Schistosoma is a nematode parasite responsible for causing the disease Schistosomiasis or ‘bilharzia’ in humans. Although most infections settle without any complications, and sometimes without any significant symptoms, some may develop life threatening events. Although the disease causing Schistosoma worms are not detected in the US, it is a global burden of disease as almost 200 million individuals suffer from this disease while almost 800,000 succumb to their illness each year.
The Schistosoma parasite starts its life cycle as eggs and these eggs usually contaminate fresh water streams through urine or feces excreted by an infected human being. Thus, good personal hygiene and appropriate sanitary practices should prevent these eggs from reaching fresh water streams that may explain the higher incidence of Schistosomiasis in developing parts of the world.
The miracidium state:
Once in the water, the eggs will hatch into an intermediate organisms known as the miracidium and it will be able to move around the water seeking out its potential host, the fresh water snail. If encountered, the miracidium has the ability to penetrate the surface tissues of the snail and enter into its system.
Within the intermediate host:
Inside the snail, miracidium will grow into a sporocyst, which will be able to produce free-swimming intermediate organisms known as the ‘cercariae’, and would be released into the water. While remaining in the intermediate host, the sporocyst will be able to shed cercariae at different times.
The cercariae can swim through fresh water seeking for its ultimate host, the humans, and it is estimated that, these organisms are able to swim around for almost 48 hours following its shedding from the fresh water snail.
Entering the human host:
If cercaria gets in contact with human skin, it can attach and later penetrate the skin while shedding the tail, which will no longer be necessary. In most instances, people who swim in fresh water lakes and streams would be vulnerable to such penetration and a characteristic itch known as ‘swimmers itch’ may be present 1 – 2 days following exposure to the cercariae penetration. Following penetration, the ‘cercariae’ will be known as ‘schistosomulae’.
Reaching sexual maturity:
The schistosomulae will travel through the venous system of the body, first to the lungs and thereafter to the portal system of the liver. It will reach the adult stage and will feed on red blood cells within the circulatory system until it reaches sexual maturity.
From its initial habitat in the liver, the adult worms will pair and relocate themselves to specific regions of the circulatory system depending on the type of Schistosoma species to which it belongs. Thus, Schistosoma mansoni worms will migrate to the blood vessels around the large intestine while Schistosoma japonicum worms will migrate to blood vessels around the small intestine. Apart from these, Schistosoma haematobium worms are known to inhabit blood vessels near the bladder.
Once they reach these specific locations, the process of laying down eggs following copulation begins and according to studies, each worm is able to shed around 3000 eggs per day. The main symptoms associated with Schistosomiasis would be due to the release of eggs and its interaction with the immune system of the body rather than due to adult worms.
Re-starting the cycle:
The released eggs will attach themselves to the intestinal or bladder surface and will penetrate into its lumen before shedding into the outside environment via feces or urine to begin the life cycle again.
Center for Disease Control : Parasites – Schistosomiasis
Roberts, Larry S. and John Janovy Jr. Foundations of Parasitology 6th Ed. Boston: McGraw Hill, 2000.