Schistosomiasis (Bilharzia)
Summary
Schistosomiasis is a parasitic disease caused by blood flukes (trematodes) of the genus Schistosoma. It affects >240 million people worldwide. Infection occurs via skin contact with contaminated freshwater containing larval forms (cercariae) released by intermediate snail hosts. The disease manifests in three stages: Dermatitis ("Swimmer's Itch"), Acute Schistosomiasis ("Katayama Fever"), and Chronic Schistosomiasis (caused by granulomatous reaction to trapped eggs in tissues). The two major forms are Urogenital (S. haematobium) and Intestinal/Hepatic (S. mansoni/japonicum). [1,2]
Key Facts
- Geographic Distribution:
- S. haematobium: Africa and Middle East. (Lake Malawi is a notorious hotspot).
- S. mansoni: Sub-Saharan Africa, South America (Brazil), Caribbean.
- S. japonicum: China, Philippines, Southeast Asia (Most virulent, zoonotic).
- Egg Identification:
- S. haematobium: Terminal spine. (Found in Urine).
- S. mansoni: Lateral spine. (Found in Stool).
- S. japonicum: Small/Rudimentary spine or round. (Found in Stool).
- The Cancer Link: Chronic S. haematobium infection is a Class 1 Carcinogen for Squamous Cell Carcinoma of the bladder (unlike TCC which is caused by smoking/dye).
Clinical Pearls
The "Midday Urine": S. haematobium egg excretion peaks between 10am and 2pm. Exercise (jumping up and down) before sampling increases the yield of eggs in urine.
Katayama Trap: Treat Katayama fever with Corticosteroids first. Praziquantel kills worms, releasing massive amounts of antigen which can worsen the hypersensitivity reaction (Jarisch-Herxheimer-like effect).
Wait to Test: Serology takes 6-8 weeks to turn positive. Testing a returning traveller the day they land is useless. Wait 3 months post-exposure to treat.
Transmission
- Vector: Freshwater Snails (Bulinus, Biomphalaria, Oncomelania).
- Activity: Swimming, bathing, washing in infested lakes/rivers.
- Not transmitted: By drinking (stomach acid kills cercariae, though buccal mucosa penetration is theoretically possible). No person-to-person spread.
Life Cycle
- Skin: Cercariae penetrate skin. Lose tails -> Schistosomula.
- Migration: Enter circulation -> Lungs -> Portal Venous System (mature into adults).
- Mating: Male and female pair up (female lives in male's Gynaecophoric canal).
- Migration to Target:
- S. haematobium: Vesical venous plexus (Bladder).
- S. mansoni/japonicum: Mesenteric veins (Bowel).
- Egg Laying: Thousands per day.
- Pathology: 50% of eggs are excreted. 50% are trapped in tissues. The host immune system forms a granuloma around the egg -> Fibrosis -> Calcification.
1. Cercarial Dermatitis ("Swimmer's Itch")
2. Acute Schistosomiasis ("Katayama Fever")
3. Chronic Schistosomiasis (Years)
- Abdomen: Hepatomegaly (firm/nodular)? Splenomegaly (can reach RIF)? Ascites?
- Chest: Signs of Cor Pulmonale (Pulmonary Hypertension from egg embolisation).
Microbiology
- Urine Microscopy: Filtration of terminal urine (10ml). Look for S. haematobium eggs.
- Stool Microscopy: Kato-Katz thick smear technique. S. mansoni/japonicum.
- Rectal/Bladder Biopsy: Higher sensitivity if microscopy negative. "Squash preparation".
Serology
- ELISA: Detects antibodies.
- Limitations: Cannot distinguish past from current infection. Positive for life. Useful for symptomatic travellers but not in endemic areas.
Imaging
- CXR: Pulmonary infiltrates (Katayama).
- Ultrasound Liver: Periportal fibrosis (distinctive "bull's eye" pattern).
- CT/MRI Brain: "Ring enhancing lesions" (granulomas).
Management Algorithm
DIAGNOSED SCHISTOSOMIASIS
(Eggs in urine/stool or serology)
↓
┌─────────────┴─────────────┐
ACUTE CHRONIC
(Katayama Fever) (Established)
↓ ↓
STEROIDS FIRST PRAZIQUANTEL
(Prednisolone) 40-60 mg/kg PO
↓ (Divided doses)
Add Praziquantel ↓
when stable Repeat in 4-6 weeks
(to kill matured forms)
1. Antiparasitic Therapy
- Drug: Praziquantel.
- Dose: 40 mg/kg (S. haematobium/mansoni). 60 mg/kg (S. japonicum - split dose).
- Mechanism: Increases calcium permeability of worm tegument -> Paralysis -> Detachment.
- Timing: Only effective against adult worms. If treated too early (e.g. during incubation), immature worms survive. Retreatment at 4-6 weeks ensures eradication.
2. Surgical
- Management of complications (Variceal banding, Shunt surgery, Ureteric stenting).
- Bladder Cancer: Squamous Cell (S. haematobium).
- Cor Pulmonale: Pulmonary hypertension from egg emboli.
- Renal Failure: Obstructive uropathy.
- Anaemia: Chronic blood loss.
- Early: 100% cure with Praziquantel.
- Late: Fibrosis (Symmers') is irreversible but progression stops with treatment.
Key Guidelines
| Guideline | Organisation | Key Recommendations |
|---|---|---|
| Schistosomiasis | WHO | Mass Drug Administration (MDA) with Praziquantel in endemic schools. |
| Travel Medicine | CDC | Avoid freshwater swimming in endemic areas. Chlorination kills cercariae. |
Landmark Knowledge
1. The Lake Malawi Cluster
- Many UK cases come from gap year students swimming in Lake Malawi.
- "Snail Fever" is a misnomer; the snail doesn't cause the fever, the worm does.
- Cercariae can penetrate intact skin within minutes. Towel drying does not prevent it.
What is Bilharzia?
It is a disease caused by tiny worms that live in fresh water in Africa and Asia. They come from snails.
How did I get it?
When you swam or waded in the lake, microscopic larvae burrowed through your skin. They travelled to your blood vessels and grew into worms. The worms are now laying eggs.
Why am I finding blood in my urine?
The eggs have sharp spines. As they try to exit your body through the bladder wall, they cause bleeding and inflammation.
Is it curable?
Yes! A tablet called Praziquantel kills the worms very effectively. We might need to give you a second dose later to catch any stragglers.
Primary Sources
- World Health Organization (WHO). Schistosomiasis Fact Sheet. 2023.
- Bocanegra C, et al. Clinical profile of Schistosomiasis in travellers. Trop Med Int Health. 2016.
- Ross AG, et al. Schistosomiasis. N Engl J Med. 2002;346:1212-1220.
Common Exam Questions
- Microbiology: "Traveller to Malawi. Haematuria. Egg with terminal spine?"
- Answer: Schistosoma haematobium.
- Pathology: "Type of bladder cancer?"
- Answer: Squamous Cell Carcinoma.
- Medicine: "Acute fever, hives, eosinophilia 4 weeks after swimming?"
- Answer: Katayama Fever (Acute Schistosomiasis).
- Pharmacology: "Treatment?"
- Answer: Praziquantel.
Viva Points
- Eosinophilia: Why is it high in Katayama but low in chronic? Acute response to antigens. In chronic phase, immune modulation ("blocking antibodies") downregulates the response.
- Co-infection: S. mansoni patients often have Salmonella co-infection (bacteria hitch a ride on the worm).
Medical Disclaimer: MedVellum content is for educational purposes and clinical reference. Clinical decisions should account for individual patient circumstances. Always consult appropriate specialists.