Bio-evaluation of larvicidal plants from the coastal parts of Kenya

Abstract

Malaria is responsible for significant morbidity and mortality, particularly among children in sub-Saharan Africa. In this area, estimates of symptomatic malaria range from one to five episodes per child per year, while deaths attributable to malaria are estimated at between 0.5 to 2.0 million, annually. Annual global clinical cases are estimated at 300-500 million with 90% of the cases in sub-Saharan Africa. Malaria remains intractable throughout most of its range, in part due to the evolution of behaviours maximizing contact of some Anopheles vectors with the human host while minimizing the risk of mosquito mortality. There is rapid development of insect resistance to conventional insecticides. Virtually all areas where the disease is endemic have seen drug-resistant strains of the malaria parasite emerge. Meanwhile, a vaccine for malaria is proving elusive. The situation of malaria worldwide is therefore one of deterioration and desperation. Therefore there is need to search for new and ecologically acceptable methods for the control of the malaria vector. Plants have provided a wide range of useful chemicals such as insecticides, pharmaceuticals, nutraceuticals, dyes, pesticides and other agro-chemicals. Many plants have been reported to have larvicidal activity against mosquitoes. Larvicides of plant origin are preferable because they are biodegradable and generally target-specific. Larval control using plant-derived compounds is a measure that could be adopted to arrest the insects before they develop into disease transmitting adults. It is easier to control the insects at the larval stage when they are most accessible, concentrated within specific habitats and less mobile. This research set out to examine the larvicidal activity of extracts of various plants from coastal parts of Kenya. Zanthoxylum paracanthum, Z. holstiana and Derris trifolia showed the highest larvicidal activity. Activity of up to 2 ppm was recorded in Derris trifolia dichloromethane extract. . Extracts from these plants were subjected to bio-assay guided fractionation and isolation of 12 larvicidal compounds from the three plants and three compounds are being reported for the first time .The isolated compounds were assayed against 3rd instar Anopheles gambiae larvae and LCso values ranging between 1.8 and 93.3 ppm were recorded.