Daniel, Barbara Elizabeth; (1990) Host-parasite interactions: Snails of the genus Bulinus and Schistosoma margrebowiei. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In Africa the schistosomes that belong to the shistosoma haematobium group are transmitted in a highly species specific manner by snails of the genus Bulinus. Hence the miracidial larvae of a given schistosome will develop in a compatible snail but upon entering an incompatible snail an immune response will be elicited which destroys the trematode. The factors governing such interactions were investigated using the following host/parasite combination; Bulinus natalensis and B. nasutus with the parasite S. mararebowiei. This schistosome species develops in B. natalensis but not in B. nasutus. The immune defence system of snails consists of cells (haemocytes) and haemolymph factors. The haemocytes from the two snail species were compared on both a quantitative and qualitative level. Cell numbers were compared between species before and following infection with S. mararebowiei. The existence of sub-populations of haemocytes was examined using both functional tests and differential binding by lectins. The triggering of the immune response requires that the invasive pathogen be recognised as foreign. Research in other fields of invertebrate immunology indicate that this may be mediated by factors, such as agglutinins, present in the haemolymph. In snails and schistosomes it is postulated that these factors recognise molecules on the surface of the larval schistosomes and the binding of factor to larvae elicits the immune response. The cell-free haemolymph of the snails was investigated for the presence of haemagglutinins. A protein capable of agglutinating vertebrate red blood cells was found in the haemolymph of B. nasutus snails. The surface carbohydrate and protein profiles of the different larval stages, miracidium, sporocyst and cercaria, were analysed using techniques of lectin binding and biotinylation of proteins followed by subsequent visualisation with labelled avidin. For these purposes the miracidia were transformed into sporocysts in vitro to ensure they were free of acquired snail proteins and sugars. The larvae were also investigated for possible interactions with the snail haemagglutinin. B. nasutus agglutinin was found to bind to the surface of S. margrebowiei miracidia but not to other larval forms.

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