This study on the epidemiology of malaria in Gabonese children is the first to assess the prevalence of plasmodial infection between symptomatic and asymptomatic children in Gabon, in three localities with different epidemiological contexts.
In this study, the average age of asymptomatic children was significantly higher than that of symptomatic children in the three localities. These data are consistent with the results of previous studies which have shown that asymptomatic children are older than symptomatic children (18, 28). This could be explained by the fact that the study was carried out in malaria-endemic areas. As such, age is an important determinant of protection against clinical malaria in endemic areas. Indeed, young children are more vulnerable, but adults and older children who have acquiered a form of immunity after cumulative exposure to the parasite are more likely to carry asymptomatic infections (18, 28). The haematological parameters varied considerably depending on the symptomatology and the socio-geographic factors. A decrease in the hemoglobin level was observed in all infected children and induced anemia in symptomatic patients, this is confirmed by the low average number of red blood cells of infected children. Although the etiology of anemia in tropical areas is multi-factorial, our data are consistent with several other studies showing that anemia during P. falciparum malaria is closely associated with malaria parasitemia (29–31). Similarly, it has been shown that in children infected with P. falciparum a combination of hemolysis of parasitized and non-parasitized red blood cells and erythropoiesis depression inducing anemia are often observed (32). Furthermore, the decrease in hemoglobin and red blood cell levels suggests the upward stimulation of TNF-α, interferon gamma (INF-γ) and interleukin-1 (IL-1) in infected patients by malaria (32). In contrast, in Franceville, the levels of red blood cells and hemoglobin were not significantly different between symptomatic and asymptomatic children. This may be due to the fact that in this locality the parasite densities were not significantly different between the two groups of children, since it was shown that parasite density has an impact on the occurrence of anemia in an individual (32). Furthermore, in the semi-rural area, although a decrease in the average hemoglobin level and in the number of red blood cells was observed in the two groups, these two haematological parameters were significantly low in symptomatic children compared to asymptomatic children. This difference could be due to a massive destruction of red blood cells in symptomatic children in this locality. The average high parasite densities observed in symptomatic children could also explain this difference (32). Moreover, the high prevalence of anemia in rural areas could be due to the high circulation of intestinal parasites in these areas (17). Although thrombocytopenia in infected children in sub-Saharan African regions is very common (32), our study did not find the presence of thrombocytopenia in the children. However, a difference in platelet count was observed between symptomatic and asymptomatic children in Makokou. The decrease in platelet counts in symptomatic children compared to asymptomatic children is believed to be the result of increased consumption of peripheral blood platelet counts in infected children. This is in accordance with studies that have shown the stimulation of cytokines in response to parasitized red blood cells. It can activate the coagulation cascade: consumption of antithrombin III, increased concentration of fibrinogen degradation products (FDP) and increased splenic clearance of platelets, leading to coagulopathy and thrombocytopenia in patients with malaria (33). In the group of symptomatic children, only the red blood cell and the hemoglobin levels were significantly different between the three localities. A difference in the white blood cell count and the number of platelets was observed in the group of symptomatic children. These results are consistent with the results of previous studies carried out in different regions of Gabon which have shown similar profiles in terms of hematological parameters according to the living area of infected children (8, 14).
We assessed the prevalence of plasmodial infection in febrile and non-febrile children in urban, semi-urban and rural settings. The data presented here highlight a large difference in the prevalence of plasmodial infection in the three localities. This makes it possible to confirm and understand the disparity in the epidemiological characteristics of plasmodial infection in Gabon. The prevalence of plasmodial infection was higher in Lastoursville (rural area) and Makokou (semi-urban area) than in Franceville. Thus, the difference in malaria prevalence between the three areas could be due to better management of children aged under 5 years old by anti-malarial programs in Gabon and on the other hand, to a poor access to malaria control measures in rural and semi-urban areas. This difference in malaria prevalence could also be explained by the socio-economic level (14) and heterogeneity of transmission in Gabon, which have been demonstrated in other endemic areas (34). This heterogeneity of transmission was also observed in the town of Libreville in Gabon, where the level of urbanization, the type of housing and the socio-economic level have an impact on the transmission of parasite (12). In urban areas, the overall prevalence of plasmodial infection was estimated at 21.6%. These results are in agreement with those of the previous studies carried out in Franceville which showed that the prevalence of plasmodial infection in children with fever was approximately 20% (7, 14). The lack of difference in prevalence between asymptomatic and symptomatic children confirms the low level of circulation of the parasite in this locality. The level of prevalence in asymptomatic children could explain the stagnation in the prevalence of malaria since 2010 despite the implementation of infection control policies in Gabon (8, 9, 14).
The overall prevalence of plasmodial infection in semi-urban and rural areas was comparable to that described above (14, 35). The proportion of febrile children with plasmodium Spp infection in semi-urban and rural areas was significantly higher than that of non-febrile children. This observation may be due to the fact that in these two localities the mean age varied significantly between the infected children, since it has been shown that asymptomatic infections are more frequent in young adolescents and adults residing in urban areas with endemic malaria. Because young adolescents and adults have acquired protective immunity through exposure to the parasite, it would limit the onset of symptoms. This is not the case for children under five years old who are at risk of developing malaria (36). These results are not in agreement with the results of a study carried out in Cameroon in a semi-urban environment which reports a significantly high prevalence in asymptomatic children (30.7%) compared to symptomatic patients (17.8%) (37). The prevalence of asymptomatic infections recorded in these 2 environments remains significant, since people with asymptomatic plasmodial infections are silent reservoirs of the parasite and pose a serious challenge to malaria health efforts because of their ability to maintain transmission among the population.
Symptomatic plasmodial infection was significantly higher in the semi-urban and rural areas than in the urban area, as shown by a recent study (37). This is due to the level of knowledge on malaria of people living in urban areas leading to a more effective use of prevention means. Preventive measures have a significant impact on the circulation of Plasmodium and reduce the prevalence of plasmodial infection. These differences could be partly attributed to the fact that urban areas provide better housing than rural and semi-urban areas, since it has been shown that better housing can reduce the transmission of plasmodial infection. On the other hand, urban environments are less favorable for vector species, in particular An. gambiae, which has a strong preference for unpolluted waters (38). The lifespan of An. gambiae in urban areas was estimated to be less than half its lifespan in rural areas (4.1 versus 11 days) in a study in Kinshasa, the capital of the Democratic Republic of the Congo (39). Mosquito dispersal is also much more limited in urban areas due to the higher housing density (40), which focuses the transmission of urban malaria (41). In Franceville, people live in better housing than in rural and semi-urban areas. Better housing reduces the risk of malaria because it minimizes mosquito entry points during the night. To illustrate this, a study in Gambia has shown that homes with children infected with malaria are more likely to have mud walls, open eaves and absent ceilings than those with uninfected children (42). Floors made of earth bricks are also associated with a lower risk of malaria, as residents are more likely to sleep on raised beds to avoid soil moisture and they are less likely to be bitten by An. gambiae mosquitoes looking for preys close to the ground (42).
It was also observed in this study that the prevalence of asymptomatic plasmodial infection was high in rural areas compared to the two other localities. This observation is supported by the fact that children in rural areas have developed non-sterilizing anti-malarial immunity due to a higher exposure to the parasite. This is not the case with children in urban areas who are overprotected. Therefore, the fact that the prevalence of asymptomatic infections is high in rural areas ensures an increased maintenance of the prevalence of plasmodial infection in these localities. This study shows that the prevalence of malaria in Gabon differs considerably depending on the local economic status, confirming previous data (35). This can be explained by the fact that in rural areas, antimalarial drugs are misused despite the availability of artemisinin-based combination therapy (ACT). Poor socio-economic conditions and insufficient knowledge of malaria could contribute to the high prevalence of malaria in rural areas, as noted in a recent study (14).
We also carried out molecular analyzes based on Snounou’s method (27) in order to determine the plasmodial species circulating. Our analysis made it possible to highlight three plasmodial species (P. falciparum, P. malariae, P. ovale) and to determine their prevalence in terms of mono- and co-infections. In general, the species were determined for 712 samples, representing a success rate of 91.4%. P. falciparum was responsible for the majority of plasmodial infections with an overall prevalence of 91.4%. This can be explained by the fact that P. falciparum is the main species in Gabon. Indeed, our results are consistent with our previous studies carried out in Gabon, which already showed that P. falciparum infections were the most widespread in urban, semi-urban and rural areas, whether in symptomatic or asymptomatic individuals (7, 10, 13). This suggests that, despite the reduction in the burden of malaria in Gabon, the distribution of species among infected children has not changed and that P. falciparum remains the main species involved. In contrast, among asymptomatic people living in rural Gabon, it has recently been reported by two studies that P. malariae accounted for 21.9–47.6% of cases of plasmodial infections (25, 43). These results are consistent with those of our study which classify this species in second position after P. falciparum in the semi-urban and rural environments (25, 43). This could suggest a selection of P. falciparum to the detriment of other species in the event of symptomatic malaria infection. Another possible explanation is the use of different diagnostic methods. Delicat-Loembet and al. used the 454 sequencing method to identify Plasmodium species while other authors used less sensitive methods (PCR and blood smears with RTD) (7, 14, 25). Furthermore, the low sensitivity of PCR can be explained by an insufficient yield and the low sensitivity of the primer set used. Though we did not report cases of monoinfection with P. ovale in our study, this species is sometimes encountered in urban areas and we diagnosed a case of co-infection. We confirm that P. vivax does not circulate in humans in south-eastern Gabon. Furthermore, our analyzes highlight a high prevalence of P. malariae among asymptomatic children in rural and semi-urban areas, these results are consistent with those of a study conducted in a rural area in Gabon (25, 43). This can be explained by the section of certain parasitic clones which could be more virulent others, and also by the fact that P. falciparum is more apt to induce symptomatology than the other species.