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Schistosomiasis Control Program

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How is schistosomiasis transmitted?

The parasite Schistosoma is transmitted through freshwater sources, such as ponds, dams, and rivers. During its larval stage, the parasite emerges from infected snails and swims in water until it can penetrate the skin of people in contact with the water. Once in the body, the larvae develop into adult male and female parasites, which pair and live together in human blood vessels for years. The female parasites release thousands of spiny eggs, some of which are passed out in the urine (in the case of urinary schistosomiasis) or feces (in the case of intestinal schistosomiasis), but some eggs remain trapped in body tissues.
 
When infected people urinate or defecate in a community water source, the eggs are immediately released. The eggs hatch and infect freshwater snails, such as the Bulinus, which then become the intermediate hosts. Inside the snails, the parasites develop and multiply; they are now able to re-enter the skin, infecting new victims and continuing the cycle.
 
Symptoms associated with urinary and intestinal schistosomiasis are caused by the eggs that remain trapped in body tissues. In the case of urinary schistosomiasis, the trapped eggs tear and scar the tissues of the bladder, ureters, and kidneys. Bladder cancer is common in advanced cases of the disease. In intestinal schistosomiasis, the disease develops more slowly. Symptoms include progressive enlargement of the liver, lungs, and spleen; intestinal damage due to fibrotic lesions around lodged eggs; and hypertension of the abdominal blood vessels. Bleeding from these vessels leads to blood in stools. The disease seriously weakens its victims and, in some cases, impairs the function of organs such as the spleen and kidneys. Death resulting from the disease is mostly due to bladder cancer associated with urinary schistosomiasis and to bleeding from varicose veins in the oesophagus associated with intestinal schistosomiasis.