Plant molluscicides for snail control in the South African context.

Abstract

Despite more than half a century of international research on schistosomiasis control, this disease remains a public health concern in many Third World countries. Four to five percent of the world's population is estimated to be infected. In South Africa, bilharzia is prevalent in rural communities which lack piped water and adequate sanitation. Transport and treatment costs limit access to Western medication. In the last decade, plant molluscicides have received considerable attention in the ongoing search for cheaper alternatives to chemotherapy and synthetic molluscicides. Plant molluscicides may be locally harvested, extracted and applied for the control of the snail host. This approach is based on a philosophy of self-reliance and self-determination. However, such involvement presupposes recognition of the infection as a real problem. Before selecting and evaluating plants for molluscicidal activity, it was first necessary to establish the status of schistosomiasis in rural communities, and the willingness of community members to become involved in control efforts. Field surveys showed that rural South African communities do not share the indifference of the past health-care system towards schistosomiasis in this country. Concern for the disease was matched by a prevalence of 75.14% for S. haematobium in the study area. The potential of the South African flora to provide suitable molluscicidal candidates has never been systematically assessed . As random screening is costly and is historically of limited success, a need has existed for the development of an objective selection procedure . A simple scoring system was devised, based on the criteria for "good" plant molluscicides as defined by the World Health Organization (WHO). Three plant species were subsequently selected for further intensive evaluation: Warburgia salutaris, Gardenia thunbergia and Apodytes dimidiata. Initially, the activity of crude aqueous suspensions of all three candidates was investigated, since this application form is the most practical for use in a rural situation. Further, the stability of extracts under changing physical and chemical conditions was assessed, as were t he effects of molluscicides on a wide range of to the toxicity of aqueous suspensions, cultivation potential, medicinal value, stability, and toxicity to non-target organisms. Studies on the mode of action of the three plant extracts revealed that death occurred via distinct physiological pathways; this despite the similarities in clinical symptoms. Such is the complexity of snail physiology that until more intensive neurological investigations are undertaken, the mechanisms of molluscicidal action can only be inferred. Chemical compounds responsible for activity were identified from Apodytes dimidiata; genipin, a monoterpene, was one such active constituent. Although not previously recorded as molluscicidal, earlier workers have isolated genipin from Genipa americana and Gardenia jasminoides. Notably, this compound has not yet been isolated from Gardenia thunbergia. Further activity in A. dimidiata extracts appeared to be the result of a synergistic effect, potentially involving (S)(-) ethyllactate. Research on mammalian toxicity, and registration procedures for plant molluscicides in South Africa should be prioritized for further investigation, before pilot field trials are initiated. Almost thirty years have passed since the first reports on the activity of Phyto!acca dodecandra. Although more than 1000 plant species have since been tested world-wide, it appears that no plant molluscicide has ever been endorsed by the WHO. More concerted efforts are necessary to ensure that appropriate molluscicidal technologies are provided to infected communities.