Infectious Diseases

The Role of Snails in Schistosomiasis



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According to the World Health Organisation, Schistosomiasis, or Bilharzia, as it is commonly known, is a parasitic disease that infects over 200 million people in 74 tropical and sub-tropical countries who do not have safe drinking water. A further 500 million are estimated to be at risk. They range from Africa to the Middle East and from the Caribbean to Cambodia.

The spread of Bilharzia is exacerbated by the many schemes intended to improve conditions for poorer people, by building dams and reservoirs, particularly along the Nile.  Over half the infected population live in Africa.


Schisostomiasis is caused by parasitic flatworms or flukes that live in the blood vessels of infected people, making them chronically ill, but not often killing them for many years. It has a complex life-cyle which involves three cyclical stages, one in humans, one in water and one in water-snails.  This makes it extremely difficult to eradicate.

It is caught from infested water, rivers, lakes, reservoirs and dams, as there are five related species found in different countries and conditions.  Children are particularly vulnerable as they play in water.  So are washerwomen and fishermen.  Even wealthy tourists who overturn their dinghies in African reservoirs can easily become infected when fluke larvae in the water penetrate their skin. They are too small to feel, but constitute a health hazard second only to malaria in importance.

Inside the blood vessels, they grow into adults or schisostomes, which mate and females release eggs.  Most of the eggs are excreted in faeces or urine, but others remain in the body tissues, especially the intestines, bladder, genital organs and liver. The symptoms they cause are immune-related reactions to the presence of the eggs, not the original worms. These include swelling, inflammation and increasing organ damage as well as sterility, pain and even paralysis.  Abdominal pains with bloody diarrhoea are a good reason to suspect schistostomiasis, which, if untreated, can lead to liver and spleen enlargement, kidney damage, cancer and eventual death. In young children it can cause stunted growth and cognitive damage. The disease is spread by poor hygeine.

Fluke eggs in untreated urine or faeces re-enter the water table where they drain into rivers, lakes and reservoirs.  Here they  hatch as miracidia, free-swimming microsopic larvae which burrow into the feet of water snails, where they form primary sporocysts.  Within these protective cysts, germ cells divide to form secondary sporocysts which migrate into the liver and pancreatic system of the snail.  Here they divide again to give rise to thousands of infectious flukes, called cercariae, which are released into the water to reinfect mammals, especially humans.  It is thought that they are stimulated into activity by turbulence in the water caused by human activity and children at play. 

They attach themselves to the skin, where they produce digestive enzymes that allow them to penetrate and rest for a couple of days while they transform into schistosomulae which can migrate through human tissue to the lungs.  Inside the lungs they change again and travel to the liver, a process that takes another eight days.  Here they feed on blood cells and mate.

In pairs, they travel to the blood vessels of the intestines, bladder and bowels where their eggs are released and find their way back into the water supply to reinfect the snails.

This incredibly complex process has evolved over many thousands of years to ensure the survival of the Bilharzia fluke. It is a very efficient parasite.  At any time, it can be found in many different stages in many different places, in human organs and blood vessels, free-swimming in water and in snails. For this reason it is extremely difficult to eradicate and since it does not kill its host immediately, it can remain there for up to 20 years, reproducing by hundreds of thousands every few days.

Schisostomiasis was described in ancient Egyptand in Roman times, but was only officially classified in 1851 by Theodor Bilharz.

It is now the prime target of international initiatives led by the World Health Organisation and many governments as well as private charitable projects by universities and colleges such as the Imperial College in London.

http://www.who.int/topics/schistosomiasis/en/

http://www.who.int/mediacentre/factsheets/fs115/en/index.html

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