Unexpectedly low Human Blood Index associated to high Plasmodium sporozoite rates in Anopheles gambiae complex species from a LINN-protected village in Burkina Faso

The global effectiveness of long-lasting insecticidal nets (LLINs) in reducing malaria transmission is indisputable. However, in areas where malaria transmission levels are extremely high, substantial reductions in transmission intensity only led to a modest reduction in human parasitaemia. A paradigmatic case is represented by Burkina Faso where, after a few years of mass distribution of LLINs, the burden of malaria has not significantly changed as highlighted by WHO Country statistics and statistical bureau of Burkina Faso. We here report the results of a longitudinal survey on host choice and Plasmodium sporozoite rate (SR) in malaria vectors belonging to Anopheles gambiae complex in a rural village of Burkina Faso where LLINs were broadly distributed the year before the sampling (August - November 2011). The human blood index (HBI) was 18.8% (N=112) and 8% (N=75), in An. coluzzii and An. arabiensis, the two most abundant malaria vectors in the area. These values are much lower than usually reported particularly for An. coluzzii, which is known as a highly anthropophilic species, but consistent with the hypothesis that LLINs reduced the availability of human hosts to mosquitoes. Unexpectedly, Plasmodium sporozoite rates (An. coluzzii: 7.6%, N=449; An. arabiensis: 5.2%, N=229) were found to be in the range of those reported in the region before LLIN implementation when much higher HBIs were observed. This suggests that, despite LLINs have significantly reduced human/vector contact, this has not apparently yielded to a substantial reduction of mosquito infection rates. Further investigations are needed to confirm these results; however, they are fully consistent