This study was conducted to assess the current P. falciparum genotypic structure in the semi-arid area in North East Ethiopia, using the highly polymorphic (block 3) region of the msp2 gene as a molecular marker. The msp2 marker is recommended for genotyping P. falciparum parasite populations compared with msp-1 and glurp [29]. P. falciparum isolates from this region were mainly monoclonal with a low MOI and limited genetic diversity. These findings are important for ongoing evaluation of the effect of malaria control strategies, as Ethiopia moves towards malaria elimination.
The 3D7/IC allelic family of msp2 was more prevalent than the FC27 allelic family. This is in agreement with previous reports from Burkina Faso [30], South West Ethiopia [22] and Sudan [31]. However, this finding differs to results from North West Ethiopia [20], and Central Sudan [32], where FC27 was the more prevalent allelic family. These differences could relate to the semi-arid geographic setting and low transmission intensity compared to the hot and humid climate in North West Ethiopia.
Limited genetic diversity of P. falciparum was observed in this study. Similar results have been reported in other areas with low P. falciparum transmission [33] and in regions with declining transmission related to malaria control efforts [34]. In contrast, a high level of genetic diversity was reported in high endemicity settings in Cameron [14] and Burkina Faso [30].
The current study found that the P. falciparum parasite population in Meleka-Werer exhibited a low heterozygosity (He = 0.5), consistent with that reported in Mubuga Rwand (He = 0.49) [35]. In areas with declining local transmission, it is expected that lower parasite diversity (heterozygosity) will be present [36]. Declining diversity and transmission have been associated with improved malaria control interventions [37, 38].
The overall mean MOI reported in this study was low (MOI = 1.2). This is in agreement to previous studies where low malaria transmission settings are commonly associated with lower MOIs [39–40]. Moreover, the low MOI observed in this study may reflects most positive samples being from adult patients. This is in agreement with previous findings that there was a higher MOI observed in adults compared with younger children [41]. In contrast, a study Cameroon reported a higher MOI in adults [16]. This difference could be due to the genotyping methods and interpretation of result, and the heterogeneity of the study populations may suggest the variability of results between studies.
The majority of participants in the current study were older than 10 years, similarly observed in area with a lower intensity of malaria transmission [30] and contrasting to reports from high transmission settings [42]. However, it is also possible that the age-related malaria risk may have been influenced by implementation of effective malaria control interventions, such as the widespread distribution of long-lasting insecticide treated nets (LLINs) and indoor residual spray (IRS), and sustained treatment of malaria patients with artemisinin-combination therapy (ACT). This supported by the 2015 MIS which found that the Afar region had the highest percentage use LLINs compared to other regions of the country [5].
Age is considered an important factor for involving in the acquisition of immunity against P. falciparum and may have also an effect on MOI [43]. Although, the influence of age on the MOI is highly affected by malaria transmission intensity [44]. Previous studies have shown an association between age and MOI in areas with intense perennial malaria transmission or hypo-meso-endemic malaria transmission [45, 46]. However, the current study found no association between age and MOI. Similar findings have been reported in other countries [47, 48].
The higher geometric mean parasite density in individuals with previous exposure to malaria attack was found. In this low endemicity setting, a lower proportion of individuals will have likely had prior immunity, meaning that infected patients will be more likely to become symptomatic at a lower parasitemia than in high endemicity settings.
It was difficult to correlate transmission levels with genetic diversity and MOI in the current study due to a lack of entomologic inoculation rate (EIR) data from the study area. However, the genetic diversity and the MOI reported in the present study supported a low average microscopy positive rate (8.5%) (unpublished Meleka-Werer rural town health office data, 2015). A limitation of this study was the small sample size, in part due to the nomadic nature of the local communities. Furthermore, due to resource restrictions, we used a lower discriminatory power agarose gel electrophoresis compared to capillary electrophoresis potentially reducing the number of alleles and parasite population that could be detected [49].