Entomological survey in a LLIN-protected village of Burkina Faso shows unexpectedly low Human Blood Index associated to high Plasmodium sporozoite rates in the malaria vector Anopheles coluzzii

Background & Objective: Despite the effectiveness of mass distribution of long-lasting insecticidal nets (LLINs) in reducing malaria transmission in the continent of Africa, in hyperendemic countries such as Burkina Faso the burden of malaria remains high. Materials & Method: We here report the results of a longitudinal survey (August - November 2011) on human blood index (HBI) and Plasmodium sporozoite rate (SR) in malaria vectors from a LLIN-protected village in Burkina Faso one year following a national bednet distribution programme. Results & Discussion: The HBI was 20.2% and 5.8% in Anopheles coluzzii (N=263) and An. arabiensis (N=103), the two most abundant malaria vectors in the area, respectively. These values are much lower than usually reported for a highly anthropophilic species such as An. coluzzii, consistent with the hypothesis that LLINs reduced the availability of human hosts to mosquitoes, and with the high presence of cattle breeding in the village. Unexpectedly, SR was found to be relatively high (An. coluzzii: 7.6%, N=503; An. arabiensis: 5.3%, N=225) particularly in relation to the low HBI observed. A regression analysis of data from a systematic review of published studies reporting HBI and SR for An. gambiae complex confirms that the observed SR values are out of the expected ranges, reinforcing the hypothesis that in the study site the reduced human/vector contact caused by LLINs was not sufficient to decrease mosquito infection rates. Conclusion & Recommendation: These results could contribute to the observed partial effectiveness of LLINs in reducing malaria transmission in the region. Moreover, they highlights the need of constant entomological surveys in these areas, in particular to assess the risk levels of outdoor transmission.