The Schistosomes

S. mansoni adults/CDC

General Information

• Trematodes from the genus Schistosoma that live in the blood vessels of humans and other mammals
• Adults have separate sexes (different from other trematodes)
• Disease called schistosomiasis or Bilharziasis, named after Theodore Bilharz
• Can cause acute and chronic disease, a disease-complex
• Most important helminth infection in terms of worldwide distribution, prevalence and morbidity
• Sometimes considered a "man-made" disease with the construction of hydroelectric dams and irrigation systems in developing nations
• The three most important human species are Schistosoma mansoni, S. haemotobium and S. japonicum

Geography

• S. mansoni is found in Africa (including Madagascar), the Arabian Peninsula, Brazil, Suriname, Venezuela and some Caribbean islands
• S. haemotobium is found in Africa and the Middle East
• S. japonicum is found in China, the Philippines and parts of Indonesia

Morphology (adults)

• Separate sexes
• Cylindrical in shape
• Male: short (10-20 mm in length) with copulatory groove
• Female: Long and slender (20-26 mm in length)

Morphology (eggs)

• S. mansoni: 180 um x 80 um, developed miracidium and a large lateral spine
• S. haemotobium: 160 um x 80 um, developed miricidium and a large terminal spine
• S. japonicum: 80 um x 60 um, developed miricidium and a small lateral knob (spine)

Life Cycle

• Eggs laid in blood vessels, pass from vessels into tissue, some reach the intestine  or bladder, excreted in the feces or urine
• Eggs that reach freshwater, miricidia hatches, swims to find appropriate snail host (intermediate) and penetrate
• In the snail- two generations of sporocysts> produce cercariae> cercariae migrate out of snail into water (Snail cycle requires 3-5 weeks)
• Cercariae: 0.1-0.2 mm in length, fork-tailed (infective stage) swims to find human host
• Penetrates through skin or mucous membranes
• Loses tail upon penetration> schistosomula
• Schistosomula enter peripheral veins> pass through heart, lungs to portal veins> to intestinal or bladder veins depending on species
• Adults have long life span 5-7 years and up to 30 years
• Eggs appear in feces or urine 1-3 months after cercarial penetration

Pathology

• Itchy papular eruption during cercarial penetration
• Response to antigenic and toxic secretions of larval and adult worms and eggs
• Acute stage- Katayama syndrome: fever, chills , nausea, abdominal pain, marked eosinophilia, enlarged and tender liver
• Daily egg outputs: S. haemotoboium 100, S. mansoni 300 and S. japonicum 1000-3500
• Late complications from chronic infections include liver fibrosis, portal hypertension and its sequelae
• Bowel obstruction, uropathy, superimposed bacterial infection (Salmonella) and bladder cancer
• Eggs depositied in ectopic sites like brain, spinal cord

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. 
• Kato-Katz, Ritchie technique
• Antibody detection

Treatment

• Infections with all major Schistosoma species can be treated with praziquantel

Epidemiology

• Exposure to contaminated freshwater that is used for agricultural, household, industrial or recreational purposes

Prevention

• Prevent exposure to contaminated freshwater
• Proper disposition of feces and urine 
• Educating the public, treatment
• Molluskicides for treating snail-breeding sites

S. haemotobium egg/CDC

S. mansoni egg/CDC

S. japonicum egg/CDC