The infected person's immune system reacts against the parasites' eggs in their blood vessels, which are recognised as 'foreign bodies'. The immune reaction causes an acute inflammation around the eggs, which can lead to chronic symptoms (see box below). Note that the clinical manifestations of schistosomiasis are mainly related to the immune response against the eggs in the intestine or bladder – the symptoms are not due to the worms themselves. The adults can survive in the person's body for up to 20 years, releasing around 300 eggs every day.

Clinical manifestations of schistosomiasis

Dermatitis (itching) where a parasite has penetrated the person's skin. This so-called 'swimmer's itch' occurs most often with Schistosoma mansoni, manifesting two or three days after invasion as an itchy rash on the affected areas of the skin.
The main symptoms of Schistosoma mansoni infection of the intestines are abdominal pain and bloody diarrhoea. A blood test usually reveals signs of anaemia and the abdomen may be swollen due to enlargement of the liver. If the infection remains untreated it can lead to permanent liver damage in advanced cases.
The main symptoms of Schistosoma haematobium infection of the bladder are pain during urination, frequent need to urinate, and blood in the urine. If the infection remains untreated it can lead to chronic bladder diseases, including cancer, and permanent kidney damage. It may also lead to infertility in men, and pain during sexual intercourse and vaginal bleeding in women.

The clinical manifestations (described above) should lead you to suspect cases of schistosomiasis. Asking children if they have seen any blood in their urine is an important way of detecting whether Schistosoma haematobium is common in the area. The diagnosis of schistosomiasis is confirmed in a laboratory by direct observation of the parasite eggs in samples of faeces or urine examined under the microscope (figure below).