It is estimated that about 600 million people in 79 countries suffer from schistosomiasis (Bilharziasis). The schistosomes cause intestinal, hepatosplenic, pulmonary, urogenital, cerebral and other forms of schistosomiasis. Schistosome is the only fluke with separate sexes. The female worm lies in the gynecophoral canal of the male. This condition is important for transportation.
There are five medically important species:
1. Schistosoma mansoni: causes intestinal schistosomiasis.
2. Schistosoma haematobium: causes vesical (urinary) schistosomiasis.
3. Schistosoma japonicum: causes intestinal schistosomiasis.
4. Schistosoma intercalatum: causes intestinal schistosomiasis.
5. Schistosoma mekongi: causes intestinal schistosomiasis. This seems to cause milder disease in man. It causes disease in other vertebrate hosts.
LIFE CYCLE OF SCHISTOSOMES
Adult worms reside in pairs: the female lying in the gynecophoral canal of the male. After fertilization, eggs are passed into the venules. A larval form – the miracidium – develops within the egg. Its lytic enzymes and the contraction of the venule rupture the wall of the venule liberating the egg into the perivascular tissues of the intestine (S. mansoni) or urinary bladder (S. haematobium). The eggs pass into the lumens and organs and are evacuated in the feces (S. mansoni) or the urine (S. haematobium).
On contact with fresh water the miracidia hatch from the eggs and swim about until they find the appropriate snail, which they penetrate. After two generations of sporocyst development and multiplication within the snail, the fork-tailed cercariae emerge. Infection to man takes place during bathing or swimming. The cercariae penetrate the skin, are carried into the systemic circulation and pass through to the portal vessels. Within the intrahepatic portion of the portal system, the worms feed and grow to maturity.
Symptoms and complications
Patients infected with S. haematobium suffer from terminal haematuria and painful micturition. There is inflammation of the urinary bladder (cystitis), and enlargement of spleen and liver. Patients infected with S. mansoni suffer from cercarial dermatitis (swimmers itch) and dysentery (mucus and blood in stool with tenesmus) as well as enlargements of the spleen and liver. S. haematobium causes squamous cell carcinoma in the bladder.
♦ Microscopic examination of the stool for eggs after concentration by sedimentation method. The egg has characteristic lateral spine.
♦ Rectal snip
♦ Examination of the urine after allowing it to sediment in a conical urinalysis glass. A drop from the sediment is taken and examined for eggs. Egg has terminal spine.
♦ Biopsy from bladder
Praziquantel: single oral dose of 40 mg/kg divided into two doses.
1. Health education: A. On use of clean latrines and safe water supply B. Avoid urination and defecation in canals, avoid contact with canal water
2. Snail control: A. Physical methods: i. Periodic clearance of canals from vegetations. ii. Manual removal of snails and their destruction. B. Biological methods: Use of natural enemies to the snails such as Marisa. C. Chemical methods: Molluscides are applied in the canals to kill the snails. E.g. Endod.