The molluscicidal properties of Apodytes dimidiata (Icacinaceae): geographical variation in molluscicidal potency.

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.

Gross histopathological effects of an extract of Agave attenuata on the epithelium of the digestive tract of Bulinus africanus.

A lethal concentration of a crude, aqueous extract of Agave attenuata was applied as a contact poison to Bulinus africanus, the intermediate host of Schistosoma haematobium, for a 24-h period. The gross histopathological effects of the extract on the epithelium of the digestive tract were then studied. A graded series of cellular injuries to the epithelial layer was observed along the length of the tract. These included the loss of cilia and brush border, disruption of the epithelial layer, cellular vacuolation, swelling and rupture, and the discharge of secretory products from mucous gland cells. The results of the microscopy show that epithelial tissue is probably a primary target of the molluscicide. The cytological injuries induced by extracts of A. attenuata indicate that the molluscicide acts by disrupting the osmoregulatory mechanisms of the epithelial cells, but further, detailed studies are required to confirm this.

Proposed standardization of assessment protocols for plant molluscicides for use in self-help control programmes.

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.

A comprehensive evaluation of Agave attenuata, a candidate plant molluscicide in South Africa.

Dried leaf material of a Mexican plant, Agave attenuata, was found to be highly toxic to the target snail, Bulinus africanus, intermediate host of Schistosoma haematobium in South Africa. The molluscicidal potency of the plant underwent seasonal variation, but remained stable over a range of pH values, after exposure to sunlight and organic matter, and during dry storage. It was unstable in aqueous suspension. The toxicity of the molluscicide to crop plants, invertebrates, fish and mammals was lacking or low. Given these properties, A. attenuata may provide a substitute for niclosamide and be used safely for snail control by rural communities.

Mammal toxicity assessment of the plant molluscicide, Apodytes dimidiata (Icacinaceae), in South Africa.

Apodytes dimidiata has recently come to the fore as a potential plant molluscicide for schistosomiasis control in rural communities in South Africa. Prior to field applications of its leaves and extract to waterbodies, selected acute and sub-acute mammal toxicity tests were conducted in accordance with the Organisation of Economic Cooperation and Development (OECD) Guidelines to identify any potential hazards that might arise form the plant's use. Acute and sub-acute mammal toxicity test results classified A. dimidiata as non-toxic and non-irritating. Based on this toxicity evaluation, the dried leaf material and aqueous extracts of this plant are considered safe for use in preliminary field trials.

Acute toxicity evaluation of the plant molluscicide, Apodytes dimidiata (Icacinaceae), to Eisenia fetida (Oligochaeta) and Oreochromis mossambicus (Cichlidae) in South Africa.

The crude aqueous leaf extract of a South African tree, Apodytes dimidiata, has been identified as a potential molluscicide for snail control in anti-schistosomiasis programmes in rural communities. Before preliminary field trials could be implemented its acute toxic effects on two non-target organisms were examined in accordance with the OECD guidelines to identify any potential hazards that might arise from the plant's use. Laboratory results showed that working molluscicidal concentrations of the plant extract were non-toxic to earthworms suggesting that it is non-hazardous to soil-dwelling fauna. The plant extract proved toxic to fish and was assigned the risk phrase 'harmful to the aquatic environment' according to the EEC classification scheme. Based on these toxicity evaluations, aqueous extracts of A. dimidiata are considered safe for use in preliminary field trials.