In this study 34,548 individuals from 6,071 households were followed. Half (50.2%) of the participants were male, and in the age group of 15 years and above (50%). Fifty one percent of the participants were illiterate, approximately 33.2% were poor, 69% of them had more than 5 household size, around 26 % of the households received both LLIN and IRS, and median (Interquartile range) of household distance from breeding site was 1.66 (0.70–2.95) km (Table 1).
Table 1
Socio-demographic characteristics of the participant south-central Ethiopia, October 2014 to January 2017
Variables (n = 34,548)
|
Number
|
Percent
|
Sex
|
|
|
Male
|
17,327
|
50.2
|
Female
|
17,221
|
49.8
|
Age group in years
|
|
|
< 5
|
6,488
|
18.8
|
5–14
|
11,136
|
32.2
|
≥ 15
|
16,924
|
49.0
|
Educational status of household head
|
|
|
Illiterate
|
19,752
|
51.2
|
Read and write
|
3,738
|
10.8
|
Primary education complete
|
8,155
|
23.6
|
Secondary and above
|
2,903
|
8.4
|
Wealth Status
|
|
|
Poor
|
11,459
|
33.2
|
Middle
|
11,578
|
33.5
|
Rich
|
11,511
|
33.3
|
Family size
|
|
|
≤ 5
|
10,664
|
30.9
|
> 5
|
23,884
|
69.1
|
Intervention arms
|
|
|
IRR + LLIN
|
9,104
|
26.4
|
LLIN alone
|
8,038
|
23.3
|
IRS alone
|
8,567
|
24.8
|
Routine
|
8,839
|
25.5
|
Median (IQR) distance from mosquito breeding site
|
1.66 (0.70–2.95) km
|
IQR: Interquartile Range; IRS: Indoor Residual Spraying; LLINs: Long Lasting Insecticidal Nets;
Incidence of P. vivax and P. falciparum
Overall, 531 episodes of P. vivax including mixed infection in 487 participants were observed during the follow up period. Out of the 487 participants, 450 developed one episode, 29 developed 2 episodes, six experienced 3 episodes and one experienced 5 episodes of P.vivax. On the other hand, 884 P. falciparum including mixed infection episodes were occurred during the whole study period. The total number of P. falciparum episode experienced by the individuals was one episode (753 individuals), two episodes (56 individuals), three episodes (6 individuals) and four episodes (1 individual). As shown in Fig. 2, malaria occurs in the study area throughout the year with varying intensity dominated by P. falciaprum. The incidence rate of P. falciaprum and P. vivax was higher during the major malaria transmission seasons (September – December).
The overall incidence rate of P. vivax was 7.4 episodes per 1,000 person-years of observation. Plasmodium vivax incidence was high among children under 5 years old (9.6 per 1000 person-years), less educated (read and write) household heads (11.4 per 1,000 person-years), house roof made of thatch or leaf (7.9 per 1,000 person-years) and living within 1 km from mosquito breeding site (10.8 per 1,000 person-years). Whereas, the overall incidence rate of P. falciparum was 12.3 episodes per 1,000 person-years of observation. The incidence of P. falciparum was high among children under 5 years old (15.9 per 1,000 person-years), less educated (read and write) household heads (10.88 per 1,000 person-years), living within 1 km from mosquito breeding site (17.00 per 1,000 person-years) and house roof made of thatch or leaf (14.22 per 1,000 person-years) (Table 2).
Table 2
Incidence rate and Incidence rate ratio (IRR) of P. vivax and P. falciparum, south-central Ethiopia, October2014 to January 2017
Variables
(n = 34,548)
|
Person-years
|
P. vivax
|
P. falciparum
|
No. of episode
|
IR/1000 PY
|
IRR
|
No. of episode
|
IR/1000 PY
|
IRR
|
All
|
71,859.1
|
531
|
7.39
|
|
884
|
12.30
|
|
Sex
|
|
|
|
|
|
|
|
Male
|
36,180.4
|
259
|
7.16
|
1.00
|
445
|
12.30
|
1.00
|
Female
|
35,678.7
|
272
|
7.62
|
1.06
|
439
|
12.30
|
1.00
|
Age group in years
|
|
|
|
|
|
|
< 5
|
12,739.8
|
122
|
9.58
|
1.37
|
203
|
15.93
|
1.12
|
5–14
|
23,727.3
|
184
|
7.75
|
1.11
|
275
|
11.59
|
0.81
|
15–24
|
13,999.7
|
76
|
5.43
|
0.78
|
101
|
7.21
|
0.51
|
≥ 25
|
21,392.3
|
149
|
6.97
|
1.00
|
305
|
14.26
|
1.00
|
Educational status of household head
|
|
|
|
|
|
Illiterate
|
41,303.9
|
288
|
6.97
|
1.03
|
457
|
11.06
|
0.99
|
Read and write
|
7,712.3
|
88
|
11.41
|
1.69
|
128
|
16.60
|
1.49
|
Primary education
|
16,918.3
|
115
|
6.80
|
1.01
|
233
|
13.77
|
1.24
|
Secondary and above
|
5,924.7
|
40
|
6.75
|
1.00
|
66
|
11.14
|
1.00
|
Wealth Status
|
|
|
|
|
|
|
|
Poor
|
23,837.3
|
171
|
7.17
|
1.05
|
301
|
12.63
|
1.10
|
Middle
|
23,994.1
|
196
|
8.17
|
1.20
|
308
|
12.84
|
1.12
|
Rich
|
24,027.7
|
164
|
6.83
|
1.00
|
275
|
11.45
|
1.00
|
Family size
|
|
|
|
|
|
|
|
≤ 5
|
22,036.5
|
164
|
7.44
|
1.01
|
280
|
12.71
|
1.05
|
> 5
|
49,822.6
|
367
|
7.37
|
1.00
|
604
|
12.12
|
1.00
|
Intervention arms
|
|
|
|
|
|
|
IRR + LLIN
|
18,712.6
|
143
|
7.64
|
0.97
|
236
|
12.61
|
1.00
|
LLIN alone
|
17,243.0
|
105
|
6.09
|
0.78
|
209
|
12.12
|
0.96
|
IRS alone
|
17,152.1
|
136
|
7.93
|
1.01
|
222
|
12.94
|
1.03
|
Routine
|
18,751.4
|
147
|
7.84
|
1.00
|
217
|
11.57
|
0.92
|
House roofing
|
|
|
|
|
|
|
|
Thatch/grass
|
33,481.50
|
266
|
7.94
|
1.15
|
476
|
14.22
|
1.12
|
Corrugated iron
|
38,377.60
|
265
|
6.91
|
1.00
|
486
|
12.66
|
1.00
|
Distance from breeding
|
|
|
|
|
|
|
<=1km
|
22,741
|
245
|
10.77
|
1.85
|
386
|
16.97
|
1.48
|
> 1km
|
49,118
|
286
|
5.82
|
1.00
|
565
|
11.50
|
1.00
|
‡PY = Person Years; ±IR = Incidence Rate; #IRR = Incidence Rate Ratio;€IRS = Indoor Residual Spraying,¥LLINs = Long Lasing Insecticidal Nets; $km = Kilo Meter
Risk factors of P. vivax and P. falciparum episodes
Multivariable generalized Poisson log–linear model analys is was fitted to measure the risk factors for P. vivax and P. falciparum. The result has shown that the same variables, distance of household from mosquito breeding site, age groups, educational status of household head and roof of housing were the risk factors for both P.vivax and P. falciparum. In this study, after controlling for potential confounding factors we observed that households closer to mosquito breeding place were more at risk of P.vivax infection (IRR: 1.33; 95% CI = 1.23 − 1.45). Furthermore, age group under 5 years (IRR: 1.40, 95% CI = 1.10 − 1.79), age group 5–14 years (IRR: 1.27, 95% CI = 1.03 − 1.57) and less educated (read and write) household head (IRR: 1.63, 95% CI = 1.10 − 2.44) were the predictors of P. vivax. The study also showed that peoples living in a house roof made of thatch/leaf were more at risk of experiencing P. vivax (IRR: 1.35, 95% CI = 1.11–1.65) than corrugated iron roof housing (Table 3 as additional file 1). Unlike P. vivax, age group 5–14 years was not a risk factor for P. falciparum.