Useful information on Schistosomiasis and STH
What is Schistosomiasis
Schistosomiasis, also known as bilharziasis, is a parasitic disease that leads to chronic ill health.
Discovery:
The disease has been recognized since the time of the Egyptian pharaohs. The worms were eventually discovered in 1851 in the Kasr El Aini Hospital, Cairo, Egypt, by Theodor Bilharz, a young German pathologist, from whom the disease took its original name, bilharziasis.
Causative agents
In terms of public health, there are five species of schistosomes which are the most important. They are found in different areas of the world and can be distinguished by the specific snails they use in their transmission routes (see below) and the different pathologies they induce. They can be divided into intestinal schistosomes (four species) and urinary schistosomes (just one species).
Transmission:
A person infected with schistosomiasis expels eggs in their faeces or urine.
In villages or communities where there is no proper latrine system or sanitation, freshwater sources around the village or community can easily become contaminated with faeces or urine containing the eggs. On contact with water, the eggs hatch and release larvae called miracidia. In order to survive, these tiny larvae must find a specific freshwater snail to inhabit. If the miracidia find the right type of snail, they use it to multiply in several cycles, eventually producing thousands of new parasites, called cercariae, which the snail then releases into the surrounding water.
Humans become infected when they come into contact with contaminated water. In a few seconds the cercariae penetrate the skin and make their way into the person's bloodstream.
Over the next 30–45 days, the parasite transforms itself into a long worm – either male or female – which makes its way to the intestine or bladder:
- intestinal schistosomes find their home in the blood vessels lining the intestine;
- urinary schistosomes live in the blood vessels of the bladder.
Once in their final location, the female lays between 200 and 2000 eggs per day over an average of 5 years, according to the species. In fact, only about half the eggs produced are excreted in the faeces or urine. The rest become trapped in the body tissues, and this is what does the major damage.
Symptoms of infection and impact on health:
:: Urinary schistosomiasis. The clearest tell-tale sign of urinary schistosomiasis is to see blood in urine. Eventually, infection can lead to bladder cancer or kidney problems.
:: Intestinal schistosomiasis. For intestinal schistosomiasis the symptoms are much less obvious and nonspecific. They include: diarrhoea, tiredness, abdominal pain or discomfort, and blood in the faeces. Infection can eventually lead to serious complications of the liver and spleen.
The severity of the disease depends on two factors: the schistosome species (the Asian species, S. mekongi and S.japonicum, cause a much more severe and rapid pathology) and the intensity of the infection (the more worms a person harbours, the more severe the disease)
Each egg has a small spine which tears the tissues it contacts, creating a lesion. The eggs can also become embedded in the tissues. Each time this happens the body tries to repair the damage and the healthy cells change to a more granular type, leading to formation of a “granuloma”. Over time the number of these granulomas can increase to the point where the organ no longer functions as it should. In urinary schistosomiasis, the bladder can calcify and become cancerous. In severe intestinal schistosomiasis, the liver and spleen can enlarge in reaction to the parasite eggs. This interferes with the body's normal blood circulation and causes portal hypertension (high blood pressure in the venous system) as the blood tries to find another route.
It is the eggs, not the worms, which cause the damage to the intestine, the bladder and other organs.
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