Species composition, vector dynamics and vectorial role of An. arabiensis
A total number of 24,125 adult mosquitoes were collected in the six districts (during 216 nights of HLC). Among these, 73.7% (n = 17786) were identified as Culex quinquefasciatus and 19.1% (n = 4599) as Anopheles gambiae s.l. In addition, Anopheles funestus, An. pharoensis and An. ziemanni were also collected but in very low proportions (< 0.5%).
Mosquitoes belonging to the Anopheles gambiae s.l. complex were identified as An. gambiae s.s. (n = 1333; 46.0%), An. coluzzi (n = 1517; 52.3%) and An. arabiensis (n = 48; 1.7%) (Table 1).
Table 1
Distribution of the species within the Anopheles gambiae complex collected by HLC in each surveyed district.
District
|
HLC
nights
|
An. gambiae s.s.
N (%)
|
An. coluzzi
N (%)
|
An. arabiensis
N (%)
|
Air France 1 & 2
|
36
|
267 (91.8)
|
24 (8.2)
|
0 (0)
|
Belleville 3
|
36
|
272 (93.5)
|
19 (6.5)
|
0 (0)
|
Koko
|
36
|
149 (40.7)
|
182 (49.7)
|
35 (9.6)
|
Sossoribougou
|
36
|
47 (21.8)
|
167 (77.7)
|
1 (0.5)
|
N’Gattakro
|
36
|
371 (30.1)
|
863 (69.9)
|
0 (0)
|
Odiennekourani
|
36
|
228 (45.0)
|
266 (52.6)
|
12 (2.4)
|
Total
|
216
|
1333 (46.0)
|
1517 (52.3)
|
48 (1.7)
|
An. arabiensis was found at the end of the rainy season (October 2019), except for one individual captured in March, at the beginning of the rainy season. It was identified only in the Odiennekourani and Koko area, in the western part of the city, around an inland valley that crosses both districts (Fig. 2). An. arabiensis was totally exophagic (48 individuals caught outdoors) and active during the second part of the night (91.7% of individuals were collected between 00:00 and 07:00). None of the 46 mosquitos of the An. arabiensis species tested positive for Plasmodium falciparum infection. However, 11 An. coluzzi and 4 An. gambiae s.s. were positive for P. falciparum.
The kdr West and East mutations were detected in 41 (85.4%) and 7 (17.1%) individuals, respectively, revealing an allelic frequency of 1 in both cases. None of the individuals carried the ace-1R mutation.
Anopheles arabiensis breeding sites
We found potential breeding habitats for Anopheles mosquitoes (i.e., larvae breeding ground filled with water) in 35 and 40 sites (out of the 42 sampled sites) in October 2020 and April 2021, respectively. Anopheles larvae were collected in 9 of the 35 sites in October 2020, and then in 24 of the 40 sites in April 2021 (Table 2).
Table 2
Characteristics of the breeding sites prospected in Bouaké.
|
October 2020
|
April 2021
|
|
N
|
Positive
n (%)
|
N
|
Positive
n (%)
|
Garden market
|
8
|
4 (44.4)
|
20
|
14 (58.3)
|
Rice field
|
9
|
3 (33.3)
|
1
|
0 (0.0)
|
Puddle
|
1
|
1 (11.1)
|
5
|
4 (16.7)
|
Bush
|
11
|
0 (0.0)
|
12
|
5 (20.8)
|
Watercourse
|
6
|
1 (11.1)
|
2
|
1 (4.2)
|
Total
|
35
|
9 (25.7)
|
40
|
24 (52.5)
|
In October 2020, 1053 larvae (min = 4, max = 444) were collected, of which 470 were reared to adults. In April 2021, 846 larvae (min = 1, max = 146) were collected, of which 193 were reared to adults. As in the HLC sampling, An. gambiae s.s., An. coluzzii and An. arabiensis were identified. Moreover, two hybrids of An. gambiae s.s. and An. coluzzi were found. An. coluzzii was predominant (64.8%) in the larval sampling (Table 3).
Table 3
Species composition of the adults reared from larvae sampling.
|
October 2020
N (%)
|
April 2021
N (%)
|
An. gambiae s.s.
|
186 (39.6)
|
66 (34.2)
|
An. coluzzi
|
277 (58.9)
|
125 (64.8)
|
An. arabiensis
|
5 (1.1)
|
0
|
An. obscurus
|
2 (0 .4)
|
0
|
An. gambiae M/S
|
0
|
2 (1.0)
|
Total
|
470
|
193
|
An. arabiensis individuals (n = 5) were only found at the end of the rainy season in 2020, and in only two sites (Fig. 2). The first site was a watercourse in the Air France district (BS1, Photo 1) and the second was located in the lowland of Odiennekourani (BS36, Photo 2), near the house where adults were caught in October 2019.
In Odiennekourani, the water was turbid and stagnant in garden markets, whereas in the Air France district, water was not turbid but seemed polluted.
An. arabiensis were found together with An. gambiae s.s. and/or An. coluzzii in the same breeding sites. In BS1, only 2 larvae were found and became one female of An. arabiensis and one male of An. gambiae s.s. In BS36, 200 larvae were collected and produced 134 adults: 2 females and 2 males of An. arabiensis, 30 females and 44 males of An. coluzzii, 29 females and 27 males of An. gambiae s.s.