Anti-larval compounds from vitex schiliebenii and vitex payos
Mokua, Gladys Nyamoita
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Over two billion people in tropical countries are at risk from mosquito-borne diseases such as dengue fever, hemorrhagic fever, malaria and filariasis. The search for effective vaccines against these diseases is still in progress. It is estimated that US $ 2 billion is spent on malaria control and treatment programmes in Africa annually. The problem has become increasingly difficult to manage because of the spread of resistance to antimalarial drugs by the parasites resulting in increased severity of the disease. Protection from mosquito bites is still the most important measure to control the disease. However, vector resistance to insecticides is a recurring problem and a threat to malaria control programmes. The safety and efficacy of N, N-diethyl-m-toluamide (DEFT), which is the most potent of the modern synthetic repellents, is questionable. Controlling mosquitoes at larval stage is an additional effort that man has put in place to try and combat malaria menace. During the past two decades, considerable progress has been made in the development of natural and synthetic compounds, which are capable of interfering with the process of growth, development and metamorphosis of the target mosquito species: insect growth regulators (IGRs). Several phytochemicals such as azadirachtin have been shown to posses larvicidal/IGR activities. World Health Organization (WHO) has recommended IGR compounds such as methroprene, diflubenzuron and triflumuron. These compounds have been approved by World Health Organization Pesticide Evaluation Scheme (WHOPES) for use against immature mosquitoes. In this study, we carried out phytochemical investigation of anti-larval compounds from Vitex payos and Vitex schilliebenii using late third instar Anopheles garnbiae larvae using WHO bioassay procedures. Monitoring their larval mortality after every 24 hours and inhibition of adult emergence to assess the impact was done. The crude extracts with larvicidal or antilarval activity were subjected to chromatographic seperation techniques to isolate the active compounds, whose structures were elucidated by the conventional spectroscopic techniques. Four compounds: 20-hydroxyecdysone (2, 3, 14, 20, 22, 25hexahydroxycholest-7-en-6-one (80) (VSCE 2), 22-epi-20-hydroxyecdysone (82) (VSCE 3), stigmast-5-en-ol (118) (VSH 2), stigmaterol (133) (VSH 1), were isolated from Vitex schiliebenii, while five compounds: VPH 1, VPH 2, stigmasterol (133) (VPH 3), secoisolariciresonol (134) GF/CE/1 and GF/CE/2 were isolated from Vitex payos. Four compounds VSCE 2, VSCE 3, VSH 1 and VSH 2, were assayed for IGR activity. They exhibited 100 % inhibition of emergence and caused morphological deformation in some pupae of Anopheles garnbiae at 50 ppm, resulting in larval-pupal intermediates with the heads of pupae and the abdomens of larvae. Most adults from larvae treated with the test compounds died within 48 hours of emergence. The compounds showed delayed larval mortality at lower doses (1-5 ppm), which indicated effective developmental inhibition potential.