Schistosomiasis

Schistosomiasis is a parasitic disease caused by schistosomes, a type of trematode/fluke. Infection occurs when skin comes in 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 present with fever, cough, and angioedema. Chronic infection by schistosomes causes a granulomatous inflammatory response to schistosome eggs. The consequences of this chronic inflammation depend on the anatomic location of the eggs and the burden of disease. Genitourinary schistosomiasis may present with hematuria and dysuria. Long-standing infection increases the risk of bladder cancer. Intestinal schistosomiasis may present with diarrhea and abdominal pain, whereas hepatic schistosomiasis can lead to hepatosplenomegaly and/or portal hypertension. Definitive diagnosis requires direct visualization of schistosome eggs in the stool or urine microscopy. Acute schistosomiasis syndrome is treated symptomatically with corticosteroids. 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 or flukes of the genus Schistosoma)
  • Schistosoma mansoni: Africa, South America, and the Carribean
  • 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
   • Patients usually spontaneously recover after 2–10 weeks. ^[3]
3. Chronic schistosomiasis: Deposition of eggs leads to chronic inflammation and granuloma formation.

• Laboratory tests: eosinophilia
• Pathogen detection
  • Serology
  • Definitive diagnosis (gold standard): direct visualization of schistosome eggs via stool or urine microscopy. Egg morphology is indicative of Schistosoma subtype:
    • Schistosoma mansoni: Eggs have a prominent lateral spine.
    • Schistosoma haematobium: Eggs have a prominent terminal spine.
    • Schistosoma japonicum: Eggs have a miniscule lateral spine.
• Further tests
  • Imaging may be required to rule out specific organ involvement.
  • Detection of antigens in urine/stool sometimes used to measure treatment efficacy.

• Acute schistosomiasis syndrome
  • Corticosteroids
  • Praziquantel should only be administered after symptoms of acute illness have resolved.
• Chronic schistosomiasis: : praziquantel

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.