Anopheles mosquito species composition and abundance
During the study period, a total of 11237 An. gambiae s.l. females were collected by PSC and HLC approaches. Among these specimens, 46.6 % (5239/11237) were collected in Dangassa and 53.4% (5998/11237) in Koïla Bamanan. Based on logistics and practical considerations, a sub-sample was randomly selected across surveys to conduct molecular species identification. Of the 679 identified by PCR in Dangassa, An. coluzzii was the predominant species with a prevalence of 91.4% followed by An. gambiae (8.0%) and An. arabiensis (< 1%). In Koïla Bamanan, 623 An. gambiae s.l. were identified by the same methods and An. coluzzii represented 99%, An. arabiensis 0.8% and An. gambiae 0.2% of the catches. Overall, the An. coluzzii frequency was higher in Koïla Bamanan (99%) than in Dangassa (91.4%) (χ2 = 38.99, P <.001). No significant difference was observed between the frequencies of An. arabiensis (χ2 = 0.001, P = 0.970) and An. gambiae (χ2 = 0.081, P = 0.770) at both sites.
Density and Human blood index
In terms of abundance, Fig. 2 shows the monthly mean densities of An. gambiae s.l. in both localities using the PSC method. The mean density of An. gambiae s.l. over the entire study period was 12.8 [95% BCI: 12.5-13.2] per room in Koïla Bamanan and 6.4 [95% BCI: 6.2-6.7] per room in Dangassa. The highest densities were observed in April and August of 2014 for both villages. However, the densities of Koïla Bamanan were considerably higher than those of Dangassa. The lowest densities were observed in April 2016 for both study sites.
Over the 12 surveys, the mean HBIs of An. gambiae s.l. for Dangassa and Koïla Bamanan were 79.4% and 15.9%, respectively (Fig. 2). In Dangassa, the highest HBI was observed in August 2014 (96.8%) and the lowest HBI (52.1%) in April 2015, while in Koïla Bamanan, the lowest (6.7%) was observed in October 2012 and the highest (46.0%) in April 2015. HBIs were significantly higher in Dangassa than in Koïla Bamanan (χ2 = 1913.6, P <0.001) during the entire study period.
Monthly human biting rates (MHBRs), sporozoite infection rates (SIR) and entomological inoculation rates (EIRs)
Fig. 3 shows the human biting rates and the entomological inoculation rates from PSC in Dangassa and Koïla Bamanan respectively. The average monthly human biting rate (MHBR) was 2.3 times higher in Koïla Bamanan (69.8 bites/person/month) than in Dangassa (30 b/p/m). Monthly variations in MHBRs were observed at both sites. The highest MHBRs were recorded in April and August 2014. However, they were higher in Koïla Bamanan (266.6 and 249.6 b/p/m) than in Dangassa (65.9 and 105.5 b/p/m) (Fig. 3).
There was no significant difference in overall (over the entire study period) sporozoite infection rates (SIR) between the two study sites (infection rates were 2.9%, [95% BCI: 2.3—3.6 for Dangassa versus 2.6% [95% BCI: 2.2—3.1] for Koïla Bamanan; Table 2). Monthly and annual variations were observed at both sites. In Dangassa, the highest SIRs were observed in October (8.9%) in both 2012 and October 2014 (5.9 %). The peaks of the infection rates were observed in October in both Dangassa Koïla Bamanan. In Koïla Bamanan, in addition to the peak in October (6.4% in 2014), there was a second peak that was observed in April (7.1% in 2014 and 78.5% in 2016).
Given the fact that mosquitoes can feed on different hosts, to have a correct estimation of the EIR, we multiplied it by the proportion of mosquitoes which have taken their blood meal on human following the ELISA test. This correction is especially required when the human blood index is less 60%. The mean EIR was significantly higher in Dangassa (0.70 infective bites/person/month = ib/p/m) than in Koïla Bamanan (0.29 ib/p/m; Fig. 3). In Dangassa, the highest monthly EIRs were observed during the months of October 2012, April and October 2014, (2.75; 1.81 and 1.61 ib/p/m). In Koïla Bamanan, the highest EIRs were recorded for April 2014 and June 2015 (3.33 and 0.59 ib/p/m, respectively). As with the sporozoite infection rates, EIRs also were subject to monthly and annual variations.
Outdoor vs indoor biting activities of An. gambiae s.l. in Dangassa and Koïla Bamanan
Data for comparing the location of biting activities (indoor versus outdoor locations) were generated using mosquitoes collected by human landing catches. HBRs showed a wide patterns of monthly variation for both indoor and outdoor locations in both villages (Fig. 4).
Fig. 4 shows HBR per hour and by season. HBR was higher during the second part of the night indoors and outdoors regardless of site and season. We observed a peak of HBR outdoors between 02:00 h and 03:00 h, and then two peaks respectively between 01:00 h - 02:00 h and 04:00 h -05:00 h during the rainy season at Dangassa (Fig. 4A). In the dry season, the peak was observed between 02:00 h and 04:00 h, but it was lower than in the wet season (Fig. 4B). At Koïla Bamanan, two peaks were observed indoor during the rainy season, where the first occurred between 23:00 h and 00:00 h, and the second occurred between 01:00 h and 02:00 h (Fig. 4C). In the dry season, a single peak was observed between 23:00 h and 00:00 h indoor and outdoor at Koïla Bamanan (Fig. 4D).
In Dangassa, the highest MHBRs were observed in July, October and November 2012, and in April and August 2014 (Fig. 5). The average HBR over the study period was significantly higher outdoors (648.9 b/p/m [BCI: 642.6—655.2]) than indoors (560.3 b/p/m [BCI: 554.7—565.9]) suggesting an exophagic coefficient of 1.2 (648.9/560.3).
In Koïla Bamanan, the highest MHBRs were observed in October 2012, April and August 2014 and in June 2016 indoor and outdoor (Fig. 5). As observed in Dangassa, in Koïla Bamanan the average HBR over the study period was significantly higher for outdoor locations (242.4 b/p/m [BCI: 238.1—246.7]) than indoor locations (228.9 b/p/m [BCI: 225.1—232.6]) with an exophagic coefficient of 1.1 (242.4 / 228.9). HBRs were significantly higher in Dangassa than in Koïla Bamanan, both indoors and outdoors.
In Dangassa, the sporozoite infection rate in An. gambiae s.l. was 3.1% [BCI: 2.1—4.3] and 3.6% [BCI: 2.4—5.0] for indoor and outdoor locations, respectively. No significant differences were observed between these two rates. The sporozoite infection rates were subject to wide month-to-month variability. In Koïla Bamanan, the average sporozoite infection rate was 4.3% [95% BCI: 2.7—6.3] and 2.4% [95% BCI: 1.1—4.2] for indoor and outdoor locations, respectively. The sporozoite infection rate indoors was 1.8 times higher than outdoors, though no significant difference was observed between the two. Similar monthly variations were observed both indoors and outdoors for Dangassa (Table 3),
EIRs showed monthly variations (Fig. 5). In Dangassa, the EIRs were estimated to be 13.74 [BCI: 9.21—19.14] and 18.66 [BCI: 12.55—25.81] ib/p/m, for indoor and outdoor locations, respectively. The EIR was 1.4 times higher outdoors than indoors, though the difference was not significant. The highest rates were observed in July and October 2012. In Koïla Bamanan, EIRs were 1.57 [BCI: 2.34 —2.72] and 0.94 [BCI: 0.43—1.64] ib/p/m for indoor and outdoor locations, respectively. The highest rates were observed for August 2014 and June 2015. In contrast to Dangassa, the EIR was 1.7 times higher indoors than outdoors. However this difference was not significant