Several candidate plant molluscicides have been identified for possible incorporation into self-help control programmes against schistosomiasis, but their full potential has yet to be realised. This has been, in the most part, due to the absence of standardized assessment and toxicity protocols, and subsequently the inability to register crude plant products in their country of origin or internationally. In an attempt to solve this dilemma, a series of protocols has been designed which will provide a useful standardized guideline for investigations into plant molluscicides, as well as precipitate moves towards the establishment of internationally accepted guidelines for the assessment of various categories of plant biopesticides. Ultimately, being able to register a crude plant extract will permit health organizations such as the World Health Organization to promote the use of such material, especially in self-help control programmes, thereby improving the health standards of rural communities.

Synthetic molluscicides have proved too expensive for most countries wanting to include snail control in their anti-schistosomiasis programmes. An alternative, which is not only cheaper but also promotes self-reliance and empowerment of the affected communities, is the use of molluscicidal plants. An often-cited limitation to using such natural products is the geographical variation in the toxicity of candidate species. The geographical variation in the molluscicidal activity of Apodytes dimidiata was investigated in South Africa. Leaves of this plant were collected from six, widely separate localities within the areas of the country where schistosomiasis is endemic. The results of bio-assays using the intermediate host snail, Bulinus africanus, clearly showed that variation in toxicity did exist and appeared to be correlated with the range in mean annual temperatures and altitude where the plants grew. Whether the variation was a phenoplastic response to the environment or genetically determined still has to be investigated. Nevertheless, a thorough knowledge of the geographical variation in the level of the active compound(s) in the candidate plants in endemic areas will be needed prior to the implementation of plant-propagation and snail-control programmes.