Ethiopia has now moved forward in targeting nationwide malaria elimination program by 2030. For effective implementation of this strategic target, one of the key intervention strategy is improving malaria surveillance and response [24]. In this regards, molecular epidemiological study approach like characterization of block 3 region of P. falciparum msp-2 provides comprehensive molecular evidence for effective disease surveillance that ultimately transformed to core interventions to the control and elimination of malaria.
The present study revealed that consistence with our previous report [25] and that of [15], incidence of P. falciparum isolates was higher in male (71%) individuals (Table 1). The major factors that may account for such higher malaria cases compared to female is that older boys and men may be at special risk for malaria from occupational and travel-related factors [24]. In addition, consistent with the report of [26] incidence of P. falciparum isolates in the present study was significantly related to occupation type (X2 = 0.017) (Table 1). This could be due to strong relation of malaria incidence with lower standard of living. This might have contributed for the occurrence of 76% of all P. falciparum isolates in the present study only from farmers, daily laborers, and students alone.
Investigation of msp-2 block-3 region of P. falciparum genetic profile revealed in this study was the first in its kind in our study area. Moreover, we examined the seasonal and spatial distribution of msp-2 allelic variants in selected sites within the designated study area, together with their urban rural counterpart. Of the total successfully genotyped msp-2 gene the monoclonal alleles of IC/3D7 and FC27 constitute 31.8% and 27.7% respectively (Table 2). Report from maritime region of Togo [27] and Ponte-Noire, Republic of Congo) [28] complement our finding. Moreover, the number of msp-2 genotypes detected for IC/3D7 and FC27 was 7 and 10 respectively (Table 2). Although the number of genotypes might have be underestimated due to the limitation of the techniques. Fragment size polymorphism described in this study is nearly comparable with the previous report from Republic of Congo [28], Nigeria [29], Sudan [17], and northeastern Ethiopia [30]. Other reports from Congo Brazzaville [31], north western Ethiopia [32] shown the predominance of IC/3D7 allelic family. Such inconsistency in P. falciparum allelic size polymorphism could be due to geographical location, transmission intensity and scope of sample population covered in the study. The rate of msp-2 polyclonal infection identified was 40.5%, with the overall MOI of 1.4 (Table 2). This finding is lower than the previous report from southwestern Ethiopia [33] northwesten Ethiopia [32], Sudan [17], Cameroon [34], and Nigeria [35]. And somewhat higher than the previous report from north eastern Ethiopia [30], and Ghana [36]. The variation in multi-clonal infection and multiplicity of infection could be due to the overall prevalence of infection in the population and the age of the individual [37]. The overall MOI identified in our study area could serve as proxy of transmission intensity for targeted intervention in the region. Moreover, in the present study we investigated the expected heterozygosity (He) of 0.49 (Table 2) that nearly indicates an intermediate transmission pattern in the study area.
In this study, analysis of the variation of msp-2 allelic frequency across different age groups generally tends to rose up with age group in parallel with P. falciparum clinical prevalence during the study period. However, the variation was not statistically significant (P = 0.09) (Table 3). The general increment observed in this study is in agreement with the previous five year retrospective study reported in Adama, where the clinical cases of P. falciparum rose up with age group [16]. In the present study, we found that MOI tends to increase with age groups until the age of late adolescent (Table 3). This finding differ from the report from hyper endemic area of Burkina Faso [38], where they reported the existence of negative relationship between MOI with patient age. On the other hand, consistent with our finding other report from Burkina Faso [37], and Tanzania [39], explained that episode of infection in children is commonly for very short duration and the duration of episode of infection increases with age contributing to the rise in MOI in other age groups. When the correlation of msp-2 allelic variant and parasitemia level was examined, we found no statistically significant correlation (Pearson correlation = -0.19, X2 = 0.07) existed between msp-2 allelic variants with parasitemia level in different age groups (Table 3). This could be due to the reduced transmission intensity of P. falciparum infection in the study area. Even though different factors may contribute to the fluctuation of parasitaemia level in symptomatic patients over time, the highest geometric mean of microscopically detected parasitemia level was in school age children (5–14 years old) (Table 3). This was due to delayed acquisition of protective immunity of this age groups [40].
Unlike the report from Burkina Faso, where there was a difference in some allelic family observed in rural and urban settings probably due to urbanization [37], in the present study, we observed no statistically significant variation in both rural and urban localities (P = 0.56). This indicates the existence of similar malaria epidemiology in both rural and urban settings of the study area. Slightly higher MOI detected school age children (Fig. 2) could be due to higher (70%) malaria case due to P. falciparum of this age group from urban area. Moreover, study site based distribution of msp-2 allelic variant (Fig. 3) showed a highly significant variation (P = 0.000). This could be largely due to the differences in local micro-ecological factors that may result in varying mosquito population density, change in parasite vector interaction, change in host immunity induced by parasite interaction, and spatial heterogeneity of the study sites under consideration [41, 42]. These major factors might have affected local transmission pattern contributing to such variation. In Ethiopia there are two malaria season. The major season follow the rainy summer season from June to August that begins from September to December. The minor malaria season follow the shorter rainy season around April and May (FMOH, 2015). Analysis of intra seasonal variation on the distribution of msp-2 allelic variant (Fig. 4) showed relatively higher variation during major malaria season, although the variation is not statistically significant (p = 0.9). In complement with this finding [37] reported that the dominance of any msp-2 allele was dependent on transmission intensity and independent of seasonal change.