A total of 535 larval habitats with 86 positive breeding habitats were surveyed from all the study sites, comprising six (6) different habitat types over the entire sampling period. Overall, the most abundant habitat type was car tyres [59.3%, n = 51/86], whereas the least were tanks [2.3%, n = 2/86] and buckets [2.3%, n = 2/86] (Table 1). A significantly higher abundance of larval habitats was encountered during the rainy season [62.8%, n = 54/86] than in the dry season [37.2%, n = 32/86] (X2 = 18.5035, df = 5, P = 0.002). Within the 4 sites sampled, a significantly higher proportion (99.8%, 85/86) of larval habitats were encountered outdoors as compared to indoors (1.2%, 1/86) (X2 = 42.4941, df = 5, P < 0.001).
Larval Indices (Stegomyia Indices)
Larval Indices calculated using the WHO formula for each study site were used to make inferences about the risk of transmission of yellow fever in the outbreak and non-outbreak zones studied. From the results obtained, in Larabanga the indices (BI: 84% CI:26.4%) exceeded the threshold and can be classified as a high-risk zone for yellow fever transmission as compared to all the other study areas, Wenchi (BI:42.3% CI:19.3% HI:23.1%), Kpalsogu (BI:37.7% CI:12.1% HI:6.7%) and Pagaza (BI: 30.2% CI:8.9% HI:9.4%) that had values within the range sufficient to promote an outbreak. These are shown in Fig. 3.
Figure 3: Stegomyia Indices per study site
Spatio-temporal distribution of adult Aedes mosquitoes
Overall, a total of 1,507 adult Aedes mosquitoes were collected from all the study areas, with Aedes aegypti [92%, n = 1386/1507] as the predominant species, followed by Aedes formosus [8%, n = 121/1507]. A high abundance of Aedes mosquitoes was collected in Larabanga, a yellow fever outbreak site [Aedes aegypti 884/1386 (63.8%); Aedes formosus 112/121 (92.3%)], whereas the least abundance was collected in Pagaza, a non-yellow fever outbreak site [Aedes aegypti 75/1386 (5.4%); Aedes formosus 9/121 (7.7%)] (X2 = 52.061, df = 3, P < 0.001).
A high abundance of Aedes mosquitoes was sampled outdoors [n = 914 (60.7%)] as compared to indoor collections [n = 593 (39.3%); X2 = 68.38, df = 1, P < 0.01)]. A high abundance of adult Aedes mosquitoes was sampled for both indoor [n = 342 (34.3%)] and outdoor [n = 654 (65.7%)] collections in Larabanga whereas, the least recorded abundance was in Pagaza [Indoor (n = 45 (53.6%); Outdoor (n = 39, 46.4%; X2 = 32.021, df = 3, P < 0.001), Table 3.
Table 3
Adult Aedes abundance per study site and location
Study Sites | Aedes aegypti | Aedes formosus | Indoor | Outdoor | Total per Site |
No. (%) | No. (%) | No. (%) | No. (%) | No. (%) |
Wenchi | 325 (100) | 0 (0.0) | 159 (48.9) | 166 (51.1) | 325 (21.5) |
Larabanga | 884 (88.0) | 112 (12.0) | 342 (34.3) | 654 (65.7) | 996 (66.6) |
Kpalsogu | 102 (100) | 0 (0.0) | 47 (46.1) | 55 (53.9) | 102 (6.7) |
Pagaza | 75 (89.0.) | 9 (11.0) | 45 (53.6) | 39 (46.4) | 84 (5.6) |
Total | 1386 (92.0) | 121 (8.0) | 593 (39.3) | 914 (60.7 | 1507 (100) |
Adult Aedes mosquitoes were more predominant in the dry season [n = 922/1507 (61.2%)] than in the rainy season [n = 585/1507 (38.8%)], (X2 = 75.36, df = 1, P < 0.001). In the dry season, the highest abundance of adult Aedes mosquitoes was recorded in Larabanga [n = 289/585, (49.4%)], followed by those from Wenchi [n = 189/585, (32.3%)], both being yellow fever outbreak sites. Mosquitoes from the non-yellow fever outbreak sites were the least, Pagaza [n = 70/585, (12.0%)] and Kpalsogu [n = 37/585, (6.3%)]. Similarly, during the rainy season, the highest abundance of Aedes mosquitoes was recorded in Larabanga [n = 707/922, (76.7%)], followed by Wenchi [n = 136/922 (14.8%)], the yellow fever outbreak sites. There were small numbers of Aedes mosquitoes sampled in the non-yellow fever outbreak sites of Kpalsogu [n = 65/922, (7.0%)] and Pagaza [n = 14/922, (1.5%)]. These are shown in Fig. 4.
Figure 4: Seasonal abundance of adult Aedes mosquitoes
Seasonal abundance of adult Aedes mosquitoes per trap type
Three separate traps namely, HLC, PPK, and BG were used to collect a total of 1507 adult Aedes mosquitoes from all the study sites. A total of 601 (39.9%) Aedes mosquitoes were collected using HLC [dry = 280 (46.6%); rainy = 321 (53.4%)], PPK 443 (29.4%) [dry = 183 (41.3%); rainy = 260 (58.7%)] and BG 463 (30.7%) [dry = 122 (26.3%); rainy = 341 (73.7%)]. Of the 796 female adult mosquitoes caught with PPK and BG, 575 [PPK = 295 (66.6%); BG = 280 (60.5%)] were unfed, 177 [PPK = 77 (17.4%) BG = 100 (21.6%)] were blood-fed, half-gravid [PPK = 12 (2.7%); BG = 0] and gravid [PPK = 18 (4.1%); BG = 14 (3.0%)], (X2 = 46.745, df = 2, P < 0.001) (Fig. 5).
Figure 5: Season abundance of Aedes mosquitoes per trap type (In-Indoor; OUT-Outdoor; BG-Biogents-2 sentinel traps; HLC-Human Landing Catches; PPK-Prokopack aspiration)
Insecticide susceptibility status of Aedes mosquitoes
Bioassay results showed resistance to deltamethrin at x1 concentration across all the study sites (33.75% − 70%) (Fig. 6). However, susceptibility to deltamethrin at x5 was observed across all sites. The mosquitoes showed possible resistance to permethrin at x1 in Kpalsogu (95%), Pagaza (96.5%), and susceptibility in Larabanga (98%). Resistance to pirimiphos-methyl at x1 was shown in Aedes mosquitoes in Kpalsogu (63.75%) and possible resistance was observed in Pagaza (93.75%) and Larabanga (92.5%). For Wenchi, for logistical challenges, not many larvae were sampled for the determination of insecticide susceptibility, so only deltamethrin was run. These results are shown in Fig. 6.
Figure 6: Mortalities of Aedes mosquitoes exposed to different insecticides in the study sites.
Genotypic mutations associated with resistance in Aedes aegypti
A subset of 242 Ae. aegypti obtained from the phenotypic assays were genotyped for the F1534C, V1016I and V410L kdr mutations. About 20–30 mosquito samples were selected from each site per insecticide paper per concentration for the genotypic resistance determination. The F1534C mutation was detected in moderate to high allele frequencies in Ae. aegypti mosquitoes exposed to pyrethroid insecticides. Aedes aegypti from Wenchi that were exposed to deltamethrin, had a significantly high allele frequency of F1534C mutation (0.92) compared to mosquitoes from the non-outbreak sites, Pagaza (0.19) and Kpalsogu (0.35) (χ2 = 50.50, df = 3, P < 0.001). For the V1016I mutation, low to moderate allele frequencies (0.23 to 0.54) were observed in Ae. aegypti mosquitoes exposed to both deltamethrin and permethrin except for Larabanga, an outbreak site, where mosquitoes had a high allele frequency of 0.77. However, there was a significant difference in the frequency of V1016I mutation in outbreak and non-outbreak sites (P < 0.05). For the V410L mutation, there was no significant differences in the frequency of the mutations between outbreak sites and non-outbreak sites with low allele frequencies ranging from 0.05 to 0.15. The genotypes and allele frequencies of each kdr mutation are shown in Table 4.
Table 4
Number of genotypes and frequencies of the F1534C, V1016I and V410L mutation in the voltage-gated sodium channel gene of Aedes aegypti mosquitoes.
| | | | | F1534C | | | | V1016I | | | | V410L | | |
Insecticide | Site Description | Study site | n | CC | FF | FC | Allele Freq. | II | VV | VI | Allele Freq. | LL | VV | VL | Allele Freq. |
Deltamethrin | Non-outbreak | Pagaza | 60 | 6 | 43 | 11 | 0.19 | 3 | 22 | 35 | 0.34 | 0 | 45 | 15 | 0.13 |
Kpalsogu | 44 | 12 | 25 | 7 | 0.35 | 1 | 1 | 42 | 0.5 | 3 | 40 | 1 | 0.08 |
Outbreak | Larabanga | 30 | 8 | 13 | 9 | 0.42 | 2 | 0 | 28 | 0.53 | 0 | 27 | 3 | 0.05 |
Wenchi | 52 | 45 | 1 | 6 | 0.92 | 8 | 4 | 40 | 0.54 | 3 | 46 | 3 | 0.09 |
| Total | 186 | 71 | 82 | 33 | | 14 | 27 | 145 | | 6 | 160 | 22 | |
Permethrin | Non-outbreak | Pagaza | 32 | 8 | 21 | 3 | 0.3 | 0 | 17 | 15 | 0.23 | 2 | 29 | 1 | 0.15 |
Kpalsogu | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
Outbreak | Larabanga | 24 | 5 | 16 | 3 | 0.27 | 13 | 0 | 11 | 0.77 | 0 | 24 | 0 | 0 |
Wenchi | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| Total | 56 | 13 | 37 | 6 | | 13 | 17 | 26 | | 2 | 53 | 1 | |
Abbreviations: VV, wild type (susceptible); VL, heterozygotes; LL, mutant (resistant); VI, heterozygotes; II, mutant (resistant); FF, wild type (susceptible); FC, heterozygotes; CC, mutant (resistant); n sample size.