Diversity of malaria parasites in great apes in Gabon

In the last few years, several new Plasmodium species were discovered in African non-human primates, especially great apes [2,3,6,10,26]. These discoveries were made possible by the development of a non-invasive method allowing detection of Plasmodium infections from faecal samples [2,6], despite inherent problems of DNA degradation with this type of biological material. This issue was overcome by the use of mitochondrial sequences to amplify the parasite, which presents several advantages: 1) Mitochondrial DNA is in multiple copy inside parasites (unlike nuclear DNA) and 2) if properly chosen, small portions of the mitochondrial genome (as small as 200 bp), can contain enough phylogenetic information to identify the different Plasmodium species. This method is now one of the main methods used to analyse Plasmodium from wild non-human primates [2,7,12,25].

In the present study, analyses were performed on a set of 1,261 faecal samples collected all over Gabon from chimpanzees and gorillas. All Plasmodium species found belonged to the subgenus Laverania and were all previously identified in Pan troglodytes troglodytes and Gorilla gorilla gorilla, respectively [2,12]. No new phylogenetic lineage or species were identified. Surprisingly, no species of the subgenus Plasmodium (non-Laverania) were identified either. This is at odds with recent observations made from ape blood samples or infected sylvatic anopheline mosquitoes collected in Gabon showing the circulation of P. vivax-like parasites in the area. [13] These results are nevertheless congruent with those from Liu et al. [12]. Although they analysed 3,000 ape faecal samples from west and central Africa, they only obtained seven sequences of Plasmodium belonging to the subgenus Plasmodium (non-Laverania).

One main factor could explain why parasites of the subgenus Plasmodium were not detected and this is most likely linked to the nature of the primers used to perform PCRs. Indeed, as in the study of Liu et al. [12], the primers used were specifically designed to amplify sequences of Laverania parasites. As a consequence, several nucleotides of differences separated them from the homologous sequences in P. vivax, P. malariae and P. ovale, thus very likely reducing the sensitivity of this PCR to detect non-Laverania species. In addition, such problem might have been amplified by the presence of co-infections with Laverania species. Indeed, it has been demonstrated that in case of co-infection, the PCR tends to favour the amplification of the parasite with the best matching sequence to the primers [35,36]. Recently, Liu et al. [14] solved this problem by designing primers specific to P. vivax. Out of the 3,000 samples previously analysed and re-analysed with other samples, they finally detected more than 87 P. vivax infections.

In this study, no Laverania species were found to infect both hosts (gorillas and chimpanzees). This reinforces the hypothesis that Laverania lineages infect specific hosts [10], a specificity that could be associated to specific ligand/receptor interactions occurring in the vertebrate host, as suggested by several studies [37], or by ecological factors such as the trophic preferences of the vectors [38] or the fact that gorillas’ and chimpanzees’ home ranges might not overlap in space and time. Additional studies would be needed to disentangle these different possibilities.

No human Plasmodium species were found. This result is congruent with other studies performed so far on wild populations of apes [8,12,39] thus confirming that, contrarily to what some authors have feared [7,40], great apes do not (and will certainly never) constitute reservoirs of Plasmodium, in particular P. falciparum, for humans. The fact that their populations are rapidly declining [41,42] is unfortunately another element in support of this prediction. Finally, no evidence of ape-to-human transfers of Laverania species was ever recorded despite efforts to find them [37]. The only documented record of this kind of transfer, in a natural context, was for a P. vivax-like (non-Laverania) parasite [13].

Regarding the prevalence of infections, more than 15% of the chimpanzee and 21% of the gorilla samples were positive to Plasmodium. Infections were detected in 17out of 23sites for chimpanzees and 16 out of 24 for gorillas. Global rates of infection found in this study are similar to those found by Prugnolle et al. [2], Kaiser et al. [8], and Liu et al. [12] in other areas. As previously discussed [2,12], it is very likely that the accurate rates of infection are higher, because the detection of Plasmodium in this kind of biological material (faecal) is expected to be less sensitive than in blood, as it is the case for urine and saliva [12,43,44], due to sample degradation or repeated sampling (faecal samples from the same individual may have been collected several times). The effect of sample degradation (and hence DNA degradation) is evident here when comparing the rates of infection detected in the faeces that were collected before and after 24 hours post excretion. The freshest (and so the less degraded) faecal samples significantly present more Plasmodium infections than the other ones (odds ratio = 2.038).