Introduction to Parasitic Trematoda Quiz

Introduction to Parasitic Organisms: Schistosomiasis

• Schistosomiasis is a parasitic disease caused by trematodes, specifically blood flukes, which are the most significant trematodes from a clinical point of view.
• Approximately 250 million people are infected with blood flukes, and 600 million are at risk, with 5% of the world population having schistosomiasis.

Parasitic Trematodes: Classes and Characteristics

• The phylum Platyhelminthes is divided into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda.
• The most significant trematodes from a clinical point of view are the blood flukes, which belong to the genus Schistosoma.

Schistosomiasis: Clinical Importance and Species

• Three species of Schistosoma are of major medical importance: Schistosoma mansoni, Schistosoma japonicum, and Schistosoma haematobium.
• Schistosomiasis, also known as bilharziasis or swamp fever, ranks second behind malaria as a cause of worldwide morbidity and mortality, resulting in approximately 800,000 deaths per year.

Symptoms and Diagnosis

• People not previously exposed to schistosomiasis may develop flu-like symptoms, including dermatitis, fever, headache, persistent cough, myalgias, and malaise, known as "Katayama disease".
• Infected individuals often develop heart, lung, and circulatory problems, with eosinophilia frequently present.
• Chronic disease often produces bloody diarrhea, bowel obstruction, hypertension, and liver cirrhosis.
• Diagnosis is confirmed by finding eggs in stool, urine, or rectal biopsy samples.

Life Cycle and Geographic Distribution

• Schistosome eggs are released from the definitive host in feces or urine, and are ingested by snails, which act as intermediate hosts.
• Free-swimming miracidia seek out and enter snail intermediate hosts, where asexual reproduction (sporocysts) occurs.
• Schistosome cercariae emerge from snails and seek vertebrate hosts, penetrating host skin.
• Schistosomules migrate via the circulation to the appropriate vascular sites, such as liver sinuses, intestinal and bladder veins.

Pathology and Complications

• Adult schistosomes and eggs can cause serious liver cirrhosis and hepatosplenic disease.
• Eggs and immature worms can also cause serious lung disease.
• Adult schistosomes can remain in host tissues for 30 years or more, feeding on red blood cells and leaving a brown hematin pigment in tissues and phagocytic cells.
• Schistosoma mansoni and Schistosoma japonicum can cause significant intestinal pathology, while S. haematobium can cause serious urinary complications.