Plasmodium vivax in Sub-Saharan Africa: An Advancing Threat to Malaria Elimination?

Circulation of Non-falciparum Species in Niger: Implications for Malaria Diagnosis

Background

Niger's National Malaria Control Programme and its partners use histidine-rich protein 2-based RDTs, which are specific to Plasmodium falciparum diagnosis. This study aimed to screen for the circulation of non-falciparum species in Zinder, a region of Niger, West Africa.

Methods

A cross-sectional study was carried out from July to December 2022 at the district hospital of the Zinder region of Niger. P falciparum histidine-rich protein 2-based rapid diagnostic tests were performed, and dried blood spot samples were collected for further laboratory multiplexed photo-induced electron transfer-polymerase chain reaction (PET-PCR) analysis on positive light microscopy from all patients with fever who attended the Zinder district hospital during the study period.

Results

In total, 340 dried blood spots were collected and analyzed by PET-PCR. Overall, 73.2% (95% CI, 68.2%-77.9%; 249/340) were positive for Plasmodium genus and species and represented the study population. Plasmodium species proportions were 89.5% (95% CI, 85.1%-93.1%; 223/249) for P falciparum, 38.5% (95% CI, 32.5%-44.9%; 96/249) for P malariae, 10.8% (95% CI, 7.3%-15.4%; 27/249) for P vivax, and 1.6% (95% CI, .4%-4.1%; 4/249) for P ovale. Single infection with Plasmodium species counted for 61.8% (95% CI, 55.5%-67.9%; 154/249), and the mixed infections rate, with at least 2 Plasmodium species, was 38.1% (95% CI, 32.1%-44.5%; 95/249). Single non-falciparum infections represented a rate of 10.0% (95% CI, 6.6%-14.5%; 25/249).

Conclusion

This study confirms the first evidence of Plasmodium vivax by PET-PCR in Niger in addition to the other 3 Plasmodium species. These findings underline the need to adapt malaria diagnostic tools and therapeutic management, as well as the training of microscopists, for recognition of non-falciparum plasmodial species circulating in the country. This will better inform the strategies toward malaria control and elimination, as well as the decision making of the health authorities of Niger.

Origin of the human malaria parasite Plasmodium vivax

The geographic origin of Plasmodium vivax, a leading cause of human malaria, has been the subject of much speculation. Here we review the evolutionary history of P. vivax and P. vivax-like parasites in humans and non-human primates on three continents, providing overwhelming evidence for an African origin. This conclusion is consistent with recent reports showing that Duffy-negative humans in Africa are, in fact, susceptible to P. vivax, with parasites invading Duffy-antigen-expressing erythroid precursors. Thus, the African origin of P. vivax not only explains the distribution of the Duffy-negative genotype but also provides new insight into the history and status of P. vivax malaria in Africa and efforts geared toward its eradication.

The two parasite species formerly known as Plasmodium ovale

Plasmodium ovale was the last of the exclusively human malaria parasites to be described, in 1922, and has remained the least well studied. Beginning in 1995, two divergent forms of the parasite, later termed 'classic' and 'variant', were described. By 2010, it was realised that these forms are two closely related, but genetically distinct and non-recombining species; they were given the names Plasmodium ovale curtisi and Plasmodium ovale wallikeri. Since then, substantial additional data have confirmed that the two parasites are indeed separate species, but the trinomial nomenclature has often led to confusion about their status, with many authors describing them as subspecies. We hereby formally name them Plasmodium ovalecurtisi and Plasmodium ovalewallikeri.

Unveiling P. vivax invasion pathways in Duffy-negative individuals

Vivax malaria has long been thought to be absent from sub-Saharan Africa owing to the high proportion of individuals lacking the Duffy antigen receptor for chemokines (DARC) in their erythrocytes. The interaction between P. vivax Duffy-binding protein (PvDBP) and DARC is assumed to be the main pathway used by merozoites to invade reticulocytes. However, the increasing number of reports of vivax malaria cases in genotypically Duffy-negative (DN) individuals has raised questions regarding the P. vivax invasion pathway(s). Here, we show that a subset of DN erythroblasts transiently express DARC during terminal erythroid differentiation and that P. vivax merozoites, irrespective of their origin, can invade DARC+ DN erythroblasts. These findings reveal that a large number of DN individuals may represent a silent reservoir of deep P. vivax infections at the sites of active erythropoiesis with low or no parasitemia, and it may represent an underestimated biological problem with potential clinical consequences in sub-Saharan Africa.