Plasmodium species occurrence, temporal distribution and interaction in a child-aged population in rural Burkina Faso

Malaria remains a huge burden for human populations living in tropical areas. More than two million malaria cases were recorded in 2010, with the heaviest mortality rates in children living in sub-Saharan Africa[1]. During the past decade, the scale-up of malaria control interventions has resulted in considerable reductions in morbidity and mortality associated with malaria in parts of Africa[2, 3]. Despite these efforts, malaria continues to pose a major public health threat in many African countries[1] and recent work suggests that control efforts can be followed by infections rebounding to pre-intervention levels[4]. Successful local strategies for malaria control could be based on malaria surveys, which are an important tool to investigate the impact of the disease on population health status in endemic areas[5]. A primary component for this kind of survey should be the determination of parasite species diversity because optimal control measures are different for the different species of malaria.

Currently, malaria can be caused by five Plasmodium species which include Plasmodium falciparum, Plasmodium malariae, Plasmodium ovale, Plasmodium vivax and, more recently, Plasmodium knowlesi[6]. Plasmodium falciparum is the most prevalent in Africa and the most pathogenic of these, but in most malaria endemic regions multiple sympatric species are found and co-infection within individual human hosts or the mosquito vector population is common. In Africa, P. malariae is the species most frequently found in sympatry with P. falciparum[7]. Plasmodium inter-species interactions have been the focus of interest of number of studies[8‐11]. The co-infecting species interactions in humans can modify within-host dynamics[12, 13] and alter transmission potential[8]. The effect of mixed species infections on clinical outcome has been described as both beneficial[14] and adverse[15]. Indeed, in areas where P. falciparum co-occurred with P. malariae, field studies reveal that this co-occurrence can reduce disease severity[16, 17], give a lower peak of parasitaemia[18] or boost P. falciparum gametocyte production[8, 9].

In Burkina Faso particularly, where P. falciparum co-exists with P. malariae and P. ovale, there is not much data about the interaction of co-infecting species. In fact, only a few studies even report the prevalence of the last two species across human population[5, 19‐21]. Most of these studies were done in the past decade before the scale up of interventions such as the availability of artemisinin combination therapy (ACT) or the mass distribution of insecticide-treated bed nets (ITNs). In addition, the country profile reveals an increase in malaria cases from 2008 to 2010 without any information on P. malariae and P. ovale[1], while some research highlighted a substantial P. ovale malaria burden in sub-Saharan Africa[22, 23]. Moreover, in Burkina Faso, where the carriage of P. falciparum gametocyte is important with a substantial submicroscopic part[24], the role of P. malariae or P. ovale to this gametocytaemia should be assessed. The aim of the current study was to determine Plasmodium species-specific parasite and gametocyte prevalence, their temporal distribution and potential interaction across a child-aged population from a rural village in Burkina Faso.

Three sympatric species, P. falciparum, P. malariae and P. ovale were observed in the study site, with P. falciparum being the most prevalent species. Plasmodium falciparum infection is widely prevalent across the entire study period with some monthly fluctuation while P. malariae and P. ovale individually peak at the end of the rainy part of the study period. Similar fluctuation between P. malariae and P. falciparum densities has been reported by several other studies[19, 27]. Offset peaks may be explained by competition between both species and the fact that immune-protection induced by infection with P. falciparum does not seem to be effective against P. malariae[19]. It has also been suggested that P. malariae can be found at higher prevalence when transmission and overall infection rates are lower and it is alleviated from the suppressive effect of P. falciparum[10, 18]. A similar phenomenon may explain P. ovale occurrence. This finding revealed a kind of alternation in the relative contribution of each species to the total parasite indexes that occurred in the study area[28].

The significant increase of P. malariae prevalence throughout the four-year study period allows speculation on positive selection of this parasite by medication. In fact, in the study site and its surrounding villages many research activities occurred with treatment of symptomatic children[24, 29, 30] or a trial for intermittent preventive treatment of malaria in children (IPTc)[31, 32]. These symptomatic cases are rarely P. malariae infected-children because of this species low density. As a result, P. malariae remains in circulation despite the treatment and its transmission goes up in the population. More, with the absence of P. falciparum due to drug, P. malariae prevalence and density goes up because of the suppressive effect of P. falciparum[10, 18]. In all cases, particular attention should be paid to this secondary parasite. These secondary parasite species should not be ignored particularly as control interventions are planned and evaluated. Entomological studies are also needed to identify anopheline species which breed around the site and assess their vectorial capacity for a better understanding of this increased P. malariae prevalence.

In addition, the high proportion of gametocyte carriers and gametocyte density observed among P. malariae-infected children during the study period, suggests that specific attention should be paid to this parasite. Indeed, gametocytes are the currency of transmission from human to mosquito for maintaining the malaria transmission cycle. The presence and infectiousness of gametocytes in circulation determines the success of transmission from humans to mosquitoes. For now, P. malariae gametocyte density and mosquito infection are not well documented. Nevertheless it is well known that lower Plasmodium density as well as submicroscopic density is infectious to mosquitoes[24, 33]. Although considered mild, P. malariae is known to cause chronic infections which can last for years and might re-occur decades after initial exposure when people have long since left endemic regions[34]. Any level of infection with this species should be taken into account with relevant treatment. Moreover, this study reports a decrease of P. malariae asexual forms density and gametocytes prevalence when P. falciparum is present. With vaccine leading to the absence of P. falciparum, the inhibitory effect on P. malariae may disappear. Then, parasitaemia and gametocyte carriage of P. malariae could increase and maintain severe malaria infection. Within the context of vaccine being developed against P. falciparum[35], this finding presents a great epidemiological issue because the presence of non-falciparum species will still be a serious health concern to the communities at risk of infection.

The current data reveal that P. falciparum- and P. malariae-specific gametocyte prevalence, gametocyte and asexual form densities were higher in mono-infection as compared to mixed infections. In summary, the implications of these observations may have a profound repercussion in the outcome of disease control. If the simultaneous infection by other species inhibits P. falciparum-gametocyte production[36] and if control measures affect and reduce more effectively Plasmodium species other than P. falciparum, this inhibitory effect may disappear[28]. This finding is in agreement with some previous studies on P. falciparum[28, 36]. Indeed in Thailand, Price et al.[36] reported that mixed infections with P. falciparum and P. vivax are associated with a reduction in the prevalence of P. falciparum gametocytes. In the same way, Marques et al.[28] found in Mozambique that P. falciparum gametocytes predominated in single infections. In contrast, it has been reported from studies conducted in Columbia and Kenya that P. malariae infection enhanced production of P. falciparum gametocytes[8, 9]. These conflicting results suggest that the relationship between mixed species infections and gametocytaemia may be different under different endemicities[9].

Then, variability in the interactions between species under different transmission intensities, coupled with different sympatric species combinations may contribute to observed differences in the epidemiology and clinical presentations of malaria between endemic regions[10, 37].

Finally, prevalences and densities recorded in the current study may underestimate the real part of each Plasmodium species. Much study report that parasite prevalence under microscopy is 2.5-fold lower compared to prevalence found with molecular method and this fold difference was consistent across the different transmission levels in individual studies[38‐40] or meta-analysis[41]. In light of these results, a longitudinal parasitological survey using both microscopy and molecular tool are necessary to assess species-specific real prevalence for a better understanding of species interaction. An entomological study is also need to evaluate minor species gametocyte infectiousness and their contribution to malaria transmission.

Three species of malaria parasite are in circulation in the study area, with a fluctuation of P. falciparum and other species prevalence. These data revealed high gametocyte prevalence in other Plasmodium species than P. falciparum. The high proportion of gametocyte carriers and gametocyte density across P. malariae-infected children as well as the increase observed across years, presents a great epidemiological issue in the context of a vaccine being developed against P. falciparum. The relevance of these data should be taken into account when general control strategies are planned.