diseases for advice. ^[4]

• Confirmatory studies ^[5][6]
  • Microscopic examination of stool or urine for eggs
  • Schistosoma subtype determined by egg morphology
    • S. mansoni: Eggs have a prominent lateral spine.
    • S. haematobium: Eggs have a prominent terminal spine.
    • S. japonicum: Eggs have a miniscule lateral spine.
• Additional studies ^[4][7]
  • CBC may show eosinophilia.
  • Serology
  • Chronic infections may require additional studies based on clinician suspicion, e.g., urinalysis, biopsy, CT scan.

There is no consensus on certain treatment details (e.g., dosages, timing); consider consulting infectious diseases for help with management. ^[4][6]

• Swimmer's itch
  • Self-resolves in 4–7 days ^[4]
  • Symptomatic management of pruritus (e.g., antihistamines, topical steroids)
• Acute schistosomiasis syndrome ^[4][6]
  • Administer glucocorticoids to reduce inflammation, e.g., prednisone (off-label). ^[4]
  • Administer praziquantel once inflammation improves. ^[4]
    • S. mansoni, S. haematobium, Schistosoma intercalatum: praziquantel (off-label) ^[7]
    • S. japonicum, Schistosoma mekongi: praziquantel ^[7]
    • Repeat therapy 2–6 weeks after symptom onset.
• Chronic schistosomiasis: : Administer praziquantel (see dosing in “Acute schistosomiasis syndrome”).
• Posttreatment monitoring: Test urine and stool 6–8 weeks after completing therapy and again 18–32 weeks later to confirm clearance.

The following applies for endemic areas:

• Avoid swimming in freshwater or wear protective clothing if contact is unavoidable.
• Boil water prior to drinking.
• Drinking water or water used for bathing should not come into contact with sewage systems, feces, or urine.