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Background  Targeting the stages of the malaria parasites responsible for transmission from the human host to the mosquito vector is a key pharmacological strategy for malaria control. Research efforts to identify compounds that are active against these stages have significantly increased in recent years. However, at present, only two drugs are available, namely primaquine and artesunate, which reportedly act on late stage gametocytes.    Methods  In this study, we assessed the antiplasmodial effects of 5 extracts obtained from the neem tree  Azadirachta indica  and  Guiera senegalensis  against the early vector stages of  Plasmodium falciparum,  using field isolates. In an  ex vivo  assay gametocytaemic blood was supplemented with the plant extracts and offered to  Anopheles coluzzii  females by membrane feeding. Transmission blocking activity was evaluated by assessing oocyst prevalence and density on the mosquito midguts.    Results  Initial screening of the 5 plant extracts at 250 ppm revealed transmission blocking activity in two neem preparations. Up to a concentration of 70 ppm the commercial extract NeemAzal® completely blocked transmission and at 60 ppm mosquitoes of 4 out of 5 replicate groups remained uninfected. Mosquitoes fed on the ethyl acetate phase of neem leaves at 250 ppm showed a reduction in oocyst prevalence of 59.0% (CI 95  12.0 - 79.0;  p  < 10 -4 ) and in oocyst density of 90.5% (CI 95  86.0 - 93.5;  p  < 10 -4  ), while the ethanol extract from the same plant part did not exhibit any activity. No evidence of transmission blocking activity was found using  G. senegalensis  ethyl acetate extract from stem galls.    Conclusions  The results of this study highlight the potential of antimalarial plants for the discovery of novel transmission blocking molecules, and open up the potential of developing standardized transmission blocking herbal formulations as malaria control tools to complement currently used antimalarial drugs and combination treatments.

parasites and vectors

Transmission blocking activity of Azadirachta indica and Guiera senegalensis extracts on the sporogonic development of Plasmodium falciparum field isolates in Anopheles coluzzii mosquitoes