Schistosomiasis is pronounced 'shy-stoh-soh-my-assis'. It is described as chronic because the symptoms develop gradually, become progressively more serious, and last for a long time unless treatment is given.
Schistosomiasis is a chronic communicable disease caused by parasitic flatworms (also known as trematodes, or blood flukes), which affect the blood vessels in the intestines or in the urinary tract of infected people. In some places, the disease is known by its alternative name – bilharzia. Two of the most common species of Schistosoma parasites are Schistosoma mansoni (figure below) which causes disease mainly in the intestines, and Schistosoma haematobium, which causes disease mainly in the bladder and sometimes also in other parts of the urinary tract such as the kidneys.
The WHO estimates that more than 207 million people worldwide are infected with Schistosoma parasites – and 85% of them are in Africa. Approximately 200,000 people die every year in Africa as a result of the complications caused by these parasites. Rural communities living near water bodies such as rivers, lakes and dams may be highly affected by the disease, because the worms have a complex lifecycle in which they spend part of their development living in freshwater snails. You will learn more about their lifecycle later in this section.