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Schistosomiasis, bilharzia or snail fever is a disease affecting many people in developing countries. Although it has a low mortality rate, schistosomiasis is very debilitating.

There are 5 species of flatworms that cause schistosomiasis. Each causes a different clinical presentation of the disease

  • Schistosoma mansoni and S. intercalatum cause intestinal schistosomiasis
  • Schistosoma haematobium causes urinary schistosomiasis
  • Schistosoma japonicum and S. mekongi cause Asian intestinal schistosomiasis

Geographical distribution and epidemiology

The disease is found in tropical countries in Africa, Caribbean, eastern South America, east Asia and in the Middle East. Schistosoma mansoni is found in parts of South America and the Caribbean, Africa, and the Middle East; S. haematobium in Africa and the Middle East; and S. japonicum in the Far East. S. mekongi and S. intercalatum are found focally in Southeast Asia and central West Africa, respectively.

An estimated 200 million people have the disease, 120 million symptomatic. A few countries have eradicated the disease, and many more are working towards it. The World Health Organization is working towards this goal. Controlled urbanization has reduced exposure sites, with a subsequent decrease in new infections. The most common way of getting schistosomiasis in developing countries is by swimming in lakes, ponds and other bodies of water which are infested with the snails (usually of the Biomphalaria genus) that are the natural reservoirs of the Schistosoma pathogen

Life cycle

The eggs of the above schistosomes open upon contact with water, releasing a miracidium parasite, which then infects a snail. After infection, the miracidium divides into thousands of new parasites, known as cercariae , which are the infective larvae. The cercariae, capable of penetrating a human being's skin, are excreted by the snail into the surrounding water. Once the infective larvae penetrate the host organism's skin and enter its blood vessels, it is 30 to 45 days before a long worm, which is capable of laying between 200 and 3000 eggs per day over the course of its 5 year lifetime, appears. S. japonicum lays about 3000 eggs daily and S. mansoni lays 350 eggs.

Upon release from the snail, the infective cercariae swim, penetrate the skin of the human host, and shed their forked tail, becoming schistosomulae. The schistosomulae migrate through several tissues and stages to their residence in the veins. Adult worms in humans reside in the mesenteric venules in various locations, which at times seem to be specific for each species . For instance, S. japonicum is more frequently found in the superior mesenteric veins draining the small intestine, and S. mansoni occurs more often in the superior mesenteric veins draining the large intestine. However, both species can occupy either location, and they are capable of moving between sites, so it is not possible to state unequivocally that one species only occurs in one location. S. haematobium most often occurs in the venous plexus of bladder, but it can also be found in the rectal venules. The females (size 7 to 20 mm; males slightly smaller) deposit eggs in the small venules of the portal and perivesical systems. The eggs are moved progressively toward the lumen of the intestine (S. mansoni and S. japonicum) and of the bladder and ureters (S. haematobium), and are eliminated with feces or urine, respectively.


Pathology of S. mansoni and S. japonicum schistosomiasis includes: Katayama fever, hepatic perisinusoidal egg granulomas, Symmers’ pipe stem periportal fibrosis, portal hypertension, and occasional embolic egg granulomas in brain or spinal cord. Pathology of S. haematobium schistosomiasis includes: hematuria, scarring, calcification , squamous cell carcinoma, and occasional embolic egg granulomas in brain or spinal cord.

Clinical Features

Many infections are asymptomatic. Acute schistosomiasis (Katayama's fever) may occur weeks after the initial infection, especially by S. mansoni and S. japonicum. Manifestations include:

Occasionally central nervous system lesions occur: cerebral granulomatous disease may be caused by ectopic S. japonicum eggs in the brain, and granulomatous lesions around ectopic eggs in the spinal cord from S. mansoni and S. haematobium infections may result in a transverse myelitis with flaccid paraplegia. Continuing infection may cause granulomatous reactions and fibrosis in the affected organs, which may result in manifestations that include:

Laboratory Diagnosis

Microscopic identification of eggs in stool or urine is the most practical method for diagnosis. Stool examination should be performed when infection with S. mansoni or S. japonicum is suspected, and urine examination should be performed if S. haematobium is suspected.

Eggs can be present in the stool in infections with all Schistosoma species. The examination can be performed on a simple smear (1 to 2 mg of fecal material). Since eggs may be passed intermittently or in small amounts, their detection will be enhanced by repeated examinations and/or concentration procedures (such as the formalin - ethyl acetate technique). In addition, for field surveys and investigational purposes, the egg output can be quantified by using the Kato-Katz technique (20 to 50 mg of fecal material) or the Ritchie technique.

Eggs can be found in the urine in infections with S. haematobium (recommended time for collection: between noon and 3 PM) and with S. japonicum. Detection will be enhanced by centrifugation and examination of the sediment. Quantification is possible by using filtration through a Nucleopore® membrane of a standard volume of urine followed by egg counts on the membrane. Tissue biopsy (rectal biopsy for all species and biopsy of the bladder for S. haematobium) may demonstrate eggs when stool or urine examinations are negative.

Antibody detection can be useful in both in clinical management (e.g., recent infections) and for epidemiologic surveys.


Chemotherapy has been both difficult and dangerous however drugs have become available that promise effective control. Praziquantel, for example, with a one-day oral administration is well tolerated and reportedly attains 95% cure of all schistosomes of humans at any stage of development.

Prevention and hygiene

The main focus of prevention is eliminating the water-borne snails which are natural reservoirs for the disease. This is usually done by indentifying bodies of water, such as lakes, ponds, etc., which are infested, forbidding or warning against swimming and adding acrolein, copper sulfate, etc., to the water in order to kill the snails.


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Last updated: 05-13-2005 07:56:04