A short introduction – what is Schistosomiasis?


Schistosomiasis (Bilharziasis) is a disease which is caused by various human-pathogenic trematodes belonging to the genus Schistosoma. According to estimates of the World Health Organisation (WHO), more than 200 million people at present suffer from disease due to this parasitosis. The disease can lead, if not treated, to life-long infirmity or death. At the moment about 500-600 million people in 74 countries, that is approximately a tenth of the world population, are living with the risk of infection. Schistosomiasis threatens - like most parasitosis - mainly the poor populations in under-developed countries of the Third World.

Due to the disease, and its life cycle, being dependant on water, its spread is often connected to irrigation projects, especially where dams are built. The trend of disease advance in towns can also be observed, in relation to population growth and migration.

Various human-pathogenic species of schistosomes are known of, which are dependent on various intermediate hosts and therefore occur in various regions of the world.

Species Schistosoma
mansoni
Schistosoma
haematobium
Schistosoma
intercalatum
Schistosoma
japonicum
Schistosoma
mekongi
Intermediate host Biomphalaria glabrata
B. alexandrina
B. pfeifferi

a.o.
Bulinus truncatus
Bulinus globosus

a.o.
Bulinus forskali Oncomelania hupensis
a.o.
Tricula aperta
a.o.
Form of the disease intestinal bilharziasis urogenital bilharziasis intestinal bilharziasis East Asian
intestinal bilharziasis
intestinal bilharziasis
Endemic Areas South America, the Carribean, Africa, the Middle East Africa, the Middle East Gabun, Cameroon, Tschad, Zaire East Asia,
South East Asia
Indochina
Source: Lang, W., Tropenmedizin in Klinik und Praxis, 1996


Is schistosomiasis with us too?


An outbreak of the disease at our degrees of latitude can be ruled out, due to the absence of the specific intermediate host in our waters and the low yearly average temperature. However a considerable risk of infection exists for travellers in tropical countries (for further information on the pathogenesis, prevention and spread, see the Internet pages e.g. of Travel Health Online, Medicine Worldwide and/or of the Robert-Koch Institute ). However, in our regions exist species of the genus Trichobilharzia, close relatives of the schistosomes, that cause a similar disease in water birds. Although the parasite cannot develop in humans, the cercaria can penetrate human skin, which leads to an itching rash, called swimmer´s itch (see trematode fauna).


Fig.1:
Biomphalaria glabrata. Intermediate host snail for Schistosoma mansoni
What´s being done?


Schistosomiasis is a combatable disease, as examples in Asia and the Caribbean show. However, a strict combat programme, which coordinates measures at various levels, is necessary for a successful and enduring disease control. A successful combat of disease includes an appropriate and lasting health education and providing of information for the population, chemotherapy for the ill and a combat against the intermediate host snail. All measures must be carried out over several years, which is dependant on considerable financial resources. An effective and lasting restriction of the disease is therefore at the moment only financially possible for the richer countries of the Third World.



The life cycle of Schistosoma mansoni provides an example for all species of schistosomes. After the eggs of the human-dwelling parasite are emitted in the faeces into the water, the ripe miracidium hatches out of the egg. The miracidium searches for a suitable fresh water snail to act as an intermediate host and penetrates it. Following this, the parasite develops via a so-called mother-sporocyst and daughter-sporocyst generation to the cercaria.The purpose of the growth in the snail is the numerical multiplication of the parasite. From a single miracidium results a few thousand cercaria, every one of which is capable of infecting man. The ceraria propel themselves in water with the aid of their bifurcated tail and actively seek out their final host. When the recognise human skin, they penetrate it within a very short time. Following a migration through the body within the bloodstream, if they meet a partner of the opposite sex, they develop into sexually mature adults.



Further information to be found under the illustration links.




Contact and feedback regarding the pages (last updated April 2003):

Christian Fuchs
Christian.Fuchs8@uni-bielefeld.de


Prof. Dr. R. Mannesmann
Rolf.Mannesmann@uni-bielefeld.de