Schistosoma spp.

Schistosomiasis is a major human disease affecting hundreds of millions of people.

It was traditionally known as Bilharzia, so-named after Theodor Bilharz, the young German parasitologist who discovered the parasitic flatworm (Schistosoma) that causes this disease. 

It is found associated with freshwater in Africa, eastern South America and South East Asia.

There are three species of Schistosoma of medical importance that infect humans. Each species tends to favour a particular location in the body:

  • Schistosoma haematobium lives in the veins surrounding the urinary bladder.
  • S. mansoni lives in the portal veins that drain blood from the large intestine.
  • S. japonicum lives in the veins of the small intestine.

To become infected, humans need to come in contact with the infective stages of these parasites (called cercariae) which emerge from an intermediate snail host in freshwater. If they make contact with human skin the cercariae quickly penetrate, in the process causing a condition known as 'swimmer's itch'.

The immature worms enter the blood and are carried to the heart and lungs. From there they migrate across the capillaries of the lungs (from pulmonary to systemic circulation), then through the mesenteric arteries to the intestines. 

Some parasites will reach the veins of the liver where the maturing males and females pair up, with the female lying in a groove in the male worm. For the 'successful' parasites, this journey may take up to 2 months.

Depending on species the worms migrate further to their final preferred location within the body. The paired worms remain together, producing eggs for up to 30 years. The eggs have to penetrate the walls of the small veins into the intestine or bladder before being expelled by the human host. If this occurs in water the eggs hatch, releasing a form (miracidium) that can penetrate the snail intermediate host thus continuing the lifecycle.

A person who has been infected may experience symptoms within a month or two, including fatigue, fever, chills, cough, muscle aches, abdominal pain, diarrhoea, dysentery and blood in the urine. Most symptoms are associated with our own immune responses to the presence of parasite eggs.

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