Organism: Schistosomiasis is caused by blood trematodes with the main species infecting humans being Schistosoma haematobium, S. mansoni, and S. japonicum. S. megonki and S. intercalatum, more localized geographically, also cause disease.

Geographic distribution: S.mansoni: Africa, Middle East, parts of South America and the Caribbean. S. haematobium: Africa and Middle East. S. japonicum: Far East. S. mekongi: Southeast Asia. S. intercalatum: central and west Africa.

Life cycle: (See figure at bottom of newsletter)

The adult worm form resides in the mesenteric or pelvic venules depending on the species. S. mansoni is thought to be more common in the superior mesenteric veins; S. japonicum the inferior mesenteric veins. S. haematobium is most often present in the pelvis in perivesicular veins (bladder venus plexus) and rectal venules. The females release eggs into these veins which move toward the lumen of the intestines (S. mansoni, S. japonicum) or bladder (S. haematobium) until they are excreted in the feces and urine, respectively. When the eggs reach the water they swell due to osmotic pressure and the shell breaks open freeing the motile, ciliated larvae (miracidia) which penetrate the slow-moving snails which serve as intermediate hosts. In the snail host, they transform into sporocysts which mature into cercaria which swim and penetrate the skin of the human host. The cercaria mature in the bloodstream and eventually set up shop in the aforementioned venules, depending on the species.

Clinical Manifestations: Infections with Schistosoma species may be asymptomatic. Acute schistosomiasis (Katayama fever) can develop in nonimmune persons six to eight weeks after infestation. Manifestations of acute infection include fever, diarrhea, abdominal pain, hepatosplenomegaly, bronchospasm, eosinophilia and occasional central nervous system lesions. Chronic infections may cause granulomatous reactions to the eggs in the affected organs: bloody diarrhea and polyps (S. mansoni predominantly); portal hypertension with varices (S. mansoni, S. haematobium, S.japonicum); cystitis, ureteritis and urothelial carcinoma (S. haematobium); pulmonary hypertension (S. mansoni, S. japonicum) and central nervous system lesions. The eggs gain entrance to the central nervous system through the venous plexus that connects the intra-abdominal and spinal veins. The small, round eggs of S. japonicum travel all the way to the brain, whereas the eggs of S. mansoni and S. haematobium which are larger and bear protruding spines get lodged in the spinal cord. Therefore, S. mansoni and S. haematobium cause spinal mass lesions and transverse myelitis, whereas S. japonicum can cause a variety of cerebral symptoms including seizures.

Diagnosis: Microscopical examination of the stool and urine for eggs are the most readily available means of diagnosis. Eggs can be present in the stool in infections with all Schistosoma species. Urine examination should be performed if S. haematobium is suspected. S. mansoni have a characteristic laterally placed spine. S. haematobium has a terminal longitudinal spine. S. japonicum has a more rounded, smaller egg without a prominent spine. A single examination of stool has a sensitivity of about 50%, but repeated examinations and concentration techniques of stool may improve the sensitivity. Biopsy of the rectum for ova is more sensitive than stool examination but more invasive. Histologic sections typically show well-formed granulomas with mixed inflammatory response including eosinophils. Schistosoma eggs are ovoid with non-staining waxy appearance on hematoxylin and eosin stains. They often elicit a multinucleated giant cell reaction. The characteristic spines of S. mansoni and S. haematobium may not be present on the tissue level examined. S. mansoni and S. japonicum have eggs that are acid-fast, while S. haematobium eggs are not. Serum antibody tests are available at the Centers for Disease Control (CDC) and may be useful to indicate schistosome infection in someone who has traveled to an endemic area in whom eggs cannot be detected in the feces or urine

Treatment: The drug of choice for schistosomiasis is praziquantel. Oxamniquine may also be used against infections caused by S. mansoni in which praziquantel is less effective.
 
 

Life Cycle:

Images courtesy of the CDC web site Schistoma mansoni- note characteristic lateral spine. Stool O&P exam