Baseline data
In Dano (unsprayed), in June 2012, indoor human biting rate of An. gambiae sl was estimated at 4.6 bites per person per night by CDC light trap collection and 13.6 bites/person/night in July 2012 (Fig. 3A). However, in Diebougou An. gambiae sl human biting rates of. by indoor CDC light trap were found to be lower at 2 and 5 b/p/n indoors in June and July respectively. The An. funestus indoor human biting rate was less than 1 b/p/n (Fig. 4A). A similar trend was recorded for indoor resting densities, with Dano having approximately double the catch size of Diebougou (Fig. 5A&B). The highest resting densities by indoor PSC collection with a mean value in July reaching 49 An. gambiae sl per house per night in Dano (Fig. 5A). The catch size was generally low in outdoor CDC light trap collections in both sites (Fig. 3B & 4B).
Post-spraying data
A summary of mean biting rates is presented in Fig. 3 for An. gambiae sl and Fig. 4 for An. funestus ss. In addition, the number of mosquitoes collected by month and by site is summarized in Table 2&3.
In the period post-IRS (August to December) the mean indoor biting rate per person per night (b/p/n) was significantly highest in the unsprayed sites (mean = 6.55 bites per person per night from August to December) compared to sprayed sites (mean = 3.18 bites per person per night) in An. gambiae sl (P = 0.015). furthermore, the peak from indoor biting density of An. gambiae sl occurred in August with about 40 bites per person per night in Dano (unsprayed) and decreased progressively to December, when it was less than 5 b/p/n towards the end of the rainy season (Fig. 3A). The similar pattern was observed in the intervention area but with less than 15 b/p/n of An. gambiae sl. The human biting rate and mean number of An. gambiae sl per house from indoor collections (CDC LT and PSC) in sprayed sites was half a time lower compared to unsprayed sites (Odds ratio (CDC LT) = 0.51 with 95% CI: [0.34–0.67] and P = 0.001 and Odds ratio (PSC) = 0.30 with 95% CI: [0.21–0.43] and P = 0.0025. Outdoor biting rates were particularly low in both sites, with a mean of < 3 bites per person per night (Fig. 3B). But, the exposure to mosquito bites outdoors was slightly, but more increased in Diebougou (sprayed area) after treatment compared to Dano, the unsprayed area but the difference was not significant (P > 0.05).
Similar results were observed in An. gambiae s.l. and An. funestus ss biting rates (Fig. 4) in PSC collection (Fig. 5B) with a mean biting rate of 2 b/p/n in Diebougou compared with 4 b/p/n in Dano during the post-spraying period August-December (Odds ratio (indoors CDC LT) = 0.28 with 95% CI: [0.11–0.35] and P = 0.035 (Table 3).
Plasmodium falciparum circumsporozoite and entomological inoculation rates (EIR) from An. gambiae sl and An. funestus ss
The results of CSP-ELISA assays and entomological inoculation rate of An. gambiae s.l. and An. funestus ss are presented in Tables 2 and 3 respectively. Overall, 2051 An. gambiae sl and 1072 An. funestus ss specimens were screened for the circumsporozoite protein from June to December 2012 in the two areas. The sporozoites and entomological inoculation rate (EIR) were calculated by grouping the indoor and outdoor collections of An. gambiae sl and An. funestus ss due to low number sampled sporozoites rates detection (Table S1 in Additional file 2; Table S2in Additional file 3). So, during the post-IRS period (August-December), the mean sporozoites rate differed between unsprayed and sprayed areas for CDC LT method. The average sporozoites rates were significantly different (more than 2-fold) between the unsprayed areas (average sporozoite rate = 15.82%; 95% CI: [8.94–23.49]) and the sprayed areas (average sporozoites rate = 6.05%; 95% CI: [3.509–12.59]) (t = 2.475; df = 9 with P = 0.022) (Table 2). The highest sporozoites rates were observed in Dano in August (average sporozoite rate = 13.6%; 95% CI: [9.68–17.33]) and September reaching an average of 14.2%. The similar trends were also observed in An. funestus ss sporozoites rate (Table 3) but in lowest proportions (average sporozoite rate = 2.14%; with 95% CI: [0.25–3.15] in unsprayed areas and average sporozoites rate = 1% with 95% CI: [0.33–1.8] in sprayed area) with a significant difference (P = 0.035).
The major contributor to the EIR, both in the control and intervention areas, was An. gambiae s.l. (70%). The indoor EIR reached 134 infective bites/person during the five-month post-IRS in the unsprayed area Dano. IRS appears to have reduced the EIR four-fold in the sprayed area (31 infective bites /person) after spraying with P = 0.0001. An. funestus ss contributed also to the transmission in the two areas, with the similar results (EIR reduced 4-fold in sprayed area) compared to An. gambiae sl after spraying (mean EIR in Dano = 13 bi/p/n vs mean EIR in Diebougou = 3 bi/p/n with P = 0.003).
An. gambiae sl and An. funestus ss blood meal sources
The results presented in Figs. 8 and 9 included data from indoor and outdoor CDC LT and indoor PSC collections, for An. gambiae sl and An. funestus ss from the two areas. Irrespective of the sampling month, the proportion of An. gambiae sl. blood- fed on human was highest, reaching more than 80% of the total of 335 females analysed, both in the sprayed and unsprayed areas. No female was recorded blood-fed only on animals. For An. funestus ss the feeding patterns were quite different, especially in the sprayed area (Diebougou) where females showed a large range of hosts. Out of 132 An. funestus ss females analysed for their blood-fed origin, about 20–40% had taken a mixed bloodmeal (human, bovine and oat). A potential effect of the IRS on An. funestus ss was that the proportion of human blood meals decreased being replaced by animal and mixed blood meals.