What is schistosomiasis?
Schistosomiasis, also known as bilharzia or snail fever, is a parasitic disease caused by tiny blood-dwelling worms. Infection occurs when individuals, particularly children, come into contact with contaminated water.
Over 200 million people worldwide are infected with over 700 million people living at risk of infection. Over 90% of infected people live in sub-Saharan Africa. It is a disease of low socio-economic status, affecting the poorest communities and most neglected, vulnerable people. Infants and children are especially prone to infection due to their less developed immune system.
Schistosomiasis is transmitted when larval forms released by freshwater snails penetrate human skin during contact with contaminated water. The infection leads to anaemia, chronic fatigue and painful urination/defaecation during childhood, later developing into severe organ problems such as liver and spleen damages, bladder cancer, genital lesions and infertility.
Life cycle and Transmission
To understand the disease and how to control/eliminate it we must first understand the parasite's life cycle and how it is transmitted.
When schistosome worms infect people they pierce through the skin into the blood system and travel with the blood to the liver. In the blood system of the liver male and female worms pair up and move to the veins around the intestinal tract or the veins around the urinary tract, depending the species of schistosomes.
The worm pair releases schistosome eggs into the blood system. The eggs pierce through the wall of the intestinal/urinary tract and exit the body through defeacation or urination. They reach fresh water and hatch out into larval stages called miracidia. Miracidia swim through the water to find a specific aquatic snail species. Once they have found the snail species they are looking for they infect them and reproduce asexually, creating thousands of clonal larval stages. These next larval stages are called cercaraie.
Thousands of cercaraie leave the snail and swim in the water searching for their next host - people! When a person enters the water to wash clothes, fish, bathe or play, the cercarie will swim to the person and pierce through their exposed skin. Being microscopic, the person is unaware that they have just been infected by a schistosome worm. The lifecycle continues.
What do schistosomes look like?
This video shows different real life biological stages of the schistosome worm under a microscope. The video was made by our partners at the Natural History Museum and shows the work of the Schistosomiasis Collection at the Natural History Museum (SCAN) team.
Infection with the schistosome blood worm causes the disease Schistosomiasis (aka Bilharzia).
The adult worms themselves are not very harmful but the eggs they produce and release into the blood system lead to a lot of damage and are the cause of the disease. Schistosome eggs will either
- Pierce the barrier between the blood system and the intestinal/urinary tract, exiting the body through defaecation/urination, causing pain and cramping. Symptoms of this often include blood in urine, blood in faeces.
- Or the eggs get transported by the blood system into neighbouring organs and tissues, getting trapped there and leading to inflammation and tissue damage
More worm pairs = more eggs = more damage to the organs and the host. This is what causes the chronic and more severe aspects of the disease such as organ damage, bladder cancer, liver fibrosis, genital lesions which progressively get worse in adulthood.
There are two forms of the disease, depending on the species of the infecting schistosome worm:
- diarrhoea, bloody stool
- anaemia, stunted growth
- enlarged liver and spleen
- severe damage to the liver leading to liver fibrosis
- portal hypertension
- blood in urine, painful urination
- anaemia, stunted growth
- damage to the genitals, kidneys and bladder
- bladder cancer
- increased risk of sexually transmitted diseases such as HIV
Schistosome species and distributions
There are 23 described species of schistosomes but only 6 species are known to infect humans. Of those 6, three are responsible for the majority of infections globally.
- Schistosoma haematobium - found in Africa, causes urogenital schistosomiasis
- Schistosoma mansoni - found in Africa, South America and the Caribbean, causes intestinal schistosomiasis
- Schistosoma japonicum - found in Asia, causes intestinal schistosomiasis
Schistosome species distributions are determined by the distribution of their intermediate aquatic snail hosts. The relationship between schistosome species and aquatic snails species is intricate and fascinating. Schistosome species will only infect specific species of aquatic snails and not others.
Three general rules on human schistosome-snail species compatibility are:
- S. haematobium will only infect aquatic snails belonging to the Bulinus genus
- S. mansoni will only infect aquatic snails belonging to the Biomphalaria genus
- S. japonicum will only infect snails belonging to the Oncomelania genus
Snail and schistosome images from the Schistosomiasis Collection at the Natural Histroy Museum of London.