Mortality rates of An. gambiae s.l.
Deltamethrin and Permethrin
In 2013, a total of 2,817 An. gambiae s.l. mosquitoes (1,943 tested; 874 control) were used for susceptibility testing, compared with 3,208 (2,515 tested; 693 control) in 2014 across the districts. Additionally, in the control districts, 299 An. gambiae s.l. mosquitoes (195 tested; 104 control) were examined in 2013 and 287 (213 tested; 74 control) were examined in 2014 in Kaffrine. In Ndoffane, the numbers tested were 244 (208 tested; 36 control) in 2013 and 321 (214 tested; 107 control) in 2014, respectively.
In 2013, susceptibility tested conducted in four IRS districts revealed that An. gambiae s.l. populations tested in Koumpentoum and Nioro were respectively susceptible to permethrin (100%) and deltamethrin (98%). However, in Koungheul district, suspected resistance to deltamethrin (95%) and permethrin (92%) was observed among An. gambiae s.l. populations, while resistance to both pyrethroids was noted in Malem Hodar (deltamethrin: 78% and permethrin: 89%) and permethrin (70%) in the Nioro district (Figure 2).
In the control districts, in 2013, suspected resistance to deltamethrin (95.5%) was recorded in An. gambiae s.l populations in Ndoffane, while those in Kaffrine exhibited resistance to the same molecule; with a mortality rate of 88% (Figure 2). Comparative analyses of deltamethrin mortality rates in An. gambiae s.l. population of transect 1 showed that although there was no statistically significant difference (Fisher's exact test: OR: 2.6 95% CI: 0.75-11.5; P=0.112), the susceptibility status of the Koungheul population to this pyrethroid was different from that of its Kaffrine control.
Similarly, on transect 2, despite the difference in phenotype status between populations tested in 2013, the mortality rates of deltamethrin between IRS area and control area were not statistically significant (Fisher's exact test: OR: 2.6 95% CI: 0.38-25.5; P=0.45). In 2014, An. gambiae s.l. populations in Koumpentoum and Nioro were resistant to deltamethrin and permethrin. Those tested in Malem Hodar and Koungheul showed suspected resistance to permethrin (95% and 96%), while mosquitoes exposed to deltamethrin were susceptible (mortality rate 100%) (Figure 2).
For the control of transect 2 (Ndoffane), populations were resistant to all pyrethroids tested, whereas in Kaffrine, control of transect 1, their populations presented both a suspected resistance to permethrin (96%) and susceptibility to deltamethrin (98%) (Figure 2).
In transect 1 of 2014, An. gambiae s.l. populations in Koungheul were susceptible to deltamethrin, whereas those in Nioro (transect 2) were resistant to both pyrethroids tested (deltamethrin and permethrin). The difference between both populations was statistically significant (χ2 = 13.184, ddl =1, P<0.01).
DDT and Dieldrin
In IRS districts, susceptibility tests for DDT and dieldrin were carried out on 780 An. gambiae s.l. mosquitoes (488 tested; 292 control) in 2013, compared to 965 (656 tested; 309 control) in 2014.
For control districts, the number of An. gambiae s.l. used for DDT and dieldrin in 2013 was 283 (187 tested; 96 control) and 302 (205 tested; 97 control) respectively for Kaffrine and Ndoffane districts. For 2014 tests, the corresponding values were 382 (254 tested; 128 control) and 313 (211 tested; 102 control) for Kaffrine and Ndoffane respectively.
Resistance of An. gambiae s.l. to DDT and dieldrin was in all districts of transect 2, with mortality rates varying from 82% to 73% and 92% (Nioro district) and from 45% to 69% and 87% to 79% (control district) in 2013 and 2014, whereas for transect 1, only Malem Hodar populations were susceptible to dieldrin (98%, CI 95% 92-99.7) in 2013 (Figure 3).
Figure 3. Susceptibility status of An. gambiae s.l. populations to DDT and dieldrin in IRS and control districts in 2013 and 2014, Green line: minimum susceptibility threshold and red line: maximum resistance threshold.
Bendiocarb and Pirimiphos-methyl
Susceptibility tests for bendiocarb were conducted on a total of 528 of An. gambiae s.l. specimens (tested: 392; control: 136) in 2013 and 544 (tested: 420; control: 124) in 2014 in the IRS districts. For pirimiphos-methyl, the corresponding values were 493 (tested: 401; control: 92) for 2013 and 568 (tested: 461; control: 107) in 2014.
With the exception of the control districts, where An. gambiae s.l. populations were resistant to bendiocarb in 2013 and suspected resistance in the Kaffrine control district in 2014, however all An. gambiae s.l. populations tested in IRS districts were susceptible to this insecticide. (Figure 4). Conversely, total susceptibility of An. gambiae s.l. populations tested for pirimiphos-methyl was observed in all districts (both controls and IRS) in 2013 and 2014. (Figure 4).
Figure 4. Susceptibility status of An. gambiae s.l. populations to bendiocarb and pirimiphos-methyl in IRS and control districts in 2013 and 2014, Green line: minimum susceptibility threshold and red line: maximum resistance threshold.
Specific composition of An. gambiae complex exposed to diagnostic doses according to year and district of collection.
The findings from the specific identification of An. gambiae s.l. specimens, from larvae collected in 2013 and 2014 in the treatment districts alongside their corresponding controls, are shown in Table 3.
A total of 1,180 adult An. gambiae s.l. specimens exposed to diagnostic doses of WHO-registered insecticides were identified by IMP-PCR.
Molecular analyses of the An. gambaie complex revealed the presence of three species (An. arabiensis, An. gambiae s.s., and An. coluzzii), as well as hybrids (An. gambiae s.s./An. coluzzii). An. arabiensis was the predominant species with a mean proportion of 87.7% (95% CI 85.6-89.5), followed of An. gambiae s.s. 7.2% (95% CI 5.8-8.8) and An. coluzzii 4.9% (95% CI 3.7-6.3). Hybrids (An. gambiae s.s./An. coluzzii) accounted for just 0.2% of the total specimens identified (Table 3).
Furthermore, specific identifications revealed an annual variation in An. gambiae s.s. populations, with a proportion of 2.6% in 2013 rising to 10.5 in 2014). This difference in proportion was statistically significant (χ2 = 25.95, ddl =1, P = 0.01). In the IRS districts and their corresponding controls, An. arabiensis was predominant across all locations. Nevertheless, An. gambiae s.s. demonstrated statistical significance only in Nioro, where it accounted for 30.9% compared to its control, where it represented only 0.06% of the 2014 total (Fisher's exact test: OR: 48.4, 95% CI: 8.2-1947.5; P=0.001) (Table 3).
Table 3. Specific composition of An. gambiae s.l. complex exposed to diagnostic doses according to year and district of collection
Year
|
District
|
An. arabiensis (%)
|
n
|
An. gambiae s.s. (%)
|
n
|
An. coluzzii (%)
|
n
|
|
Transect 1
|
89.6
|
346
|
3.1
|
10
|
7.3
|
30
|
2013
|
IRS
|
88.7
|
252
|
2.1
|
6
|
9.2
|
26
|
|
- Koungheul
|
73.8
|
76
|
2.9
|
3
|
23.3
|
24
|
|
- Koumpentoum¥(1)
|
96.7
|
116
|
2.5
|
3
|
0.8
|
1
|
|
- Malem Hodar
|
98.4
|
60
|
0.0
|
0
|
1.6
|
1
|
|
Control
|
97.9
|
94
|
0.0
|
0
|
2.1
|
2
|
|
- Kaffrine
|
97.9
|
94
|
0.0
|
0
|
2.1
|
2
|
|
|
|
|
|
|
|
|
|
Transect 2
|
90.6
|
78
|
4.7
|
4
|
4.7
|
4
|
|
IRS
|
90.6
|
78
|
4.7
|
4
|
4.7
|
4
|
|
- Nioro
|
90.6
|
78
|
4.7
|
4
|
4.7
|
4
|
|
Control
|
|
|
|
|
|
|
|
- Ndoffane
|
-
|
-
|
-
|
-
|
-
|
-
|
|
Transect 1
|
90.5
|
334
|
4.1
|
15
|
5.4
|
20
|
2014
|
IRS
|
88.2
|
253
|
5.2
|
15
|
6.6
|
19
|
|
- Koungheul
|
84.9
|
73
|
0.0
|
0
|
15.1
|
13
|
|
- Koumpentoum
|
84.2
|
85
|
9.9
|
10
|
5.9
|
6
|
|
- Malem Hodar
|
95.0
|
95
|
5
|
5
|
0.0
|
0
|
|
Control
|
|
|
|
|
|
|
|
- Kaffrine
|
98.8
|
81
|
0.0
|
0
|
1.2
|
1
|
|
Transect 2
|
81.0
|
286
|
17.3
|
61
|
1.7
|
6
|
|
IRS
|
66
|
128
|
|
60
|
2
|
6
|
|
- Nioro¥(2)
|
65.9
|
128
|
30.9
|
60
|
3.2
|
6
|
|
Control
|
|
|
|
|
|
|
|
- Ndoffane
|
99.4
|
158
|
0.6
|
1
|
0.0
|
0
|
Abbreviations: ¥Hybrids: An. gambiae s.s. /An. coluzzii, n: number
Specific composition of An. gambaie complex according to status (Surviving and dead), year and collection district
The specific composition of An. gambaie complex varied according to status (Surviving and dead), year and collection district.
The analysis revealed the presence of three species, namely An. arabiensis, An. gambiae s.s. and An. coluzzii, as well as hybrids (An. gambiae s.s./An. coluzzii), in both surviving and dead specimens. Notably, An. arabiensis was predominantly found among surviving specimens of the populations in transect 2, constituting 67.9% in 2013 and 80.8% in 2014 (Table 4).
In 2014, An. coluzzii was more prevalent among dead specimens compared to surviving ones in the districts of Nioro, Koumpentoum and Kaffrine, with proportions of 66.7% versus 33.3%, 83.3% versus 16.7% and 100% versus 0%), respectively. Conversely, in Koungheul, the average proportion of this species was higher among surviving specimens (66.7% versus 33.3%; Fisher's exact test: OR: 0.3, 95% CI: 0.05-6.0, P=0.55) (Table 4). Notably, for these districts, there was no statistically significant difference in the mean proportions of the other two species (An. arabiensis, An. gambiae s.s) between dead and surviving specimens (Nioro: Fisher's exact test: OR: 3.73, 95% CI: 0.52-42.28; P=0.189, Koumpentoum: Fisher's exact test: OR: 4.94, 95% CI: 0.53-240.7; P=0.21 and Kaffrine: Fisher's exact test: OR: Inf, OR: inf, 95% CI:0.01-inf; P=1).
For the districts of Koungheul and Kaffrine in 2013, similar proportions of An. coluzzii were observed in both surviving and dead specimens.