Malaria vector densities and species composition
A total of 26.276 mosquitoes (13.555 anopheline and 12.721 other culicines) were collected from June to December 2012 using both CDC LT (9158 mosquitoes) and PSC collection methods (17.118 mosquitoes). In addition, 9.404 mosquitoes were collected in Diebougou (sprayed area) between June and December (Table 1) whose 3040 mosquitoes collected in baseline (June-July 2012) and 6.364 mosquitoes during post-spraying period (Augut-December) compared to unsprayed area with 16.872 collected mosquitoes whose 4303 mosquitoes at baseline and 12569 mosquitoes in post-spraying period (P = 0.0012). a significant difference was observed between the total number collected in sprayed and unsprayed areas. According to species composition, An. gambiae sl (69.34%) and An. funestus ss (24.16%) were the most predominant Anopheline species collected in sprayed area (Diebougou) compared to 45% An. gambiae s.l., 19% An. funestus s.s., and 36% other culicines (Culex sp, Aedes sp., Anopheles sp etc) (Additional file 1: Figure S1). Their proportion were significantly reduced between sprayed and unsprayed areas (P = 0.039). In addition, there was a greater number of culicids collected in sprayed areas compared to unsprayed areas certainly due to impact of IRS. Overall, the total number of collected mosquitoes in sprayed areas (6364 mosquitoes) compared to unsprayed area (12569 mosquitoes) was significant (P = 0.001) after spraying period.
Table 1
Seasonal variation of major vectors densities in sprayed (Diebougou) and unsprayed (Dano) sites
Samples | CDC indoor collections | CDC outdoor collections | Pyrethrum indoor collections | Total | % |
Sprayed area | June | July | Aug | Sept | Oct | Nov | Dec | June | July | Aug | Sept | Oct | Nov | Dec | June | July | Aug | Sept | Oct | Nov | Dec | | |
An. gambiae s.l. | 33 | 76 | 213 | 106 | 76 | 40 | 22 | 8 | 35 | 28 | 2 | 3 | 2 | 18 | 234 | 317 | 338 | 335 | 141 | 140 | 122 | 2289 | 24,34 |
An. funestus | 0 | 0 | 27 | 47 | 33 | 28 | 31 | 5 | 15 | 62 | 9 | 31 | 36 | 41 | 10 | 11 | 8 | 11 | 20 | 35 | 25 | 485 | 5,16 |
Other culicids | 256 | 220 | 678 | 246 | 309 | 206 | 92 | 15 | 131 | 433 | 37 | 203 | 183 | 135 | 809 | 865 | 667 | 149 | 588 | 191 | 217 | 6630 | 70,50 |
Total | 289 | 296 | 918 | 399 | 418 | 274 | 145 | 28 | 181 | 523 | 48 | 237 | 221 | 194 | 1053 | 1193 | 1013 | 495 | 749 | 366 | 364 | 9404 | 100,00 |
Unsprayed area | June | July | Aug | Sept | Oct | Nov | Dec | June | July | Aug | Sept | Oct | Nov | Dec | June | July | Aug | Sept | Oct | Nov | Dec | Total | % |
An. gambiae s.l. | 74 | 218 | 640 | 244 | 72 | 36 | 14 | 17 | 31 | 21 | 14 | 2 | 2 | 4 | 469 | 777 | 1926 | 1160 | 1337 | 323 | 198 | 7579 | 44,92 |
An. funestus | 15 | 98 | 94 | 109 | 158 | 110 | 50 | 7 | 25 | 2 | 3 | 3 | 4 | 26 | 118 | 244 | 256 | 547 | 557 | 468 | 308 | 3202 | 18,98 |
Other culicids | 150 | 263 | 413 | 198 | 172 | 159 | 170 | 128 | 77 | 25 | 20 | 28 | 17 | 74 | 1145 | 447 | 839 | 844 | 275 | 323 | 324 | 6091 | 36,10 |
Total | 239 | 579 | 1147 | 551 | 402 | 305 | 234 | 152 | 133 | 48 | 37 | 33 | 23 | 104 | 1732 | 1468 | 3021 | 2551 | 2169 | 1114 | 830 | 16872 | 100,00 |
During the post IRS study period, indoor resting densities of malaria vectors were significantly lower in sprayed villages (n = 1,798) compared with unsprayed villages (n = 8,607) P = 0.0051 (Table 1). There was also a significant difference for total Anopheline catch by indoor CDC LT, with 1,527 in the untreated area compared with 623 in the IRS area (P = 0.0069). When broken down to species, An. funestus indoor resting (PSC) and host-seeking (CDC LT) densities (CDC LT: sprayed = n = 166 vs unsprayed n = 521 with P = 0.004; PSC: sprayed n = 99 vs unsprayed n = 2,136; P = 0.0079) and An. gambiae s.l. indoor resting densities (PSC: sprayed n = 1,076 vs unsprayed n = 4,944 11; P = 0.0005 were significantly lower in sprayed sites compared with control villages (Table 1).
Monthly biting and resting behaviour of Anopheles gambiae sl and An. funestus ss following IRS
Baseline data
In Dano (unsprayed), in June 2012, the An. gambiae s.l. indoor human biting rate was estimated at 4.6 bites per person per night by CDC LT collection and 13.6 bites/person/night in July 2012 (Fig. 3A). However, in Diebougou the An. gambiae s.l. human biting rates by indoor CDC LT 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 s.l. per house per night in Dano (Fig. 5A). The catch size was generally low in outdoor CDC collections in both sites (Fig. 3B & 4B).
Post-spraying data
A summary of mean biting rates is presented in Fig. 3 for An. gambiae s.l. 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.
Table 2
Monthly An. gambiae s.l.sporozoite rate and entomological inoculation rate from Dano unsprayed area and Diebougou (sprayed area) from June to December, 2012.
| June | July | August | September | October | November | December | 2012 Total |
Dano (unsprayed area) |
Total An. gambiae s.l. (CDC-LT) collected | 91 | 249 | 661 | 258 | 74 | 38 | 18 | 1389 |
CDC trap-nights | 32 | 32 | 32 | 32 | 32 | 32 | 32 | 224 |
(indoors + outdoors) |
HBR per night | 2.84 | 7.78 | 20.66 | 8.06 | 2.31 | 1.19 | 0.56 | 6.2 |
Total An. gambiae s.l. tested by CSP | 78 | 45 | 44 | 14 | 21 | 27 | 13 | 242 |
Sporozoites rate | 0 | 6.7 | 13.6 | 14.2 | 09.5 | 11 | 30.8 | 7 |
EIR p/night | 0 | 0.521 | 2.809 | 1.145 | 0.219 | 0.131 | 0.173 | 0.714 (mean) |
EIR p/month* | 0 | 15.64 | 84.28 | 34.35 | 6.591 | 3.92 | 5.19 | 21.42 (mean) |
Dano 5-month EIR post-IRS August-December 2012 = 134 infectious bites per person |
Diebougou (sprayed area) |
Total An. gambiae s.l. (CDC-LT) collected | 41 | 111 | 241 | 108 | 79 | 42 | 40 | 662 |
CDC trap-nights | 32 | 32 | 32 | 32 | 32 | 32 | 32 | 224 |
(indoors + outdoors) |
HBR per night | 1.28 | 3.47 | 7.53 | 3.38 | 2.47 | 1.31 | 1.25 | 2.95 |
Total An. gambiae s.l. tested by CSP | 42 | 49 | 56 | 25 | 114 | 62 | 30 | 378 |
Sporozoites rate | 0 | 6.1 | 5 | 8 | 14 | 3.2 | 0 | 0.052 |
EIR p/night | 0 | 0.212 | 0.377 | 0.27 | 0.346 | 0.042 | 0 | 0.1778 (mean) |
EIR p/month* | 0 | 6.35 | 11.29 | 8.1 | 10.36 | 1.26 | 0 | 5.34 (mean) |
Diebougou 5-month EIR post-IRS August-December 2012 = 31 infectious bites per person |
Table 3
Monthly An. funestus ss sporozoite rate and entomological inoculation rate from Dano (unsprayed area) and Diebougou (sprayed area) from June to December, 2012.
| June | July | August | September | October | November | December | 2012 Total |
Dano (unsprayed area) |
Total An. gambiae s.l. (HLC) collected | 22 | 123 | 96 | 112 | 161 | 114 | 76 | 704 |
CDC trap-nights | 32 | 32 | 32 | 32 | 32 | 32 | 32 | 224 |
(indoors + outdoors) |
HBR per night | 0.69 | 3.84 | 3 | 3.5 | 5.03 | 3.56 | 2.38 | 3.14 |
Total An. gambiae s.l. tested by CSP | 18 | 78 | 77 | 24 | 58 | 23 | 26 | 304 |
Sporozoites rate (%) | 0 | 1.3 | 1.3 | 4.2 | 5.2 | 0 | 0 | 1.7 |
EIR p/night | 0 | 0.05 | 0.039 | 0.147 | 0.262 | 0 | 0 | 0.071 (mean) |
EIR p/month* | 0 | 1.5 | 1.17 | 4.41 | 7.85 | 0 | 0 | 2.13 (mean) |
Dano 5-month EIR post-IRS August-December 2012 = 13 infectious bites per person |
Diebougou (sprayed area) |
Total An. gambiae s.l. (HLC) collected | 5 | 15 | 92 | 56 | 64 | 64 | 72 | 368 |
CDC trap-nights | 32 | 32 | 32 | 32 | 32 | 32 | 32 | 224 |
(indoors + outdoors) |
HBR per night | 0.16 | 0.47 | 2.88 | 1.75 | 2 | 2 | 2.25 | 1.64 |
Total An. gambiae s.l. tested by CSP | 8 | 11 | 50 | 44 | 43 | 37 | 16 | 209 |
Sporozoites rate (%) | 0 | 9.1 | 0 | 0 | 2.3 | 2.7 | 0 | 2 |
EIR p/night | 0 | 0.043 | 0 | 0 | 0.046 | 0.054 | 0 | 0.02 (mean) |
EIR p/month* | 0 | 1.2796875 | 0 | 0 | 1.38 | 1.62 | 0 | 0.61 (mean) |
Diebougou 5-month EIR post-IRS August-December 2012 = 3 infectious bites per person |
In the period post-IRS (August to December) the mean indoor biting rate per person per night (bpn) was significantly different for An. gambiae s.l. females between the two areas, with the highest mean biting rate observed 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). A significant difference was observed (P = 0.015)
The peak from indoor biting density of An. gambiae sl occurred in August with about 40 bites/person/night in Dano (unsprayed) and decreased progressively to December, when it was less than 5 b/p/n at the end of the rainy season (Fig. 3A). The same pattern was observed in the intervention area but with less than 15 An. gambiae sl bites per person per night. 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, 95% CI= [0.34–0.67] with P = 0.001 and Odds ratio (PSC) = 0.30, 95% CI= [0.21–0.43]. 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 2.00 bpn in Diebougou compared with 4.00 in Dano during the post-spraying period August-December (Odds ratio (CDC LT indoors) = 0.28, 95% CI= [0.11–0.35] with P = 0.035 (Table 3).
Plasmodium falciparum circumsporozoite and entomological inoculation rates (EIR) in 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 Table 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 indoor and outdoor collections of An. gambiae s.l. and An. funestus ss due to low number sampled in indoor and outdoor collection (Additional file 2: Table S1; Additional file 3: Table S2). So, during the post-IRS period (August-December), the mean CSP 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 include data from indoor and outdoor CDC collections and indoor PSC collections, for An. gambiae sl and Anopheles 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 females analysed for their blood-fed origin, about 20–40% of An. funestus ss females had taken a mixed blood meal (human, bovine, and goat). A potential effect of the IRS on An. funestus ss is that the proportion of human blood meals decreased being replaced by animal and mixed blood meals.