Schistosomiasis

Where is the disease found? Why?

Schistosomiasis is both a water-borne, and a vector-borne disease. Water is the medium where transmission occurs. Both people and the freshwater snails (carriers of schistosome worms) become infected in water. It is not surprising that a common characteristic of communities plagued by this disease is the lack of access to safe, clean water. These communities turn to open, natural waterbodies to wash, clean, drink and bathe, increasing the risk of exposure. Areas that lack access to safe, clean water also tend to not have safe, easily accessible and easily maintained sanitation and hygiene infrastructure. This means that waterbodies used by communities and near households are easily contaminated by human waste which gives the schistosome parasite the opportunity to find and infect its preferred freshwater snail (the vector) and multiply. This makes that freshwater body a transmission site, and everyone who comes into contact with it is at risk of getting schistosomiasis.

There are 23 species of schistosome parasitic worms, six infect people, but many others infect domestic animals. The relationship between schistosomes and freshwater snails is intricate and complex. Schistosome parasites will only infect specific species of freshwater snails and not others.

Of the six species that infect people, three are responsible for the majority of human infections globally:

• Schistosoma haematobium - causes urogenital schistosomiasis and is carried by freshwater snails belonging to the Bulinus genus found in Africa and the Mediterranean.
• Schistosoma mansoni - causes intestinal schistosomiasis is carried by freshwater snails belonging to the Biomphalaria genus found in Africa, South America and the Caribbean.
• Schistosoma japonicum - causes intestinal schistosomiasis and is carried by amphibious snails belonging to the Oncomelania genus found in Asia.

• Another species of regional importance is Schistosoma mekongi which is carried by the freshwater snail Neotricula aperta, found in the Mekong River Basin of Cambodia and Lao People’s Democratic Republic.

The risk of infection is linked to how often people come into contact with infested freshwater. Whilst partial immunity can develop after repeated exposure to the parasite, regular contact with infested freshwater creates a high risk of severe disease at any age. Children who do not have partial immunity are particularly at risk of becoming infected from swimming and playing in the water; bathing and cleaning themselves in the water; or from doing household chores such as washing dishes and collecting water for the household. Children can become infected repeatedly with many schistosome parasites living in their blood system causing anaemia and tissue damage.

Families and communities that rely on fishing in freshwater bodies and on agricultural activities such as rice production are often burdened by schistosomiasis. Exposure to schistosomiasis can be linked to the social context of a community. In communities where fishing and farming are mainly done by males, men and boys are exposed to infections through these occupations. Women and girls are at risk when carrying out household chores such as washing clothes, dishes, collecting water etc.