WHO susceptibility test, intensity, and synergist assays
Susceptibility, intensity, and synergist tests results for 15 sites were reported by Kouassi et al. [16] in 2020, showing high pyrethroid resistance at all sites tested and a substantial increase in vector mortality after pre-exposure to PBO. In addition to the 15 sites already reported, high resistance was also recorded against pyrethroid insecticides in Bocanda and Jacqueville, similar to the other sites [16]. Pre-exposure to PBO increased vector mortality when exposed to deltamethrin, alpha-cypermethrin, and permethrin in Bocanda and Jacqueville as observed in the other reported sites.
Susceptibility status of An. gambiae s.l. to clothianidin and pirimiphos-methyl
For clothianidin, susceptibility was recorded in 9 out of the 17 sites including Nassian and Sakassou, both initially targeted districts for IRS. Resistance to clothianidin was observed in Abengourou, Aboisso, Bettié, Bouaké, Bouna, Daloa, Odienné, and Yamoussoukro (Fig. 3). Susceptibility to pirimiphos-methyl was recorded at the diagnostic dose in 11 sites, while low resistance intensity was observed in 5 of the remaining sites (Aboisso, Adzopé, Bettié, Bocanda, and Daloa). Only Jacqueville recorded a moderate resistance to pirimiphos-methyl (Fig. 2).
Adult Mosquito Fauna Across The Four Bionomic Monitoring Sites
A total of 105,225 mosquitoes, including 12,206 (11.6%) culicines and 93,019 Anopheles (88.4%) were collected over the year (January through December 2019) using the three collection methods (HLC, PSC and CDC light trap). Anopheles gambiae s.l. was the most predominant malaria vector species, representing 94.4% (87,765/93,019) of the total Anopheles mosquitoes collected across all sites.
Using HLC, a total of 77,122 Anopheles mosquitoes were collected. With PSC and CDC light trap methods, 10,254 and 5,643 mosquitoes were collected, respectively. Anopheles gambiae s.l. was the most collected vector using all three methods (Table 1).
Out of the 93,019 Anopheles, 8,757 were collected in Béoumi, 16,732 in Dabakala, 4,343 in Nassian from May through December and 63,187 in Sakassou from January through December 2019. Anopheles gambiae s.l. was the most collected malaria vector species in the four sites representing 90.6% (n = 7,930), 88.8% (n = 14,864), 91.4% (n = 3,968) and 96.5% (n = 61,003) of the total Anopheles collected in Béoumi, Dabakala, Nassian and Sakassou, respectively. Anopheles funestus s.l. represented the other malaria vectors found in Beoumi (0.1%; n = 7) and Nassian (8.5%; n = 368) while both An. funestus s.l. and An. nili were found in Dabakala (6%; n = 998 and 3.4%; n = 560), and Sakassou (0.45%; n = 282 and 0.1%, n = 13), respectively (Table 1).
Table 1
Number and percentages of mosquitoes collected in all sites using the three collection methods in 2019
Collection method | Species | Béoumi # (%) | Dabakala # (%) | Nassian # (%) | Sakassou # (%) | Total # (%) |
All collection methods | An. gambiae s.l. | 7,930 (90.6) | 14,864 (88.8) | 3,968 (91.4) | 61,003 (96.5) | 87,765 (94.4) |
An. funestus s.l. | 7 (0.1) | 998 (6.0) | 368 (8.5) | 282 (0.45) | 1,655 (1.8) |
An. nili | 1 (0.2) | 560 (3.4) | 0 (0) | 13 (0.1) | 574 (0.6) |
Other Anopheles | 819 (27.1) | 310 (10.2) | 7 (0.2) | 1,889 (62.4) | 3,025 (3.3) |
Total | 8,757 | 16,732 | 4,343 | 63,187 | 93,019 |
HLC | An. gambiae s.l. | 6,102 (88.5) | 10,643 (92.7) | 3,408 (92.5) | 53,179 (96.5) | 73,332 (95.1) |
An. funestus s.l. | 7 (0.1) | 281 (2.4) | 270 (7.3) | 159 (0.3) | 717 (0.9) |
An. nili | 1 (0.0) | 426 (3.7) | 0 (0) | 13 (0.0) | 440 (0.6) |
Other Anopheles | 783 (11.4) | 135 (1.2) | 7 (0.2) | 1708 (3.1) | 2633 (3.4) |
Total | 6,893 | 11,485 | 3,685 | 55,059 | 77,122 |
PSC | An. gambiae s.l. | 1,183 (97.0) | 2,381 (69.9) | 501 (83.6) | 4,725 (94.0) | 8,790 (85.7) |
An. funestus s.l. | 0 (0) | 717 (21.0) | 98 (16.4) | 123 (2.4) | 938 (9.1) |
An. nili | 0 (0) | 134 (3.9) | 0 (0) | 0 (0) | 134 (1.3) |
Other Anopheles | 36 (3) | 175 (5.1) | 0 (0) | 181 (3.6) | 392 (3.8) |
Total | 1,219 | 3,407 | 599 | 5,029 | 10,254 |
CDC light trap | An. gambiae s.l. | 645 (100) | 1840 (100) | 59 (100) | 3099 (100) | 5643 (100) |
Total | 645 | 1,840 | 59 | 3,099 | 5,643 |
*Other Anopheles included An. pharoensis, An. ziemanni and An. coustani |
Molecular species identification of adults An. gambiae s.l. collected for bionomic monitoring
A subsample of 694 An. gambiae s.l. from Béoumi, 698 from Dabakala, 608 from Nassian, and 731 from Sakassou were molecularly identified to the sub-species level. Anopheles coluzzii represented the predominant species in Béoumi and Dabakala (all collection methods included) and was the only species found in Sakassou. In Nassian, An. gambiae s.s. was the only vector species found using the three methods. Few hybrids of the two species were found in the HLC samples from Béoumi (0.2%) (Fig. 4).
Anopheles gambiae s.l. human biting rate
The human biting rates (HBRs) were higher in Sakassou, with an overall rate ranging from 62.7 b/p/n in January to 255.8 b/p/n in October indoors and from 49.1 b/p/n in January to 211.5 b/p/n outdoors compared to Dabakala (9.9 b/p/n in June to 75.0 b/p/n in July indoors and 1.1 b/p/n in June to 78.9 b/p/n in September outdoors) and Béoumi (4.3 b/p/n in June to 80.9 b/p/n in September indoors and 5.8 b/p/n in June to 69.4 b/p/n in September outdoors b/p/n). The biting rates were lowest in Nassian (from 0.4 b/p/n in May to 59.1 b/p/n in October indoors and 1.2 b/p/n in May to 41.7 b/p/n in October outdoors) regardless of the months and the collection place (indoor or outdoor) (Fig. 5). Overall, An. gambiae s.l. showed similar biting behavior across the four districts with peak biting time observed between 10:00 p.m-3:00 a.m. (Supp data 1). However, the HBR of An. gambiae s.l. were higher outdoor in Sakassou than indoor. Additionally, An. gambiae s.l. was found endophilic in Béoumi (53.2% indoors) and Nassian (57% indoors) while similar densities were collected both outdoors and indoors (50%) in Dabakala
Plasmodium falciparum sporozoite infection and entomological inoculation rate
Table 2 shows the overall infection rate and EIRs for An. gambiae s.l. and An. funestus s.l. collected by HLC in the four districts. Nassian recorded the highest Plasmodium falciparum sporozoite infection rate (SR) among An. gambiae s.l. (0.055), followed by Sakassou (0.021) and Dabakala (0.018). The lowest rate (0.010) was recorded in Beoumi. Additionally, 16 An. funestus s.l. mosquitoes were found with sporozoite infections among the samples collected in November and December 2019, representing a SR of 7.4% of the 215 analyzed mosquitoes in Nassian.
Sakassou still recorded the highest EIR (3.59 ib/p/n) of An. gambiae s.l., while Nassian and Dabakala had similar EIRs of 0.73 and 0.75 ib/p/n respectively. Béoumi recorded the lowest EIR among the four sites, with 0.24 ib/p/n. May and November were the months with the highest number of An. gambiae s.l. infected bites in Sakassou, while Dabakala recorded a single peak in July, Béoumi, and Nassian in October (Fig. 6). An average EIR value of 1.21 ib/p/n was recorded for An. funestus s.l. in Nassian.
Table 2
Sporozoite infection rate and EIR of malaria vectors collected using HLC in the four vector bionomics sites
| An. gambiae s.l. | An. funestus s.l. |
District | TC | TA | P | SR | HBR (bpn) | EIR (ibpn) | TC | TA | P | SR | HBR (bpn) | EIR (ibpn) |
Béoumi | 7,930 | 900 | 9 | 0.010 | 24.26 | 0.24 | 7 | 0 | - | - | - | - |
Dabakala | 14,864 | 868 | 16 | 0.018 | 40.71 | 0.75 | 998 | 0 | - | - | - | - |
Nassian | 3,968 | 675 | 37 | 0.055 | 13.33 | 0.73 | 368 | 215 | 16 | 0.074 | 16.4 | 1.21 |
Sakassou | 52,484 | 1541 | 25 | 0.021 | 170.10 | 3.59 | 282 | 0 | - | - | - | - |
TC = total collected; TA = total analyzed; P = positive; SR = sporozoite rate; HBR = Human Biting Rate; EIR = Entomological Inoculation Rate; (-) = not analyzed |
Epidemiological Data Of Targeted Sites
HMIS data from 2017 and 2018 were averaged to provide monthly estimates of the number of confirmed cases within the population that were recorded from all health facilities of each district.
For Sakassou, a peak of number of positive malaria cases was observed in August 2017 and in April 2018. A certain stability of malaria cases was observed between May and December in 2017 while all months had cases above 3000 from January to December 2018 with the peak of more than 7000 malaria cases in April 2018. The lowest number of malaria cases was recorded between January and March 2017 in Sakassou (Fig. 7).
In Nassian, even though the number of cases was slightly lower than Sakassou, an incidence peak was observed in August 2017 (3,041 malaria cases) while two similar peaks were recorded in April 2018 (2,544 cases) and in June 2018 (2,432 cases). However, the number of malaria cases were slightly higher in 2018 than 2017 except a drop in cases that occurred in July 2018 (Fig. 8).