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Bilharziosis or Bilharzias is a tropical illness named after the German doctor Theodor Bilharz, who found it in 1852.
Picture left: Schistosoma mansoni – one of the pathogen causes responsible for the illness Bilharziosis or Schistosomiasis. Light: male, dark: female.
The cause of Bilharziosis is a round worm (Trematoda) by the scientific name of Schistosoma – the blood fluke. The most important species are Schistosoma mansoni, Schistosoma haematobium and Schistosoma japonicum.
In fact the blood fluke usually is two animals, a male and a female living as a pair. The female, a bit longer than the male, in whose ventral groove (gynecophoral canal) it remains. Both, male and female, measure about 1 to 2 cm (0.5 to 1 inch) in length.
A bilharziosis infection may occur in shallow fresh waters in the tropics, where an animal lives, that is indispensable as the fluke's alternate host. This intermediate host always is a fresh water snail.
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Species |
Schistosoma mansoni | Schistosoma haematobium | Schistosoma japonicum |
| Alternate host | Biomphalaria glabrata | Bulinus truncatus | Oncomelania hupensis |
| Distribution | Africa, Saudi-Arabia, South America, Caribbean | Africa | Japan, China, Philippines |
For example the alternate host to Schistosoma mansoni is Biomphalaria glabrata and other members of this genus, part of the ram's horn snail family (Planorbidae).
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Picture right: Biomphalaria glabrata – the alternate host to Schistosoma mansoni.
Source: Bielefeld University.
The fluke's eggs are spread in the water together with its final host's droppings. The final host may be a human or another species of mammal. In the water, the so called miracidia, the basic larval generation, hatch from the fluke's eggs. The miracidia only live for 48 hours, during which time they have to find an alternate host. Unless they find a suitable host organism, they die. Inside the host organism, which has to be of a suitable snail species, the miracidia perform a metamorphosis, during which they develop into cercariae.
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Picture left: Miracidium und Cercaria of Schistosoma
mansoni.
Source: Bielefeld university.
The cercariae leave the host organism by passing the body wall and are able to survive another two days swimming freely in the water. In this time they infect mammals standing in the water by passing through their body wall into the legs. Through the blood vessels the cercariae wander into the host's liver, which is where they develop into adult flukes, mate and lay eggs. Those are transported away in the blood stream and distributed over intestine and ureter.
Humans are harmed by skin rashes caused by the cercariae entering the skin during infection. They also hamper the organism causing heavy, possibly fatal fevers. The eggs lastly may block blood vessels. The eggs leaving the body may harm intestine, ureter and bladder walls even causing cancer symptoms.
Measures are taken to minimize the risk of bilharziosis infection in affected regions. Besides directly killing suitable fresh water snails by poisoning them, also their habitats are destroyed by removing the waterside vegetation they live in.
Especially dangerous areas are shallow warm standing waters, such as rice fields and irrigation projects.