Nigeria is one of the countries with higher malaria prevalence worldwide, and is one of the four countries accounted for almost half of all cases globally [1]. This study detected high malaria prevalence in asymptomatic school children in southwestern Nigeria. The prevalence is higher than expected according to the prevalence in the state (32.6%) [12]. Moreover, asymptomatic malaria was also recognized as important malaria reservoir in Nigeria contributing to maintain transmission [16]. However, the prevalence shown in this study is higher than previous studies done in general asymptomatic population in other areas of Nigeria [17]. The present work supports studies that described schoolchildren as hotspot for malaria transmission [3, 4]. In fact, a study carried out in Ondo state, bordering Osun state, reports malaria prevalence over 90% for febrile children [18]. Considering the high prevalence of malaria in asymptomatic children and its potential impact as reservoir, screening of school children could be a recommendable strategy for malaria control.
Diagnosis of asymptomatic patients is usually a challenge for different reasons, as lower parasitemia. Nevertheless, our study found better agreement in RDT and microscopy results that the previous ones reported in adults in southwestern Nigeria [19]. According to previous results, RDT had better specificity than microscopy, that could be explained because of professional requirements for a high-quality microscopy increase the likelihood of false positives. Contrastingly, RDT had lower sensitivity than microscopy [16, 20]. That could be due to different reasons, but our study supports that deletions could be causing false negatives by RDT. However, the result is not conclusive according to WHO guidelines, as the threshold 5% is included in the confidence interval [21]. Surprisingly, in our study double deletions are the second most common combination of deletions, after presence of both genes, but only a minority of them were associated with false negatives by RDT. So, RDT could be reacting with other proteins or maybe there are HRP2 or HRP3 proteins still in blood from previous infections [22].
However, the frequency of deletions detected (41.53% pfhrp2 deletion, 52.72% pfhrp3 deletion) in this study is higher than the reported in previous studies in Nigeria [9, 10, 17] and in its border countries, such as Chad (12.3% pfhrp2 deletion)[23], Cameroon (1.5% pfhrp2 deletion) [24] and Benin (0% pfhrp2 deletion) [10]. That could be explained by different factors. Firstly, samples were taken at the end of the dry season and beginning of rainy season when less polyclonal infections and malaria diversity are expected and, consequently, there is more likely to detect deletions and false negatives by RDT [25, 26]. Secondly, age could influence the likelihood of deletion detection, being more probable in young patients [26]. Increasing the knowledge of pfhrp2 and pfhrp3 deletion dynamics could be essential for the design of public health strategies that target asymptomatic children.
This study presents several limitations, first of all, the study area covered only a rural area in one state, so results are not representative national wide, and the study population, including only school children, is not representative of the total population. Secondly, the study did not include analysis of gametocytes, therefore real contribution to malaria transmission could not be assessed although it will be highly recommended to clarify the real contribution of this group to the malaria transmission. However, this study provides important information about the prevalence of malaria in asymptomatic school children in Nigeria and the potential threat of their diagnosis by RDT due to pfhrp2 and pfhrp3 deletions. Considering all the above, RDT could still be a feasible strategy for malaria screening in asymptomatic population although confirmation using microscopy is highly recommended.