What is Schistosoma?

What is Schistosoma?

Schistosoma is a genus of parasitic flatworms (blood flukes) that causes schistosomiasis, a neglected tropical disease with significant morbidity and mortality affecting over 236 million people worldwide, primarily in sub-Saharan Africa. 1

Key Species and Distribution

• The three major schistosome species affecting humans are Schistosoma haematobium, S. mansoni, and S. japonicum, each with distinct geographical distributions and organ tropism 2, 1
• S. haematobium primarily affects the urogenital system, while S. mansoni and S. japonicum typically affect the intestinal tract and liver 1
• Schistosomiasis is prevalent in tropical and subtropical regions, with 90% of infections occurring in sub-Saharan Africa 3
• Natural hybrids between human and animal schistosome species (such as S. haematobium x S. bovis) are emerging, presenting potential zoonotic risks and diagnostic challenges 4

Life Cycle and Transmission

• Schistosoma has a complex life cycle involving both an intermediate host (freshwater snails) and a definitive host (humans) 1
• The parasite's infectious larvae (cercariae) develop in freshwater snails before penetrating human skin upon contact with infested water 1
• After skin penetration, the parasites mature into adult worms that reside in specific blood vessels: mesenteric veins (S. mansoni, S. japonicum) or pelvic veins (S. haematobium) 1
• Adult female worms produce eggs that are either excreted in stool or urine or become trapped in surrounding tissues, causing inflammatory responses and disease 1
• Even a small number of infected individuals can maintain transmission due to the multiplicative portion of the parasite's life cycle in snail vectors 2

Clinical Manifestations

• Schistosomiasis presents with varied clinical manifestations affecting multiple organ systems 3
• Urogenital schistosomiasis (S. haematobium) can cause hematuria, dysuria, bladder pathology, and increased risk of bladder cancer 5, 6
• Intestinal and hepatosplenic schistosomiasis (S. mansoni, S. japonicum) can lead to abdominal pain, diarrhea, and liver fibrosis 1
• Eggs trapped in tissues cause granulomatous inflammation, leading to organ-specific pathology 1
• Kidney disease, including perirenal adenopathy and glomerulonephritis, can occur as complications of chronic infection 5

Diagnosis

• Traditional diagnosis relies on detecting eggs in stool (for S. mansoni and S. japonicum) or urine (for S. haematobium) 2, 1
• Conventional microscopy methods include Kato-Katz (KK) stool examination and urine filtration, but these have limited sensitivity in low-prevalence settings 2
• Newer diagnostic approaches include detection of circulating worm antigens (CCA and CAA) in urine, which offer improved sensitivity 2
• Point-of-care Circulating Cathodic Antigen (POC-CCA) tests provide advantages including urine-based sampling and no requirement for microscopy 2
• Molecular methods like LAMP (Loop-mediated isothermal amplification) assays enable species-specific detection, including identification of hybrid species 7, 4

Treatment and Control

• Praziquantel (40 mg/kg as a single oral dose) is the recommended treatment for all Schistosoma species, being safe and effective against adult worms 5, 1
• Treatment should be sufficient in dosage and duration to completely eradicate the organism 5
• Serology may remain positive for years after successful treatment and cannot be used to assess treatment efficacy 5
• Patients with schistosomiasis should be evaluated for common co-infections such as Salmonella, HBV, HCV, and HIV 5
• Elimination efforts require a multifaceted approach including treatment, snail control, improved water and sanitation, education, and surveillance 1

Clinical Monitoring

• Patients with S. haematobium infection should be monitored for bladder cancer and urinary obstruction 5
• Those with hepatic fibrosis from schistosomiasis should be evaluated for the development of kidney disease 5
• Kidney biopsy may be considered in patients with progressive kidney disease but can be deferred in mild cases with empirical antiparasitic treatment 5
• No immunosuppressive agents are indicated in schistosomal nephropathy 5