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Schistosoma species

Common name. Blood flukes

Disease. Schistosomiasis; schistosomal hematuria, vesical schistosomiasis, urinary bilharziasis (S. haematobium); intestinal bilharziasis, schistosomiasis mansoni (S. mansoni); Oriental schistosomiasis, Katayama disease, schistosomiasis japonica (S. japonicum)

Geographic distribution. Schistosoma mansoni is found from the areas of South America and the Caribbean, Africa, and the Middle East; S. haematobium, from Africa and the Middle East; and S. japonicum from the Eastern countries including China, Japan, Philippines, Indonesia and Malaysia.

Infection rate. It is a parasitic disease with a wide range of clinical manifestations that affects more than 200 million people in 77 countries. Only 10% of infected individuals have severe clinical symptoms and this still represents 20 millions of seriously ill people worldwide.

Life cycle. The three common parasites of man, S. haematobium, S. japonicum and S. mansoni have similar life cycles. Eggs are passed out from the urine (S. haematobium) or from the feces (S. japonicum and S. mansoni); they hatch in aggregations of water such as ponds, lake edges, streams and canals. From the eggs, miracidia hatch into the water where they penetrate into the suitable snails. In the snails, they develop two generations of sporocysts; the second of which produces fork-tailed cercariae. These penetrate the skin when a new host comes into contact with contaminated water. Once get into the skin, the cercariae shed their tails to become schistosomulae, which migrate through the tissues until they reach the portal venous system of the liver. The males and females copulate before settling down in pairs in the venous system of the liver. S. haematobium usually migrates to the venous plexus of the bladder; other species (including the geographically localized S. intercalatum and S. mekongi), to the rectum where spiny eggs are laid. The eggs penetrate into the bladder or rectum.

Morphology. Schistosomes are dioecious and measure 10 to 20 mm in length and 0.5-1.0 mm in width. The male has a deep ventral groove known as the gynaecophoric canal, in which the female lies during copulation. Both sexes have 2 suckers, an anterior and a ventral sucker. The gut of the female worm appears dark because it is filled with deposits of haematin (breakdown product of haemoglobin). The life span may extend to 30 years but the mean longevity is about 5 years.

Pathology and clinical symptoms. The adults do not multiply and the eggs are the main cause of pathology in schistosomiasis. The eggs penetrate the blood vessels and the host tissues by secreting proteolytic enzymes through ultramicroscopic pores in their shell. However, many eggs become stranded in the tissues or are carried via the blood stream to other organs of the body. The host reaction to the eggs may vary from small granulomas to extensive fibrosis. The extent of damage is generally related to the number of eggs present in the tissues. Complete immunity does not occur with initial infection, and repeated infection is common in endemic areas.
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 fever, cough, abdominal pain, diarrhea, hepatospenomegaly, and eosinophilia. Occasionally central nervous system lesions occur. Continued infection may cause granulomatous reactions and fibrosis in the affected organs, which may result in manifestations that include: colonic polyposis with bloody diarrhea (Schistosoma mansoni mostly); portal hypertension with hematemesis and splenomegaly (S. mansoni, and S. japonicum); cystitis and urethritis (S. haematobium) with hematuria, which can progress to bladder cancer; pulmonary hypertension (S. mansoni, S. japonicum, more rarely in S. haematobium); glomerulonephritis; and central nervous system lesions.

Diagnosis. Microscopic identification of eggs in stool or urine is the most practical for the 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. 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. Various immunodiagnostic tests are available including intradermal and ELISA.

Prevention. Prevention of schistosomiasis includes (1) elimination of the infection source, (2) protection of snail-bearing waters from contamination with infectious urine or feces, (3) control of snail hosts, and (4) protection of persons from cercariae-infested waters.

Treatment. Praziquantel is now the drug of choice and is given as a single oral dose of 40 mg/ kg in case of S. mansoni and S. haematobium and 20 mg/ kg t.i.d in S. japonicum infection. Oxamniquine has been effective in treating infections caused by S. mansoni in some areas in which praziquantel is less effective.

Soon-Hyung Lee