Studies on molluscicidal properties of some South African medicinal plants used in the control of schistosomiasis in KwaZulu-Natal.
Schistosomiasis is an important public health issue for rural communities located near,or around slow moving water bodies in the tropical and subtropical areas. Successful control of the disease involves multifaceted approaches, which include snail control, environmental sanitation, health education and chemotherapy. Although snail control might be an effective method of controlling schistosomiasis, there has been a general lack of control initiatives, largely due to the cost of available molluscicides. Plants offer a wide array of compounds which, on extraction, may show molluscicidal activity. If molluscicidal compounds that occur in indigenous plants can be extracted using local labour and simple technology, then there should be culturally acceptable and inexpensive molluscicides. The aim of this study was, therefore, to screen some Zulu medicinal plants for molluscicidal activity. We have also attempted to isolate the active chemical compounds from such plants. Aqueous and methanolic crude extracts of ten (10) Zulu medicinal plants, used for different medicinal and domestic purposes, were screened for molluscicidal activity on Biomphalaria pfeifferi and Bulinus africanas snails reared in the laboratory during the time of bioassay. Bayluscide® (niclosamide) was used as a positive control for comparison, while de-chlorinated tap water was used as the negative control. Six of the plants were not active against the snails. Extracts from four of the plants demonstrated weak to moderate molluscicidal activities. These plants are: (i) Sclerocarya birrea stembark, (ii) Psidium guajava (hybrid) leaves, (iii) Leonotis leonurus aerial parts and (iv) Ekerbegia capensis stem-bark. The LC50 values of the plant extracts were 78 ppm, 100 ppm, 398 ppm and 600 ppm respectively. Of the 4 plants that showed molluscicidal activity, S. birrea aqueous and methanol extracts were the most active against the snails, with LC50 values of 82 ppm and 78 ppm respectively. For the other plant extracts, only the methanolic extracts showed activity. Brine shrimp toxicity assay was performed with all the active extracts. Psidium guajava showed 10% survival of the shrimps at 1000 ppm, whereas no survival was observed for the other plant extracts at this concentration (1000 ppm). The results obtained in this study indicate that further studies have to be conducted, especially with S. birrea extracts, whose both aqueous and methanolic extracts showed significant activity against the snails.