Schistosomiasis

• dams and irrigation schemes,
• fresh water reservoirs,
• artificial lakes,
• cultivation of rice,

where the snail intermediate hosts of the parasite breed. 

(courtesy WHO/TDR).

	Responsible organisms Regions Schistosoma haematobium Africa, East Mediterranean Schistosoma mansoni Africa, South America Schistosoma japonicum Asia
Endemic in 74 developing countries with more than 80% of infected people living in sub-Saharan Africa (courtesy WHO/TDR).

600 million people are exposed to the risks of schistosomiasis of whome 200 million are infected. Schistosomiasis is after malaria the most important parasitic disease of mankind.

Trematode flatworms (flukes) of the genus Schistosoma.

S. haematobium is the most prevalent and widespread species in Africa and the Middle East S. intercalatum occurs in 10 countries in the rainforest belt of Africa S. mansoni is found in Africa and is the only species seen in Latin America S. japonicum is restricted to the Pacific region including China and the Philippines S. mekongi is found in limited areas of Laos and Cambodia.
	Adult male and female Schistosoma haematobium (courtesy WHO/TDR)

S. mansoni adult female worms have a very long longevity of between 10 and 30 years

• male worms are 10-15 mm long and 1 mm large
• female worms are 15-18 mm long and less tahn 1 mm large

Schistosomula   Adult worms in: blood vessel around rectum (mansoni) bladder (haematobium) mesenteric veins and pulmonary arteries (japonicum) Egg production (10-30 per day)		Cercaria (500 per year per snail)     Sporocysts (several per miracidium)       Miracidium in water

Occurs in freshwater when intermediate snail hosts release infective forms of the parasite. People are infected by contact with water where infected snails live. 

Intermediate hiots are molluscs	schistosome
Bullinus globusus	S. haematobium
Bullinus truncatus
Biomphalaria pfeifferi	S. mansoni
Biomphalaria glabratta

Bulinus globosus, the intermediate host snail for Schistosoma haematobium (courtesy WHO/TDR)

Larval forms of the parasites (known as cercariae), released by the snails, penetrate the skin of people in the water. The snails themselves become infected by another larval stage of the parasite, known as a miracidium, which develops from eggs passed out in the urine or faeces of infected people.

• S. haematobium is mainly transmitted by Bulinus snails,
• S. mansoni by Biomphalaria
• S. japonicum by amphibious Oncomelania snails.  

Adult male and female schistosomes pair and live together in human blood vessels. The females release eggs, some of which are passed out in the urine (in S. haematobium infection) or stools (S. mansoni, S. japonicum), but some eggs are trapped in body tissues. Immune reactions to eggs lodged in tissues are the cause of disease. 

Types of disease

S. haematobium	urogenital
S. mansoni	intestinal
S. japonicum	arteries and veins

Adult male and female schistosome worms (courtesy WHO/TDR)

In urinary schistosomiasis (due to S. haematobium) damage to the urinary tract is revealed by blood and schistosome eggs in the urine. Eggs which get stuck in the tissues may calcify and lead to the formation of granulomas and superinfections. Urination becomes painful and is accompanied by progressive damage to the bladder, ureters and then the kidneys. Bladder cancer is common in advanced cases. 

In intestinal schistosomiasis (infection with S. mansoni, S. japonicum, S. mekongi) disease is slower to develop. There is progressive enlargement of the liver and spleen, intestinal damage due to fibrotic lesions around eggs lodged in these tissues, and hypertension of the abdominal blood vessels. Bleeding from these vessels leads to blood in stools, and can be fatal. Sufferers become seriously weakened by the disease and, in some cases, the functioning of organs such as spleen and kidneys becomes impaired.

Hepato-splenomegaly in S. mansoni schistosomiasis (courtesy WHO/TDR)	Calcified eggs in the tissues
	Formation of granulomas

Death is mostly due to bladder cancer associated with urinary schistosomiasis and to bleeding from varicose veins in the oesopahagus associated with intestinal schistosomiasis. Children are especially vulnerable to infection, which develops into chronic disease if not treated.  

Prevention and control

• Diagnosis:

  	Diagnosis using urine filtration and faecal smear techniques: haematobium search for eggs in urine mansoni search for eggs in stool Antigen detection in endemic areas Antibody tests in non-endemic areas
  Urine is collected in empty beer bottles for further analysis (courtesy WHO/TDR)

• Drug treatment

  Drug treatment is the only way to reduce disease symptoms and improve the situation of the patients. Drugs available are: praziquantel: effective in a single dose against all species oxamniquine: effective in a single dose, but only against S. mansoni

• Snail control through focal mollusciciding

  China, Jiangsu province: Molluscicide (niclosamide) to kill intermediate host snails (Oncomelania) involved in the transmission of Schistosoma japonicum, being sprayed over riverbank and flood-prone land using a high-pressure hose (courtesy WHO/TDR).	An irrigation canal.can create ideal habitats for the freshwater snails that are intermediate hosts of schistosomes (courtesy WHO/TDR).

• Provision of safe, adequate water supply and sanitation
• Health education 

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