Definition and epidemiology
Schistosomiasis (bilharzia) is a chronic parasitic disease caused by trematodes (blood flukes) of the genus Schistosoma. The major species are:
- S. haematobium - urinary/genital tract (Africa, Middle East)
- S. mansoni - intestinal/hepatic (Africa, South America, Caribbean)
- S. japonicum - intestinal/hepatic, more severe (East Asia)
- S. mekongi and S. intercalatum - regional, less common
It is estimated to affect over 200 million people worldwide, with the heaviest burden in sub-Saharan Africa. Transmission requires freshwater contact (bathing, wading, washing clothes, farming) in areas where infected intermediate host snails (Biomphalaria, Bulinus, Oncomelania) are present. School-age children and agricultural or fishing communities carry the highest prevalence and intensity of infection. It is one of the WHO-designated neglected tropical diseases and a major target of mass drug administration programs.
- Cercariae penetrate skin from freshwater contact; no mosquito or vector bite involved
- Pathology is driven by the host granulomatous response to eggs, not the adult worms
- S. haematobium causes painless hematuria and is linked to bladder squamous cell carcinoma
- S. mansoni/S. japonicum cause periportal (Symmers') fibrosis with presinusoidal portal hypertension
- Katayama fever is an acute serum-sickness-like reaction with fever and marked eosinophilia
- Diagnosis is by finding species-specific eggs in urine or stool; serology helps in early infection
- Praziquantel is first-line for all species, but is only effective against mature adult worms, not immature stages
- Snail intermediate hosts (Biomphalaria, Bulinus, Oncomelania) are essential to transmission
Etiology and risk factors
Infection occurs when free-swimming larvae called cercariae, released from infected freshwater snails, penetrate intact human skin during water contact.
Risk factors:
- Residence in or travel to endemic regions (Africa, parts of South America, the Middle East, Southeast Asia, the Caribbean)
- Occupational or recreational freshwater exposure (farming, fishing, swimming, laundering)
- Poor sanitation infrastructure that allows human waste to contaminate water sources, perpetuating the snail-human transmission cycle
- Children are disproportionately affected due to frequent water play
Humans are the main reservoir for S. haematobium and S. mansoni; S. japonicum also infects animal reservoirs (dogs, cattle, water buffalo), making control more difficult.
Pathophysiology
The life cycle: cercariae penetrate skin and become schistosomulae, migrate through venous/lymphatic circulation to the lungs and then the liver, mature into adult worms, and migrate to their target venous plexus (mesenteric veins for S. mansoni/japonicum, vesical/pelvic venous plexus for S. haematobium). Adult worm pairs produce eggs that are meant to pass into the bowel or bladder lumen for excretion.
Key mechanism
Most pathology results not from the adult worms (which evade immune destruction by coating themselves with host antigens) but from the host immune response to deposited eggs. Eggs that fail to pass into the lumen become trapped in tissue, triggering a granulomatous (Th2-mediated) reaction with eosinophils, leading to:
- Fibrosis and scarring over years of chronic infection
- S. mansoni/japonicum: periportal ("pipestem" or Symmers') fibrosis, portal hypertension, hepatosplenomegaly with relatively preserved liver function (presinusoidal block)
- S. haematobium: bladder wall granulomas, calcification ("sandy patches"), ureteral strictures, hydronephrosis, and squamous cell carcinoma of the bladder with chronic infection
An early hypersensitivity reaction to migrating schistosomulae and early egg deposition produces Katayama fever, a serum-sickness-like syndrome.
Clinical features
Acute phase
- Swimmer's itch: pruritic papular rash at site of cercarial penetration, within hours to days
- Katayama fever (weeks after exposure): fever, urticaria, myalgia, hepatosplenomegaly, cough, and marked eosinophilia; more common in non-immune travelers
Chronic phase
| Species | Classic findings |
|---|---|
| S. haematobium | Painless terminal hematuria, dysuria, urinary frequency, bladder/ureteral fibrosis, hydronephrosis |
| S. mansoni/japonicum | Abdominal pain, bloody diarrhea, hepatosplenomegaly, portal hypertension, variceal bleeding |
Other chronic manifestations include female genital schistosomiasis (vulval/cervical lesions, increased HIV susceptibility), male genital involvement, and rarely CNS schistosomiasis (seizures, myelopathy from ectopic egg deposition).
Diagnosis
Confirmatory tests
- Microscopy: identification of species-specific eggs
- S. haematobium: terminal spine, eggs in urine (best collected midday)
- S. mansoni: lateral spine, eggs in stool
- S. japonicum: small lateral knob, eggs in stool
- Multiple samples over several days increase sensitivity because egg excretion is variable
Supportive/adjunct tests
- Serology (antibody detection): useful in travelers or low-intensity infection, especially before eggs appear (can take 6-12 weeks post-exposure); cannot distinguish active from past infection
- Eosinophilia on CBC, especially pronounced in acute Katayama fever
- Urinalysis: microscopic or gross hematuria in S. haematobium
- Imaging: pelvic ultrasound for bladder wall thickening/calcification and hydronephrosis; abdominal ultrasound for periportal fibrosis, splenomegaly, portal vein dilation
- Cystoscopy with biopsy or rectal/bladder mucosal biopsy if stool/urine microscopy is negative but suspicion remains
- PCR-based stool/urine assays are increasingly used in reference laboratories
Differential diagnosis
- Urinary schistosomiasis vs urinary tract infection, bladder carcinoma (non-schistosomal), nephrolithiasis, tuberculosis of the urinary tract
- Intestinal/hepatic schistosomiasis vs viral hepatitis, other causes of portal hypertension (cirrhosis), amoebiasis, inflammatory bowel disease
- Katayama fever vs typhoid fever, acute viral hepatitis, other causes of eosinophilic febrile illness (e.g., strongyloidiasis, fascioliasis, acute toxoplasmosis)
- Swimmer's itch vs cercarial dermatitis from non-human schistosomes (self-limited, no systemic disease), contact dermatitis
Management
First-line therapy
- Praziquantel is the drug of choice for all Schistosoma species, given as oral therapy. It is effective against mature adult worms but has little to no activity against migrating schistosomulae (immature stages), so it is not reliable very early in infection or as prophylaxis; retreatment after several weeks is often recommended in acute infection to catch worms that mature after the first dose
- WHO recommends mass drug administration with praziquantel in endemic communities, particularly school-age children, as the backbone of control programs
Acute Katayama fever
- Corticosteroids may be used adjunctively to control the hypersensitivity reaction. Praziquantel is given but is less effective at this stage against immature worms, so it is often delayed or repeated several weeks later once worms have matured
Supportive and disease-specific care
- Management of complications: endoscopic variceal banding or beta-blockers for portal hypertension-related varices, urologic intervention for ureteral strictures or hydronephrosis
- Public health measures: snail control, improved sanitation and water access, health education, and periodic deworming programs are essential for long-term control and reinfection prevention
- Oxamniquine (historically used for S. mansoni) and artemisinin derivatives (adjunctive, limited role) are alternatives in some settings but praziquantel remains universally first-line
Complications and prognosis
- Urinary tract: hydronephrosis, chronic kidney disease, recurrent UTIs, and increased risk of squamous cell carcinoma of the bladder (classic association, distinct from the usual transitional cell carcinoma seen in smokers)
- Hepatosplenic disease: portal hypertension, esophageal varices with risk of life-threatening variceal hemorrhage, hypersplenism
- Pulmonary hypertension: from embolized eggs reaching the lungs via portosystemic shunts
- Female genital schistosomiasis: infertility, increased susceptibility to HIV acquisition
- CNS disease: seizures (S. japonicum), transverse myelitis (S. mansoni/haematobium)
Prognosis is generally good with early treatment, since praziquantel is highly effective against adult worms and fibrotic changes may partially regress if treated before advanced disease. Delayed diagnosis in endemic, resource-limited settings allows progression to irreversible organ damage.
"Hematobium = Hematuria, Bladder" - S. haematobium causes blood in urine and bladder pathology; the other species (mansoni, japonicum) go to the "gut and liver" (mesenteric veins, hepatosplenic disease).
Practice questions
1. A 12-year-old boy from rural Zimbabwe presents with several months of painless hematuria at the end of urination. Urinalysis shows red blood cells, and microscopy of a urine sample reveals an oval egg with a prominent terminal spine. Which of the following is the most likely long-term complication if this condition remains untreated?
2. A 28-year-old aid worker returns from Malawi three weeks after swimming in Lake Malawi. She reports fever, urticarial rash, myalgias, and dry cough. Labs show marked eosinophilia and mild hepatosplenomegaly. Stool and urine microscopy are negative for ova. Which of the following best explains the negative microscopy findings?
3. A 45-year-old farmer from the Nile Delta with a long history of untreated schistosomiasis presents with hematemesis. Physical exam reveals splenomegaly without jaundice, and liver function tests are relatively normal. Which of the following best describes the mechanism of portal hypertension in this patient?
Frequently asked questions
What causes schistosomiasis?
Schistosomiasis is caused by parasitic flatworms (blood flukes) of the genus Schistosoma, most commonly S. haematobium, S. mansoni, and S. japonicum. Infection occurs when larval forms called cercariae, released by infected freshwater snails, penetrate human skin during contact with contaminated water such as lakes, rivers, or irrigation canals in endemic tropical and subtropical regions.
What are the first symptoms of schistosomiasis?
The earliest symptom is often an itchy rash (swimmer's itch) at the skin penetration site within hours to days of water exposure. Weeks later, some people develop Katayama fever, with fever, hives, muscle aches, cough, and abdominal discomfort. Many infections, however, remain asymptomatic for years until chronic urinary or intestinal symptoms develop.
How is schistosomiasis diagnosed?
Diagnosis is confirmed by identifying characteristic parasite eggs under a microscope in urine (for S. haematobium) or stool (for S. mansoni and S. japonicum). Blood tests for antibodies can detect infection before eggs appear, and ultrasound can show bladder or liver changes. Eosinophilia on a blood count is a supportive clue, especially in travelers.
Is schistosomiasis curable?
Yes, schistosomiasis is curable with the antiparasitic drug praziquantel, which is highly effective against adult worms in a single or short course of oral treatment. However, it does not reverse established fibrosis or organ damage from long-standing infection, so early treatment gives the best outcomes. Reinfection is common in endemic areas without ongoing prevention.
Schistosomiasis vs liver fluke infection - what is the difference?
Schistosomiasis is caused by blood flukes acquired through skin contact with contaminated freshwater, with adult worms living in veins around the bladder or intestines. Liver fluke infections (such as Fasciola or Clonorchis) are acquired by eating contaminated raw fish, crustaceans, or aquatic plants, and the adult worms live in the bile ducts or liver parenchyma rather than blood vessels.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: B65
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.