What is schistosomiasis?

What is Schistosomiasis?

Schistosomiasis (bilharzia) is a neglected tropical disease caused by parasitic blood flukes of the genus Schistosoma that infect over 250 million people worldwide, transmitted through skin contact with freshwater contaminated by larvae released from infected snails. 1, 2

The Parasite and Transmission Cycle

• The disease is caused by parasitic flatworms (blood flukes) of the genus Schistosoma, with the main disease-causing species being S. haematobium, S. mansoni, and S. japonicum 1, 3
• Transmission requires three key elements: contamination of surface water by human excreta, specific freshwater snails serving as intermediate hosts, and human water contact 3, 2
• Infective larvae (cercariae) are released by freshwater snails and penetrate human skin during water exposure 1, 4
• Mature adult worms reside in the mesenteric veins (S. mansoni and S. japonicum) or pelvic veins (S. haematobium), where female worms lay eggs that are secreted in stool or urine 1

Geographic Distribution and Epidemiology

• The disease is widely distributed in the Middle East, South America, Southeast Asia, and particularly sub-Saharan Africa 1, 2
• Schistosomiasis affects over 250 million people worldwide, leading to an estimated loss of 1.9 million disability-adjusted life years (DALYs), though this figure is likely underestimated 5
• The disease shows focal epidemiology with overdispersed population distribution, with higher infection rates in children than adults 3, 5

Clinical Disease Patterns

Acute Schistosomiasis (Katayama Syndrome)

• Occurs 2-8 weeks after exposure in newly infected travelers, characterized by fever, dry cough, urticarial rash, diarrhea, and headache 6
• This represents a self-limiting hypersensitivity reaction and is most evident in travelers following primary infection 3, 2

Chronic Intestinal Schistosomiasis

• Caused primarily by S. mansoni and S. japonicum, presenting with chronic or intermittent abdominal pain, weight loss, and diarrhea 6
• Heavy infections can cause dysenteric illness, intestinal obstruction, or bleeding 6
• Eggs trapped in the liver cause inflammatory immune responses (including granulomas) leading to hepatosplenic inflammation, liver fibrosis, portal hypertension, splenomegaly, and variceal bleeding 1, 2

Chronic Urogenital Schistosomiasis

• Caused by S. haematobium, presenting with hematuria, proteinuria, dysuria, and may cause hematospermia and inter-menstrual bleeding 6
• Complications include renal failure and squamous-cell carcinoma of the bladder 2
• Eggs trapped in the bladder cause inflammatory and obstructive disease in the urinary system 1, 5

Neurological Involvement

• Schistosomiasis is a rare cause of myelopathy and CNS space-occupying lesions, reported in returning travelers 7
• Acute myelopathy is the commonest neurological presentation (S. mansoni or S. haematobium), presenting as rapidly progressive transverse myelitis affecting the conus medullaris and cauda equina with lower limb pain, motor dysfunction, and bladder/bowel dysfunction 7
• Cerebral disease (S. japonicum) presents with seizures, motor or sensory impairment, or cerebellar syndrome 7

Pathophysiology

• The primary pathology results from eggs trapped in surrounding tissues and organs, not from the adult worms themselves 1
• Immunopathological reactions against schistosome eggs trapped in host tissues lead to inflammatory responses, granuloma formation, and subsequent fibrosis 1, 5
• Complex immune mechanisms lead to slow acquisition of immune resistance over time, though innate factors also contribute 3, 5

Diagnostic Challenges

• Conventional diagnosis relies on microscopic detection of eggs in feces or urine, though sensitivity may be low, especially in low-prevalence areas 8, 2
• Newer, more sensitive methods include detection of worm antigens in urine or serum, such as circulating cathodic antigen (CCA) and circulating anodic antigen (CAA) 8
• Recent studies show conventional methods may significantly underestimate infection prevalence, with newer antigen detection tests revealing 6-10 times more infections in low-prevalence areas 8
• The existence of "egg-negative/worm-positive schistosomiasis" presents challenges for elimination programs, as these individuals may still contribute to transmission 8

Treatment

• Praziquantel is the first-line treatment for all Schistosoma species, typically administered as a single oral dose of 40 mg/kg 8, 9, 2
• The drug is safe and efficacious against adult worms of all six Schistosoma species infecting humans 1
• However, praziquantel does not prevent reinfection, and the emergence of drug resistance is a concern 1
• For neuroschistosomiasis, expert opinion advises prompt corticosteroid therapy with dexamethasone starting one day before praziquantel 7

Public Health Significance

• Schistosomiasis elimination requires treatment, snail control, improved water/sanitation/hygiene, accurate diagnostics, and surveillance-response systems tailored to social-ecological settings 1
• Great advances have been made through population-based chemotherapy and mass drug administration campaigns, but these require political commitment and strong health systems 3
• The discovery that schistosomiasis is more prevalent than previously thought has significant implications for public health goals and elimination strategies 8