Contribution of different Anopheles species to malaria transmission
The comprehensive mosquito survey conducted throughout the study period yielded 20,752 adult female Anopheles. all Anopheles collected, An. funestus constituted 80% (n=16,870), followed by An. gambiae s.l. at 18% (n=3,662). The remainder comprised various species, such as An. coustani (n=60), An. maculipalpis (n=27), An. ziemanni (n=6), and An. pharoensis (n=3).
Based on the ELISA tests for sporozoite infections, malaria transmission within the study villages was primarily attributed to An. funestus and An. arabiensis. (Table 1). Indoor mosquito collections using CDC-light traps yielded an overall annual EIR of 20.1 infectious bites per person per year (ib/p/y), while outdoor collections using DN-Mini traps resulted in an overall annual EIR of 6.5 ib/p/y. In the comparative analysis of species contribution to EIR, it was observed that An. funestus accounted for 97.6% and 95.4% of indoor and outdoor malaria transmission, while An. arabiensis contributed 2.4% and 4.6% to indoor and outdoor malaria transmission, respectively (Table 1). No Plasmodium infections were detected in the other Anopheles species mosquitoes collected in the study area (Table 1).
Table 1. Analysis of malaria vectors, transmission indices, and contribution of different vector species to EIR in the study area.
Parameter
|
Trap & trap positions
|
An. funestus
|
An. arabiensis
|
Other Anopheles
|
Total Trap nights
|
CDC light traps (Indoors)
|
4,464
|
4,464
|
4,464
|
DN-mini traps (Outdoors)
|
1,422
|
1,422
|
1,422
|
Number caught
|
CDC light traps (Indoors)
|
15,343
|
2,771
|
84
|
DN-mini traps (Outdoors)
|
1,527
|
891
|
12
|
No. tested pools
|
CDC light traps (Indoors)
|
1,134
|
3,632
|
70
|
DN-mini traps (Outdoors)
|
446
|
716
|
8
|
No. pools with P. falciparum sporozoites
|
CDC light traps (Indoors)
|
240
|
6
|
0
|
DN-mini traps (Outdoors)
|
24
|
1
|
0
|
Minimum prevalence of P. falciparum
|
CDC light traps (Indoors)
|
1.5%
|
0.2%
|
0
|
DN-mini traps (Outdoors)
|
1.5%
|
0.1%
|
0
|
Annual entomological inoculation rate (EIR)
|
Indoors (based on CDC-light traps data)
|
19.6
|
0.5
|
0
|
Outdoors (based on DN-Mini data)
|
6.2
|
0.3
|
0
|
Overall EIR
|
Indoors (based on CDC-light traps data)
|
20.1
|
Outdoors (based on DN-Mini data)
|
6.5
|
Percentage contribution of species to EIR
|
Indoors (based on CDC-light traps data)
|
97.6%
|
2.4%
|
0%
|
Outdoors (based on DN-Mini data)
|
95.4%
|
4.6%
|
0%
|
Potential aquatic habitats in the rainy and dry season
Across the five villages visited during the rainy season (Chikuti, Chirombola, Ebuyu, Kichangani, and Mzelezi), a total of 1,923 water bodies (potential habitats) were identified; 48 ground pools (2.5%), %), 225 human-made habitats (11.7%), 103 rice fields (5.4%), 1,104 segments of river streams (57.4%), 351 springs/ wells (18.3%), and 92 other small habitats (4.8%). The median number of potential aquatic habitats identified was 342 (range: 204-644) and the median surface area of the habitats was 62,930 (range: 19,913-1,197,974). During the dry season, seven villages were visited (Chirombola, Ebuyu, Iragua, Kichangani, Kidugalo, Mwaya, and Mzelezi), where a total of 1,528 potential habitats were identified. These included 74 ground pools (4.8%), 67 human-made habitats (4.4%), 26 rice fields (1.7%), 910 stream segments (59.6%), 401 springs/ wells (26.2%), and 50 other small habitats (3.3%). In this season the median number and area of potential aquatic habitats identified were 238 (range: 103-331) and 46,525 (range: 3,068-486,362), respectively.
Infestation of aquatic habitats by Anopheles mosquitoes in wet and dry seasons
During the rainy season, across five villages, 37.4% (n=719) of the 1,923 water bodies assessed contained Anopheles mosquito larvae or pupae (Table 3). The median number of habitats in these villages was 66.5 (range: 27-341) and the median surface area of habitats was 22,338 (range: 804-1,099,282). The analysis of the rainy season data also revealed that 317 of the 1,923 habitats were infested with late instars of An. funestus, covering an area of 1,127,069 m² out of all potential habitats (1,698,442 m²) (Table 3). When the prevalence of infestation in different habitat categories was assessed, the ground pools had the highest infestation, reaching 35.4% (17 out of 48 ground pools), which constituted 92.7% of the surface area of total ground pool area (1,160,762 m²); since the infested pools were mostly the large ones. Among all An. funestus-infested habitats rivers and streams were the main contributors in terms of the number of habitats (65.9%, 209 rivers and stream segments out of all 317 An. funestus-infested habitats) and ground pools were the primary contributor in terms of total surface area (95.5%, 1,075,964 m² out of 1,127,069 m²). For An. arabiensis, 215 of the 1,923 habitats were infested with late instars of An. arabiensis, covering an area of 1,158,574 m² out of the total 1,698,442 m² of potential aquatic habitats. Springs/ wells were the most frequent aquatic habitats for An. arabiensis in terms of the number of locations (45.1%, n = 97), but ground pools still contributed the most surface area (93.8%) for larval development (among a total An. arabiensis-infested surface area of 1,160,762 m²).
In the dry season, among seven villages and 1,528 potential habitats, 31.1% (n=475) contained Anopheles mosquito larvae or pupae (Table 3). The median number of habitats in these villages was 36.5 (range: 7-334) and the median surface area of habitats was 23,707 (range: 214-576,249) m². Further analysis of this dry season data showed that 233 of the 1,528 habitats were infested with late instars of An. funestus, covering an area of 360,597 m² out of the total 887,662 m² of potential habitats (Table 3). In the analysis of the prevalence of An. funestus infestation in each habitat category, ground pools again had the highest infestation rate, reaching 27% (20 out of 74 ground pools). Among all An. funestus-infested habitats, rivers and streams were the main contributors in terms of the number of An. funestus-infested aquatic habitats in the dry season (80.7%, 188 out of 233 An. funestus-infested habitats), and ground pools were the primary contributor in terms of total surface area, contributing 90.2% (325,325 m2 out of 360,597 m² An. funestus-infested habitats). For An. arabiensis, 148 of the 1,528 habitats were infested with late instars, covering an area of 301,248 m² out of the total 887,662 m² of potential habitats. For this species too, ground pools had the highest infestation rate reaching 20.3% (15 out of 74 ground pools). These infested ground pools represented 41.5% (268,374 m²) of the total ground pool area (646,363 m²). Among all An. arabiensis-infested habitats, rivers and stream segments remained significant, contributing 45.1% (n = 97) to the total number of An. arabiensis-infested habitats. However, in terms of surface area, 93.8% of the An. arabiensis-infested habitats surface area (1,158,574 m²) was attributed to ground pools (Table 3).
Table 2. Water bodies (potential mosquito aquatic habitats) found in the study villages.
Season
|
Habitat category
|
Total number of habitats, N (%)
|
Total area of habitats, m2 (%)
|
Median No. habitats/village, m2 (range)
|
Median area of habitats/village, m2 (range)
|
Rainy season
|
Ground pools
|
48 (2.5%)
|
1,160,763 (68.3%)
|
9 (0-25)
|
12,598 (0-988,309)
|
Human-made habitats
|
225 (11.7%)
|
28,378 (1.7%)
|
28 (19-115)
|
3,585 (1,108-16,903)
|
Rice fields
|
103 (5.4%)
|
370,740 (21.8%)
|
22 (0-42)
|
27,123 (0-175,335)
|
River/ stream segments
|
1,104 (57.4%)
|
125,169 (7.4%)
|
192 (150-300)
|
21,236 (17,122-35,847)
|
Springs/ wells
|
351 (18.3%)
|
9,128 (0.5%)
|
54 (6-150)
|
275 (12-8,186)
|
Other small habitats
|
92 (4.8%)
|
4,264 (0.3%)
|
5 (4-60)
|
62 (58-3,705)
|
Subtotal*
|
1,923 (100%)
|
1,698,441 (100%)
|
342 (204-644)
|
62,930 (19,913-1,197,974)
|
Dry season
|
Ground pools
|
74 (4.8%)
|
646,362 (72.8%)
|
15 (0-22)
|
17,828 (0-426,003)
|
Human-made habitats
|
67 (4.4%)
|
15,682 (1.8%)
|
9 (2-25)
|
260 (11-7,594)
|
Rice fields
|
26 (1.7%)
|
87,926 (9.9%)
|
5 (0-9)
|
2,066 (0-61,305)
|
River/ stream segments
|
910 (59.6%)
|
135,407 (15.3%)
|
135 (28-194)
|
11,028 (2,548-59,918)
|
Springs/ wells
|
401 (26.2%)
|
1,349 (0.2%)
|
65 (5-100)
|
191 (9-374)
|
Other small habitats
|
50 (3.3%)
|
936 (0.1%)
|
5 (1-16)
|
89 (1-322)
|
Subtotal*
|
1,528 (100%)
|
887,662 (100%)
|
238 (103-331)
|
46,525 (3,068-486,362)
|
*Numbers and areas of habitats were calculated from a different number of villages in rainy (5 villages: Chikuti, Chirombola, Ebuyu, Kichangani, and Mzelezi) and dry (7 villages: Chirombola, Ebuyu, Iragua, Kichangani, Kidugalo, and Mwaya and Mzelezi) seasons.
Table 3. Number and area of aquatic habitats infested by different immature stages of different Anopheles in the rainy and dry seasons.
Season
|
Habitat types
|
Total number of potential habitats
|
No. habitats with Anopheles immatures, (% contribution)
|
Proportion of habitats infested by habitat category
|
Median No. habitats with Anopheles immatures per village (range)
|
Total area of potential habitats
|
Total area (m2) of habitats with Anopheles immatures, (% contribution)
|
Proportion of area infested by habitat category
|
Median area (m2) of habitats infested with Anopheles immatures per village (range)
|
Any Anopheles
|
Rainy season
|
Ground pools
|
48 (2.5%)
|
27 (3.8%)
|
56.2%
|
7 (0-10)
|
1,160,762 (68.3%)
|
1,099,282 (88.3%)
|
94.7%
|
1,113 (0 (-988,309)
|
Human-made habitats
|
225 (11.7%)
|
88 (12.2%)
|
39.1%
|
11 (8-45)
|
28,378 (1.7%)
|
14,430 (1.2%)
|
50.8%
|
1,182 (313 (-10,232)
|
Rice fields
|
103 (5.4%)
|
45 (6.3%)
|
43.7%
|
9 (0-23)
|
370,741 (21.8%)
|
98,407 (7.9%)
|
26.5%
|
9,628 (0 (-48,699)
|
River/ stream segments
|
1,104 (57.4%)
|
341 (47.4%)
|
30.9%
|
38 (5-205)
|
125,169 (7.4%)
|
30,246 (2.4%)
|
24.2%
|
4,903 (450 (-16,026)
|
Springs/ wells
|
351 (18.3%)
|
186 (25.9%)
|
53.0%
|
25 (1-96)
|
9,128 (0.5%)
|
804 (0.1%)
|
8.8%
|
106 (1 (-401)
|
Other small habitats
|
92 (4.8%)
|
32 (4.5%)
|
34.8%
|
3 (1-22)
|
4,264 (0.3%)
|
1,387 (0.1%)
|
32.5%
|
53 (4 (-951)
|
Subtotal*
|
1,923 (100%)
|
719 (100%)
|
37.4%
|
66.5 (27-341)
|
1,698,441 (100%)
|
1,244,555 (100%)
|
73.3%
|
22,338 (804-1,099,282)
|
Dry season
|
Ground pools
|
74 (4.8%)
|
41 (8.6%)
|
55.4%
|
7 (0-13)
|
646,363 (72.8%)
|
576,249 (87.6%)
|
89.2%
|
17,549 (0 (-392,015)
|
Human-made habitats
|
67 (4.4%)
|
32 (6.7%)
|
47.8%
|
5 (0-9)
|
15,680 (1.8%)
|
14,884 (2.3%)
|
94.9%
|
193 (0 (-7,519)
|
Rice fields
|
26 (1.7%)
|
7 (1.5%)
|
26.9%
|
0 (0-3)
|
87,926 (9.9%)
|
32,530 (4.9%)
|
37%
|
0 (0 (-27,273)
|
River/ stream segments
|
910 (59.6%)
|
334 (70.3%)
|
36.7%
|
39 (21-91)
|
135,408 (15.3%)
|
33,468 (5.1%)
|
24.7%
|
4,623 (2,234 (-6,733)
|
Springs/ wells
|
401 (26.2%)
|
43 (9.1%)
|
10.7%
|
8 (1-10)
|
1,350 (0.2%)
|
214 (0%)
|
15.9%
|
30 (2 (-80)
|
Other small habitats
|
50 (3.3%)
|
18 (3.8%)
|
36.0%
|
1 (0-7)
|
936 (0.1%)
|
495 (0.1%)
|
52.9%
|
33 (0 (-191)
|
Subtotal*
|
1,528 (100%)
|
475 (100%)
|
31.1%
|
36.5 (7-334)
|
887,662 (100%)
|
657,841 (100%)
|
74.1%
|
23,707 (214-576,249)
|
Late instars of An. funestus
|
Rainy season
|
Ground pools
|
48 (2.5%)
|
17 (5.4%)
|
35.4%
|
4 (0-7)
|
1,160,762 (68.3%)
|
1,075,964 (95.5%)
|
92.7%
|
1,052 (0 (-965,258)
|
Human-made habitats
|
225 (11.7%)
|
35 (11.0%)
|
15.6%
|
5 (4-16)
|
28,378 (1.7%)
|
6,664 (0.6%)
|
23.5%
|
700 (190 (-4,555)
|
Rice fields
|
103 (5.4%)
|
12 (3.8%)
|
11.7%
|
2 (0-5)
|
370,741 (21.8%)
|
25,637 (2.3%)
|
6.9%
|
1,648 (0 (-14,310)
|
River/ stream segments
|
1,104 (57.4%)
|
209 (65.9%)
|
18.9%
|
16 (5-146)
|
125,169 (7.4%)
|
17,954 (1.6%)
|
14.3%
|
1,584 (450 (-12,628)
|
Springs/ wells
|
351 (18.3%)
|
38 (12.0%)
|
10.8%
|
7 (1-16)
|
9,128 (0.5%)
|
225 (0%)
|
2.5%
|
30 (1 (-89)
|
Other small habitats
|
92 (4.8%)
|
6 (1.9%)
|
6.5%
|
1 (0-2)
|
4,264 (0.3%)
|
625 (0.1%)
|
14.7%
|
16 (0 (-558)
|
Subtotal*
|
1,923 (100%)
|
317 (100%)
|
16.5%
|
26 (6-209)
|
1,698,442 (100%)
|
1,127,069 (100%)
|
66.4%
|
12,309 (225-1,075,964)
|
Dry season
|
Ground pools
|
74 (4.8%)
|
20 (8.6%)
|
27.0%
|
3 (0-7)
|
646,363 (72.8%)
|
325,325 (90.2%)
|
50.3%
|
4,618 (0 (-245,934)
|
Human-made habitats
|
67 (4.4%)
|
10 (4.3%)
|
14.9%
|
1 (0-5)
|
15,680 (1.8%)
|
13,995 (3.9%)
|
89.3%
|
52 (0 (-7,503)
|
Rice fields
|
26 (1.7%)
|
3 (1.3%)
|
11.5%
|
0 (0-2)
|
87,926 (9.9%)
|
1,370 (0.4%)
|
1.6%
|
0 (0 (-1,272)
|
River/ stream segments
|
910 (59.6%)
|
188 (80.7%)
|
20.7%
|
19 (6-79)
|
135,408 (15.3%)
|
19,803 (5.5%)
|
14.6%
|
1,924 (1,147 (-6,132)
|
Springs/ wells
|
401 (26.2%)
|
11 (4.7%)
|
2.7%
|
3 (1-4)
|
1,350 (0.2%)
|
104 (0%)
|
7.7%
|
22 (1 (-58)
|
Other small habitats
|
50 (3.3%)
|
1 (0.4%)
|
2.0%
|
0 (0-1)
|
936 (0.1%)
|
1 (0%)
|
0.1%
|
0 (0 (-1)
|
Subtotal*
|
1,528 (100%)
|
233 (100%)
|
15.2%
|
10.5 (1-188)
|
887,662 (100%)
|
360,597 (100%)
|
40.6%
|
7,683 (1-325,325)
|
Late instars of An. arabiensis
|
Rainy season
|
Ground pools
|
48 (2.5%)
|
12 (5.6%)
|
25.0%
|
1 (0-8)
|
1,160,762 (68.3%)
|
1,086,758 (93.8%)
|
93.6%
|
100 (0 (-977,509)
|
Human-made habitats
|
225 (11.7%)
|
26 (12.1%)
|
11.6%
|
5 (0-12)
|
28,378 (1.7%)
|
6,537 (0.6%)
|
23.0%
|
321 (0 (-5,435)
|
Rice fields
|
103 (5.4%)
|
23 (10.7%)
|
22.3%
|
1 (0-17)
|
370,741 (21.8%)
|
55,814 (4.8%)
|
15.1%
|
5,068 (0 (-30,745)
|
River/ stream segments
|
1,104 (57.4%)
|
42 (19.5%)
|
3.8%
|
8 (0-20)
|
125,169 (7.4%)
|
8,062 (0.7%)
|
6.4%
|
1,139 (0 (-4,050)
|
Springs/ wells
|
351 (18.3%)
|
97 (45.1%)
|
27.6%
|
24 (1-48)
|
9,128 (0.5%)
|
518 (0%)
|
5.7%
|
87 (22 (-322)
|
Other small habitats
|
92 (4.8%)
|
15 (7.0%)
|
16.3%
|
0 (0-13)
|
4,264 (0.3%)
|
886 (0.1%)
|
20.8%
|
0 (0 (-527)
|
Subtotal*
|
1,923 (100%)
|
215 (100%)
|
11.2%
|
24.5 (12-97)
|
1,698,442 (100%)
|
1,158,574 (100%)
|
68.2%
|
7,299 (518-1,086,758)
|
Dry season
|
Ground pools
|
74 (4.8%)
|
15 (10.1%)
|
20.3%
|
0 (0-7)
|
646,363 (72.8%)
|
268,374 (89.1%)
|
41.5%
|
0 (0 (-247,540)
|
Human-made habitats
|
67 (4.4%)
|
6 (4.1%)
|
9.0%
|
0 (0-3)
|
15,680 (1.8%)
|
151 (0.1%)
|
1.0%
|
0 (0 (-126)
|
Rice fields
|
26 (1.7%)
|
2 (1.4%)
|
7.7%
|
0 (0-1)
|
87,926 (9.9%)
|
20,211 (6.7%)
|
23.0%
|
0 (0 (-16,324)
|
River/ stream segments
|
910 (59.6%)
|
100 (67.6%)
|
11.0%
|
11 (1-40)
|
135,407 (15.3%)
|
12,048 (4.0%)
|
8.9%
|
1,234 (118 (-3,865)
|
Springs/ wells
|
401 (26.2%)
|
12 (8.1%)
|
3.0%
|
1 (1-5)
|
1,350 (0.2%)
|
56 (0%)
|
4.2%
|
12 (0 (-30)
|
Other small habitats
|
50 (3.3%)
|
13 (8.8%)
|
26.0%
|
1 (0-4)
|
936 (0.1%)
|
408 (0.1%)
|
43.6%
|
33 (0 (-191)
|
Subtotal*
|
1,528 (100%)
|
148 (100%)
|
9.7%
|
12.5 (2-100)
|
887,662 (100%)
|
301,248 (100%)
|
33.9%
|
6,228 (56-268,374)
|
*Numbers and areas of habitats were calculated from a different number of villages in rainy (5 villages: Chikuti, Chirombola, Ebuyu, Kichangani, and Mzelezi) and dry (7 villages: Chirombola, Ebuyu, Iragua, Kichangani, Kidugalo, and Mwaya and Mzelezi) seasons.
Availability of aquatic habitats in the wet season versus the dry season
Of all the surveyed villages, four were visited in both wet and dry seasons, namely Chirombola, Ebuyu, Kichangani, and Mzelezi (Figure 2). In these four villages, habitats with late instar An. arabiensis and An. funestus comprised only a small subset of 9.1%-20.8% of all water bodies, in the rainy season; and these figures slightly decreased to 5.8% -19.2%, in the dry season. In terms of size, these habitats covered 68.7%-70.2% of the total habitat areas in the rainy season, reducing to 25.6%-8.1% in the dry season (Table 4, Fig. 3). The number of habitats declined by 32.8% between the rainy season and the dry season (1279 vs 860 habitats). This decrease was even more pronounced in terms of surface area, which was reduced by 78.5%. The reduction in both number and surface area varied across habitat types, ranging from 12.4% to 75.9% for the number and from 30.0% to 91.8% for the surface area. For habitats infested with the An. funestus larvae, there was a 38.0% decrease in the number of habitats and a substantial 92.0% reduction in the surface area of the infested habitats in the dry season compared to those found infested in the rainy season. Rice fields, puddles, and ground pools experienced the most significant decline, with both the number and surface area of infested habitats dropping by over 50% and 90%, respectively. While the number of human-made habitats infested by An. funestus decreased by 50% during the dry season, the surface area of these infested habitats increased by a factor of 1.3 compared to the rainy season.
An. arabiensis exhibited a similar pattern. The number of infested habitats declined by 57.3% in the dry season, with a corresponding decrease of 97.5% in surface area. Ground pools, human-made habitats, puddles, rice fields, and springs/ wells all showed reductions, ranging from 76.9% to 92.5% for habitat numbers. River and stream segments also showed a slight increase in the number of infested habitats (factor of 0.68) during the dry season. However, the reduction in surface area for these habitat types was more pronounced, exceeding 60% for ground pools, human-made habitats, puddles, and rice fields. Notably, the surface area of rivers and stream segments with An. arabiensis increased in the dry season compared to the rainy season.
Table 4. Comparison of the number and surface area of potential habitats and habitats infested by An. funestus and An. arabiensis in the four villages surveyed in both dry and rainy seasons. Percentage reductions in both number and area are included.
Habitats
|
Habitat category
|
Rainy season
|
Dry season
|
Reduction in number of habitats from rainy season to dry season
|
Reduction in surface area of habitats from rainy season to dry season
|
Total number of habitats, N
|
Total area of habitats found, m2
|
Total number of habitats, N
|
Total area of habitats found, m2
|
Late instars of An. funestus
|
Ground pools
|
16
|
1,075,814.3
|
7
|
62,405.9
|
56.2%
|
94.2%
|
Human-made habitats
|
19
|
5,964.3
|
9
|
13,942.8
|
52.6%
|
133.8%*
|
Rice fields
|
10
|
25,247.6
|
3
|
1,369.9
|
70%
|
94.6%
|
River/ stream segments
|
195
|
16,862
|
135
|
12,044.6
|
30.8%
|
28.6%
|
Springs/ wells
|
22
|
194.7
|
10
|
103
|
54.5%
|
47.1%
|
Other small habitats
|
4
|
66.6
|
1
|
0.7
|
75%
|
98.9%
|
Overall
|
266
|
1,124,149.5
|
165
|
89,866.9
|
38%
|
92%
|
Late instars of An. arabiensis
|
Ground pools
|
12
|
1,086,758.3
|
3
|
3,916.4
|
75%
|
99.6%
|
Human-made habitats
|
14
|
5,913.9
|
1
|
3.9
|
92.9%
|
99.9%
|
Rice fields
|
18
|
50,745
|
2
|
20,211.3
|
88.9%
|
60.2%
|
River/ stream segments
|
22
|
4,011.9
|
37
|
4,399.5
|
68.2%*
|
9.7%*
|
Springs/ wells
|
49
|
383.6
|
2
|
13.1
|
95.9%
|
96.6%
|
Other small habitats
|
2
|
358.7
|
5
|
82.1
|
150%*
|
77.1%
|
Overall
|
117
|
1,148,171.4
|
50
|
28,626.3
|
57.3%
|
97.5%
|
All potential habitat
|
Ground pools
|
44
|
1,158,902.1
|
35
|
202,464.9
|
20.5%
|
82.5%
|
Human-made habitats
|
110
|
24,792.9
|
54
|
15,422.3
|
50.9%
|
37.8%
|
Rice fields
|
79
|
350,810.5
|
19
|
68,495
|
75.9%
|
80.5%
|
River/ stream segments
|
813
|
91,851.8
|
554
|
64,257.4
|
31.9%
|
30%
|
Springs/ wells
|
201
|
8,595.8
|
176
|
706.8
|
12.4%
|
91.8%
|
Other small habitats
|
32
|
558.6
|
22
|
360.3
|
31.2%
|
35.5%
|
Overall
|
1279
|
1,635,511.7
|
860
|
351,706.7
|
32.8%
|
78.5%
|
+ Indicates an increase in the proportion of the number or surface area in the dry season compared to the rainy season.
Cohabitation of aquatic habitats by An. funestus and An. arabiensis
Among all infested habitats across all seasons, we observed only a limited degree of habitats being co-infested by late instars of both An. funestus and An. arabiensis. Overall, only 8.4% of the habitats were infested with both the malaria vector species. This proportion varied from 6% (n=43) in the rainy season to 12% (n=57) in the dry season.
Characteristics of aquatic habitats inhabited by the two main malaria vectors
Multivariate logistic regression analysis revealed distinct aquatic habitat characteristics for An. funestus and An. arabiensis across seasons (Table 5). In both rainy and dry seasons, An. funestus exhibited a strong preference for the larger habitats (> 100m²) characterized by clear water, the presence of floating or emergent vegetations, and the presence of green algae compared to smaller (< 100m²), opaque habitats lacking these features (p < 0.05). An. arabiensis, on the other hand, displayed contrasting seasonal preferences. During the rainy season, clear water was favored compared to opaque habitat, while the presence of shade and flowing water significantly reduced the likelihood of An. arabiensis presence (p < 0.05) compared to unshaded stagnant water bodies. In the dry season, An. arabiensis habitats tended to be found in larger habitats with clear water, floating or emergent vegetation, and green algae (p < 0.05) (Table 5).
Table 5. Characteristics of different aquatic habitat types found with late instars of An. funestus and An. arabiensis in the rainy and dry season.
Season
|
Habitat characteristics
|
All water bodies n (%)
|
No. habitats with late instars of An. funestus (%)
|
No. habitats with late instars of An. arabiensis (%)
|
An. funestus
|
An. arabiensis
|
Odds ratio (95%CI)
|
p-value
|
Odds ratio (95%CI)
|
p-value
|
Rainy season
|
Water movement
|
Stagnant
|
807 (42%)
|
102 (32.4%)
|
170 (79.1%)
|
1
|
|
1
|
|
Flowing
|
1113 (58%)
|
213 (67.6%)
|
45 (20.9%)
|
1.1(0.8-1.4)
|
0.70
|
0.2 (0.1-0.2)
|
<0.01
|
Presence of emergent or floating vegetations
|
Absent
|
1182 (61.6%)
|
166 (52.7%)
|
110 (51.2%)
|
1
|
|
1
|
|
Present
|
738 (38.4%)
|
149 (47.3%)
|
105 (48.8%)
|
1.4 (1.1-1.8)
|
0.01
|
1.3 (1.0-1.8)
|
0.06
|
Water clarity
|
Opaque
|
379 (19.7%)
|
30 (9.5%)
|
49 (22.8%)
|
1
|
|
1
|
|
Clear
|
1541 (80.3%)
|
285 (90.5%)
|
166 (77.2%)
|
2.4 (1.6-3.7)
|
<0.01
|
1.8 (1.2-2.6)
|
<0.01
|
Presence of green algae
|
Absent
|
1738 (90.5%)
|
251 (79.7%)
|
188 (87.4%)
|
1
|
|
1
|
|
Present
|
182 (9.5%)
|
64 (20.3%)
|
27 (12.6%)
|
2.8 (2.0-4)
|
<0.01
|
1.5 (0.9-2.4)
|
0.09
|
Habitat shading
|
None
|
1217 (63.4%)
|
202 (64.1%)
|
176 (81.9%)
|
1
|
|
1
|
|
Shaded
|
703 (36.6%)
|
113 (35.9%)
|
39 (18.1%)
|
0.9 (0.7-1.2)
|
0.69
|
0.4 (0.3-0.6)
|
<0.01
|
Distance to nearest household
|
Greater than 100m
|
874 (45.5%)
|
157 (49.8%)
|
68 (31.6%)
|
1
|
|
1
|
|
Less than 100m
|
1046 (54.5%)
|
158 (50.2%)
|
147 (68.4%)
|
0.9 (0.7-1.1)
|
0.28
|
1.3 (0.9-1.7)
|
0.17
|
Habitat size
|
Less than 100m2
|
307 (16%)
|
24 (7.6%)
|
67 (31.2%)
|
0.9 (0.7-1.1)
|
0.28
|
1
|
|
Greater than 100m2
|
1613 (84%)
|
291 (92.4%)
|
148 (68.8%)
|
2.1 (1.3-3.4)
|
<0.01
|
NI
|
-
|
Dry season
|
Water movement
|
Stagnant
|
758 (49.6%)
|
85 (36.6%)
|
65 (43.9%)
|
NI
|
-
|
1
|
|
Flowing
|
769 (50.4%)
|
147 (63.4%)
|
83 (56.1%)
|
NI
|
-
|
1.0 (0.7-1.5)
|
0.97
|
Presence of emergent or floating vegetations
|
Absent
|
1150 (75.3%)
|
102 (44%)
|
84 (56.8%)
|
1
|
|
1
|
|
Present
|
377 (24.7%)
|
130 (56%)
|
64 (43.2%)
|
3.3 (2.4-4.5)
|
<0.01
|
2.0 (1.4-3.0)
|
<0.01
|
Water clarity
|
Opaque
|
573 (37.5%)
|
51 (22%)
|
66 (44.6%)
|
1
|
|
1
|
|
Clear
|
954 (62.5%)
|
181 (78%)
|
82 (55.4%)
|
2.3 (1.6-3.2)
|
<0.01
|
0.7 (0.5-1.0)
|
0.03
|
Presence of green algae
|
Absent
|
1381 (90.4%)
|
187 (80.6%)
|
119 (80.4%)
|
1
|
|
1
|
|
Present
|
146 (9.6%)
|
45 (19.4%)
|
29 (19.6%)
|
1.9 (1.2-2.9)
|
<0.01
|
2.0 (1.3-3.3)
|
<0.01
|
Habitat shading
|
None
|
1014 (66.4%)
|
103 (44.4%)
|
90 (60.8%)
|
1
|
|
1
|
|
Shaded
|
513 (33.6%)
|
129 (55.6%)
|
58 (39.2%)
|
2.3 (1.7-3.1)
|
<0.01
|
1.1 (0.8-1.6)
|
0.56
|
Distance to nearest household
|
Greater than 100m
|
685 (44.9%)
|
91 (39.2%)
|
64 (43.2%)
|
1
|
|
1
|
|
Less than 100m
|
842 (55.1%)
|
141 (60.8%)
|
84 (56.8%)
|
1.5 (1.1-2.1)
|
<0.01
|
1.2 (0.8-1.7)
|
0.32
|
Habitat size
|
Less than 100m2
|
281 (18.4%)
|
5 (2.2%)
|
10 (6.8%)
|
1
|
|
1
|
|
Greater than 100m2
|
1246 (81.6%)
|
227 (97.8%)
|
138 (93.2%)
|
8.8 (3.5-21.9)
|
<0.01
|
2.6 (1.3-5.2)
|
<0.01
|
NI - Variable not included in the final model