Bilharzia - Causes of schistosomiasis

Schistosomiasis is caused by tiny parasitic worms called schistosomes. The worms live in fresh water in tropical and subtropical countries.

As well as rivers and lakes, the worms can also live in man-made structures such as reservoirs, irrigation ditches and canals.

Schistosomiasis is most common in rural environments that have high levels of poverty and poor sanitation, particularly where fresh water is used for irrigation.

It is possible to develop schistosomiasis if you have contact with infected water – for example, when paddling, swimming, washing or showering.

Read about preventing schistosomiasis.

Life cycle of schistosomes parasites

Schistosomes are an unusual type of parasite in that they use a two-stage system to infect humans. An infected human can pass out eggs in their urine or faeces (stools), which can survive in water for up to seven days.

Once the eggs are in the water, they hatch, releasing larvae that then swim and look for freshwater snails. When they find a snail, the larva enters its tissue where it matures into the next stage of development, known as a cercaria. These are just visible without a microscope. 

After four to six weeks, the cercariae leave the snail and can survive in the water for up to 72 hours. If during that time the cercariae come into contact with human skin, they burrow in and develop into the next stage, known as a schistosomule. These are able to move around the body through the blood vessels, eventually reaching the organs of the abdomen. 

About four to six weeks after infection, the schistosome is mature, the males and females mate, and the female worm starts to lay eggs. Some of these eggs remain in the body and move through the blood to other organs, some remain in organs close to the adult worms, where the body mounts an immune reaction, and some pass out in urine and faeces, allowing the life cycle to begin again if suitable snails are present. Direct infection from human to human does not occur.

Adult worms can remain active for five years or more, and there may have been cases where worms have lived in a human host for up to 30 years. The female worm will continue to lay eggs throughout her lifespan.