What is schistosomiasis?

Freshwater snails of the genera Bulinus and Biomphalaria shedding cercariae, the schistosome life stage that infects humans.

Schistosomiasis infects approximately 200 million people in more than 70 countries leading to a chronically debilitating disease causing up to 300,000 deaths per year. 

More than 700 million people live at risk of infection in areas with poor water and sanitation infrastructure. Infection rates are highest in sub-Saharan Africa, where more than 85 per cent of infections occur.

Schistosomiasis is classed as one of the major Neglected Tropical Diseases (NTD).

Causes

Schistosomes

Paired blood flukes, Schistosoma spp.

Schistosomiasis is caused by parasitic flatworms called schistosomes. The parasites spend an earlier part of their life cycle in freshwater snails before they transfer to humans.

Cercariae, the infectious stage, burrow through the skin eventually reaching blood vessels around the intestine or bladder. They mature into adults that release eggs into the blood.

Eggs that become trapped in the liver or intestinal walls cause reactions leading to disease.

Eggs that pass out of the body in urine or faeces may re-enter the water supply and start the life cycle all over again.

Lifecycle diagram

Schistosomiasis lifecycle
  • Adults live in human blood vessels and release eggs there
  • Miracidia live for a short time in fresh water before infecting snails
  • Cercariae develop in the snail and are released into freshwater before infecting humans

Symptoms

Hepatic-intestinal form

Intestinal schistosomiasis is mainly caused by Schistosoma mansoni in sub-Saharan Africa and South America, whereas in East Asia it is mainly caused by S. japonicum. The cercariae of S. mansoni develop in Biomphalaria snail species, whereas those of S. japonicum develop in Oncomelania snails.

Intestinal schistosomiasis affects approximately 85 million people and is associated with:

  • bloody diarrhoea
  • anaemia and fatigue
  • stunted growth
  • liver and spleen enlargement potentially progressing to hepatosplenomegaly, a severe clinical manifestation of the disease linked to irreversible damage to the liver and spleen
Urogenital form

The main species that causes the urogenital for of the disease is Schistosoma haematobium. The cercariae of this species develop in snails of the Bulinus genus.

Urogenital schistosomiasis affects over 112 million people and is associated with: 

  • haematuria (blood in the urine)
  • bladder damage
  • hydronephrosis (swelling of the kidney due to build-up of urine)
  • risk of progression to severe disease such as kidney failure and bladder cancer
  • genital lesions with increased risk of infertility and sexually transmitted diseases

Distribution

Schistosomiasis distribution map