Plant molluscicides for snail control in the South African context.
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Date
1994
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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.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1994.
Keywords
Plant molluscicides., Schistosomiasis--South Africa., Theses--Zoology.