Demographic and Parasitological characteristics of the study participants
In this study, a total of 4881 schoolchildren (age 5–15 years) were screened using RDT and the parasite status confirmed by microscopy. The total number of the female and male participants were 2459 and 2422, respectively. Regarding the parasitological characteristics of the study participants, significant differences were observed among the male and female gender with higher P. falciparum prevalence among the male gender [Male: 32% (764/2422); Female: 27% (657/2457); p-value < 0.001]. There was no statistically significant difference in P. falciparum parasite carriage between the age groups [5–9 yrs.: 28% (712/2545); 10–15 yrs.: 30.4% (709/2336); p-value = 0.068]. The total number of mixed infections (P. falciparum plus P. ovale and/or P. malariae) detected in the study population was 204 with a non-significant difference between the age groups 5–9 years 15.73% (112/712) and 10–15 years 12.98% (92/709); (p-value = 0.139), while the single infections were 1217. Most of the mixed infections were found among the female gender 16.74% (110/204) in comparison to the males 12.30% (94/764); (p-value = 0.017) (Table 1 and Additional file 1).
Table 1
Parasitological characteristics of the study participants.
Variables
|
Age group (years)
|
Gender
|
5–9
|
10–15
|
Female
|
Male
|
Positive
|
28% (712/2545)
|
30.4% (709/2336)
|
27% (657/2459)
|
32% (764/2422)
|
Negative
|
72% (1833/2545)
|
69.6% (1627/2336)
|
73% (1802/2459)
|
68% (1658/2422)
|
χ2 (p-value)
|
3.328 (0.068)
|
13.770 (< 0.001)*
|
Mixed Infection
|
15.73% (112/712)
|
12.98% (92/709)
|
16.74% (110/657)
|
12.30% (94/764)
|
Single Infection
|
84.27% (600/712)
|
87.02% (617/709)
|
83.26% (547/657)
|
87.70% (670/764)
|
χ2 (p-value)
|
2.192 (0.139)
|
5.661 (0.017)*
|
Negative
|
72% (1833/2545)
|
69.6% (1627/2336)
|
73.3% (1802/2459
|
68.5% (1658/2422)
|
Asexual
|
25.7% (655/2545)
|
29.2% (682/2336)
|
25.4% (625/2459)
|
29.4% (712/2422)
|
Gametocyte
|
2.2% (57/2545)
|
1.2% (27/2336)
|
1.3% (32/2459)
|
2.1% (52/2422)
|
χ2 (p-value)
|
14.602 (0.001)*
|
16.137 (< 0.001)*
|
Asexual
|
91.99% (655/712)
|
96.19% (682/709)
|
95.13% (625/657)
|
93.19% (712/764)
|
Gametocyte
|
8% (57/712)
|
3.81% (27/709)
|
4.87% (32/657)
|
6.80% (52/764)
|
χ2 (p-value)
|
11.253 (0.001)*
|
2.380 (0.123)
|
Population gametocyte prevalence 2% (84/4881)
|
Gametocyte prevalence (P. falciparum positives) 6% (84/1421)
|
Population gametocyte prevalence is the percentage of gametocyte carriers among the total study population (P. falciparum positive and negative samples together) while the gametocyte prevalence among the P. falciparum positive samples is the percentage of gametocyte carriers among the P. falciparum positive samples only (excluding P. falciparum negatives). χ2 = Pearson's chi-squared test and (*) indicates statistical significance.
The population P. falciparum prevalence in this study calculated as the percentage of P. falciparum infections within the study sample was 29.11% (1421/4881). The level of P. falciparum carriage varies among study sites (range: 0-65.2%, p-value < 0.001) and within sampling periods (range: 11-78.4%, p-value < 0.001) (Figs. 1 and 2).
Prevalence of gametocyte carriage and associated risk factors in the study population
In Table 1, the total number of gametocyte carriers was 84 with 57 of the carriers found within the age group 5–9 years as compared to 27 in the age-group 10–15 years (p-value = 0.001). There was 2% (84/4881) population gametocyte prevalence in the study population. While the prevalence of gametocyte carriage among the P. falciparum malaria carriers (only P. falciparum positive individuals) was found to be 6% (84/1421). These represent a minimum gametocyte prevalence level since the sensitivity of microscopy is relatively low. There was significant difference in the population gametocyte carriage between the female 1.3% (32/2459) and male gender 2.1% (52/2422) (p-value < 0.001). The gametocyte prevalence was relatively stable with no statistically significant difference throughout the sampling periods (Fig. 2).
The analysis showed that risk of P. falciparum infection was highest among the male gender as compared to the females (OR = 0.8 [95% CI = 0.7–0.9], P < 0.001) while the age of an individual was not an independent risk factor. However, children between the ages of 5–9 years have a higher risk of gametocyte carriage when infected with P. falciparum as compared to those between the ages 10–15 years (OR = 2.1 [95% CI = 1.3–3.4], P = 0.002) (Table 2).
Table 2
Risk factors of P. falciparum infection and gametocyte carriage.
|
P. falciparum parasite carriage
|
Gametocyte carriage
|
Variable
|
OR
|
95% CI
|
P-value
|
OR
|
95% CI
|
P-value
|
Age (Years)
|
|
|
|
|
|
|
5–9
|
0.9
|
0.80, 1.02
|
0.101
|
2.14
|
1.34, 3.43
|
0.002*
|
10–15
|
1
|
|
|
1
|
|
|
Gender
|
|
|
|
|
|
|
Female
|
0.79
|
0.70, 0.90
|
< 0.001*
|
0.71
|
0.45, 1.12
|
0.136
|
Male
|
1
|
|
|
1
|
|
|
Risk factors analysis of P. falciparum infection and gametocyte carriage among the study population using binary logistic regression model. OR, odds ratio; CI, confidence interval. (*) indicates statistical significance.
Relationship between gametocyte density and multiplicity of Plasmodium falciparum infections (MOI) and Mosquito infection prevalence
The total number of samples used in assessing the relationship between gametocyte density, MOI and mosquito infection prevalence was 37. However, 15 of the 37 samples failed to amplify during the microsatellite amplification PCR and are recorded as missing data. The mean mosquito infection rate was 12.71 (SE: 2.63, SD: 16.1) and mean gametocyte density was 59.89 (SE: 12.28, SD: 74.71), respectively while the mean number of distinct alleles per isolate was 7.30 (SE: 0.80, SD: 3.76) (See Additional file 2). In this study, a significant positive correlation was found between P. falciparum gametocyte densities in the patient blood samples and mosquito infection prevalence (0.682, p-value < 0.0001). In addition, a positive correlation between multiplicity of P. falciparum infection (MOI) and mosquito infection prevalence was reported (0.451, p-value = 0.035). However, the correlation between MOI and gametocyte density was not statistically significant (0.167, p-value = 0.459). The mosquito infection prevalence is defined as the percentage of infected mosquitoes after day 10th of the membrane-feeding assay (Table 3 and Fig. 3).
Table 3
Multiple correlation analysis of gametocyte density and multiplicity of P. falciparum infection with the infection prevalence in the mosquitoes.
Parameters
|
Infection rate (P-value)
|
Gametocyte density (P-value)
|
MOI (P-value)
|
Infection prevalence
|
1 (Ref)
|
0.682 (< 0.0001)*
|
0.451 (0.035)*
|
Gametocyte density
|
0.682 (< 0.0001)*
|
1 (Ref)
|
0.167 (0.459)
|
MOI
|
0.451 (0.035)*
|
0.167 (0.459)
|
1 (Ref)
|
The dependent variable in this analysis is the infection prevalence. Ref represents the reference (*) denotes statistical significance.
In Table 4, a multiple regression was run to predict mosquito infection prevalence from gametocyte density (gametocyte/µl) and multiplicity of infection (MOI). These variables statistically significantly predicted mosquito infection prevalence, F(2, 19) = 20.235, p < 0.0001, R2 = 0.681. All the two variables added statistically significantly to the prediction, p < 0.05.
Table 4
Parameter of multiple linear Regressions Analysis.
Parameters
|
Coefficients
|
Std. Error
|
t-statistic
|
P-value
|
(Constant)
|
-6.644
|
5.564
|
-1.194
|
0.247
|
Gametocyte density
|
0.151
|
0.028
|
5.328
|
< 0.001
|
MOI
|
1.707
|
0.672
|
2.54
|
0.020
|
R = 0.825, R-SQR. = 0.681, Adj. R-SQR = 0.647, SE = 11.418. R is the multiple correlation coefficient, R-SQR. (R-square) is the multiple coefficient of determination, Adj. R-SQR represents the adjusted R-square, and SE is the standard error.
The multiple coefficient of determination (R-SQR. = 0.681) indicated that, about 68.1% of the variation in mosquito infection prevalence is accounted for by the gametocytes density and MOI. Thus, the formulated equation for mosquito infection prevalence in this study is:
Where is the expected Mosquito Infection prevalence, X1 and X2 are the Gametocyte density and MOI, respectively.