This community-based study assessed malaria infection among adults residing in the health district of Lukelenge which locates in the DRC, one of the most malaria-endemic countries in the world [4]. Therefore, the prevalence of malaria was estimated 60.2% [95%CI: 55.5; 64.8], comprising mainly sub-clinical infections. Recent reports have showed varying prevalence estimates of malaria parasite at community level (ranging from 1.9 to 63.7%) across the DRC [23–27]. Differences in prevalence estimates may result from variations in studied populations (e.g., studied age categories), local variations of malaria epidemiology (e.g., endemicity level), evolution dynamics overtime (e.g., seasonality), applied diagnostic method (e.g., microscopy, RT-PCR, PCR), approaches for surveying (e.g., community-based or hospital-based surveys), and local implementation of control measures (e.g., use of LLINs or antimalarial drugs). Overall, these estimates suggest that asymptomatic parasite carriers persist broadly through the Congolese population and may be an important latent reservoir for human-to-mosquito malaria transmission at community level [28–32]. Therefore, any prospect of controlling and eliminating malaria in the region could only be considered by taking into account these latent infections at the community level. Beyond the need for population-based public health actions, clinical measures are required at an individual level with regard to the serious medical risks that subjects with asymptomatic malaria incur (e.g., anemia, adverse pregnancy outcome, invasive systemic bacterial diseases, impaired cognitive functions, reduced school performance, Burkitt's lymphoma, etc.) [29, 33]. This underscores the need to design new strategies and allocate adequate resources to specifically address asymptomatic malaria in communities in the DRC, a country where current treatment policies are still primarily focused on actively detected in-hospital symptomatic malaria. Obviously, controlling asymptomatic malaria carriage in such an endemic region would raise significant operational challenges of a financial, pragmatic and political nature; but it is probably a necessary condition for the NMCP to make substantial progress towards malaria control and elimination. Among public health measures, indoor insecticide spraying could potentially reduce asymptomatic malaria carriage in the study population. However, caution is required given the very limited number of insecticide users on the study site (7 out of 420 participants) probably reflecting the lack of a large-scale insecticide application program formally established by the NMCP in the DRC. The local susceptibility of mosquitoes to insecticide products and percepts for its monitoring over the time should also be considered to appropriately recommend this strategy [34]. Furthermore, although based on subjects aged 18 years and above, this analysis showed that younger individuals should be prioritized in public health measures to effectively control asymptomatic malaria in the study population. This is in broad agreement with observations from other countries in sub-Saharan Africa which have demonstrated that asymptomatic carriage of the parasite is highest at young ages [28, 32]. Indeed, this age-dependent relative protection is known to result from the immunity acquired over time with repeated exposures to malaria [9–11]. Nevertheless, several other factors (e.g., intensity of parasite exposure, blood parasite densities, drug use, and implemented preventive measures) not explored in this study are probably crucial to asymptomatic malaria carriage [29]. Additional observations combining the age distribution with malaria acquisition are therefore warrant to provide further insights in this direction. Otherwise, this work highlights also the need for health policies to target also non-falciparum species. In fact, while, in agreement with previous reports [23–27], P. falciparum was still the most widespread species, non-falciparum species, namely P. malariae and P. ovale, were involved as single or mixed species infections in nearly half of infected cases. This parasite composition profile tends to deviate from traditional reports from the DRC that show mainly P. falciparum infections and very limited infections with mixed species, but almost no infections with single non-falciparum species [23, 25, 27, 35, 36]. Likewise, non-falciparum malaria infections have been increasingly reported in several African regions originally infected with P. falciparum species [37–39]. The absence of P. vivax at the current study site is consistent with the general trend in Africa and can be explained by a mechanism widely discussed in the literature that relates to a defect of the Duffy receptor on erythrocytes of Africans and their descendants [40]. However, emerging reports demonstrate that P. vivax is diffusely spread across the DRC [25, 36, 41] which is also understandable given growing evidences for alternative mechanisms that would enable P. vivax to infect Africans [42]. Health policies that are more informed on non-falciparum species are thus urgently required, especially since these parasites may endear substantial clinical and epidemiological implications for malaria control and management (e.g., change in intrinsic drug susceptibility patterns, falsely negative RDT outcomes, propensity for liver-stage hypnozoites and clinical relapses) [39]. Typically, more potent testing tools that can also cover non-falciparum malaria should be considered as HRP2-based RDTs in force locally are basically specific to P. falciparum and have now showed low diagnostic performances for detecting malaria parasite carriers. In this regard, molecular approaches similar to those used in this research could be recommended [13, 14].
This study comprised several limitations. Firstly, the community-based survey captured only apparently adults from a specific location within the entire country. Studies conducted in Eastern African countries had found that school-aged children, which were not part of the current study, develop more asymptomatic malaria than adults [28]. Generalizing current outcomes would thus need to be treated with cautions. Second, despite the potential usefulness of the molecular tests used, misdiagnosis cannot be completely ruled out. For instance, beside possible floating HRP-2 antigens arising from past infections [43], some of the RDT-positive but PCR-negative cases observed in this study would have been truly misclassified for some reasons (e.g., technical errors during PCRs). Similarly, some excess PCR positivity could have resulted from the persistence of parasite DNA without viable parasites weeks after treatment [43]. Moreover, microscopic examination which could have been informative for parasite density and correlates of malaria immunity and transmissibility, is sorely lacking in this study [28]. Finally, this survey conducted interviews with the participants, a process that can involve information and memory biases.