Schistosomiasis

Schistosomiasis is caused by infection by parasitic worms of the Schistosoma species.

Massive global health issue – up to 150 000 people estimated to die annually from renal failure secondary to S. haematobium, with up to 70 million in Sub-Saharan Africa with haematuria.

Main risk factor is exposure to infected water.

3 relevant medical species of schistosoma:

• S. mansoni – Africa, Arabian peninsula, South America (bowel disease)
• S. japonicum – China, south east Asia (bowel disease)
• S. haematobium – Sub-Saharan Africa, Arabian peninsula (urological disease)

 

Life cycle

1. Cercariae penetrate human skin in fresh water
2. Cercariae lose their forked tail and become schistosomulae, travelling through veins through the lung to the portal circulation
3. Over 3 – 4 months they mature to become adult worms, and form pairs
4. Pairs migrate to the venous plexus of the bladder
5. Females lay 100s to 1000s of eggs in the pelvic veins
6. The eggs penetrate through veins into the urothelium and to the urine, so eggs are excreted into the urine
7. Eggs hatch into the water, releasing miracidia
8. Miracidia infect snails which are the intermediate host, and miracidia undergo asexual reproduction
9. Snails release the cercariae with their forked tails into the water

 

Pathogenesis

Penetration of the skin by cercariae and their maturation causes minimal human immune reaction.

• There may be localised skin inflammation
• During maturation, the cercariae generate an outer bilipid layer, helping evade any immune recognition or response

Egg laying and deposition does generate a significant immune response -> granuloma and fibrosis formation

• This can obstruct both urine and blood flow

Eggs which are not promptly excreted then calcify and cause further immune fibrotic reaction.

Chronic schistosomiasis increases risks of recurrent/chronic UTI

• ?Bacterial adherence to eggs/worms
• Urinary obstruction and stasis

Long term infection and inflammatory response leads to urothelial hyperplasia, squamous metaplasia, dysplasia and eventually malignancy – SCC or urothelial carcinoma.

 

Clinical features

Acute phase

• Swimmer’s itch
  • Pruritic macular rash related to skin penetration, lasting only a few days

 

• Katayama fever
  • More common with S. japonicum cf. S. haematobium
  • Fever
  • Dry cough
  • Headache, diarrhoea, eosinophilia

 

Chronic phase

• 8 – 12 weeks after initial infection the eggs are shed into the bladder – causing haematuria +/- dysuria and frequency
• Chronic schistosomiasis is characterised by fibrosis and calcification
• Egg deposition in the ureters with subsequent granulomas and fibrosis can cause ureteric obstruction
• 33 – 75 % of women infected can have involved of female genital pelvic organs with egg deposition leading to PV bleeding, discharge, dyspareunia and pelvic pain
• Men can carry eggs in the ejaculatory ducts and seminal vesicles – leading to scrotal masses or semen discolouration
• Recurrent UTIs common with particularly higher rates of Salmonella UTIs
• End stage/advanced disease can cause thimble bladders with high pressure

 

Malignancy and schistosomiasis

S. haematobium is a recognised carcinogen.

60 – 90 % of schistosomiasis cancers are SCC; 5 – 15 % adenocarcinoma; remainder urothelial carcinoma.

Earlier onset at 40 – 50 years.

Half on the posterior wall, a third on the lateral wall.

SCC associated with schistosomiasis are often well differentiated with reasonable prognosis.

 

Diagnosis

Gold standard diagnosis of active infection is visualisation and count of eggs in urine cultures (or stool cultures) – characteristic finding microscopically the eggs have a ‘terminal spine’

Peak shedding of the eggs is midday so recommended to take urine samples at this time.

Biopsy of the bladder and histological assessment can aid in diagnosis, as can serology (but serology will not distinguish between active and chronic or historical infection).

 

Imaging

Mural calcification of the bladder is the hallmark characteristic of schistosomiasis on imaging.

Other organs – prostate, SVs, distal ureters, colon and gynae organs may also show calcification. Ureteric dilation may also be apparent.

 

Cystoscopy can demonstrate both mucosal and submucosal lesions – classically “sandy patches” but also “weeping ulcers” and malignant lesions.

 

Treatment

Treatment recommended by WHO is praziquantel – 2 x 20 mg/kg doses, 6 – 8 hours apart on the same day.

This results in egg reduction and cure in 60 – 90 %.

Well tolerated with minimal side effects – abdo pain, nausea – which settle quickly.

Artemisinin and its analogues can be useful for prevention.

Medical therapy is administered early may help reverse early stage disease.

 

Surgical therapy as required (don’t forget medical therapy concurrently).

• Symptomatic surgery
  • Resection, partial cystectomy for bleeding ulcers
• Functional surgery
  • Augmentation, ureteric re-implant
• Oncological surgery
  • Radical cystectomy