Parasitological correlates of Plasmodium ovale curtisi and Plasmodium ovale wallikeri infection

Abstract

BackgroundMalaria, due to Plasmodium ovale, can be challenging to diagnose due to clinically mild disease and low parasite burden. Two genetically distinct sub-species of P. ovale exist: Plasmodium ovale curtisi (classic) and Plasmodium ovale wallikeri (variant). It is presently unknown if the sub-species causing infection affects performance of malaria diagnostic tests. The aim of this work was to understand how the genetically distinct sub-species, P. o. curtisi and P. o. wallikeri, affect malaria diagnostic tests.Methods Plasmodium ovale-positive whole blood specimens were sub-speciated by PCR and sequencing of 18S rRNA and dhfr-ts. Parasitaemia, morphology, pan-aldolase positivity, 18S copy number, and dhfr-ts sequences were compared between sub-species.ResultsFrom 2006 to 2015, 49 P. ovale isolates were identified, of which 22 were P. o. curtisi and 27 P. o. wallikeri; 80% were identified in the last five years, and 88% were acquired in West Africa. Sub-species did not differ by parasitaemia, 18S copy number, or pan-aldolase positivity. Lack of Schüffner’s stippling was over-represented among P. o. wallikeri isolates (p = 0.02). Several nucleotide polymorphisms between the sub-species were observed, but they do not occur at sites believed to relate to antifolate binding.Conclusions Plasmodium ovale is increasing among travellers to West Africa, although sub-species do not differ significantly by parasitologic features such as parasitaemia. Absence of Schüffner’s stippling may be a feature specific to P. o. wallikeri and is a novel finding.

Prospective comparative multi-centre study on imported Plasmodium ovale wallikeri and Plasmodium ovale curtisi infections

“…As in previous studies, no differences were detected in parasitaemias between both groups [ 6 , 9 , 32 ]. Most of them had low parasite counts clearly below 1%, making it even more difficult to find statistically real differences.…”

“…Lastly, the incidence of imported P. ovale spp. infection seems to be increasing among travellers returning from West Africa [ 9 – 11 ], in whom chemoprophylaxis failed to work more frequently [ 12 ] compared to P. falciparum or P. vivax [ 13 ].…”

“…Once established the genetic differences, the next question was to find out if there were any other differences in epidemiology, microbiology or clinical features between both subspecies. Plasmodium o. curtisi and P. o. wallikeri do not seem to differ in parasitaemia levels [ 5 , 9 , 13 ] and the absence of Schüffner’s stippling may be a feature specific to P. o. wallikeri but only in 30% of blood samples [ 9 ]. Clinical differences between both species have not been clearly established, but P. o. wallikeri may have a shorter period of latency [ 13 ].…”

Distinction of Plasmodium ovale wallikeri and Plasmodium ovale curtisi using quantitative Polymerase Chain Reaction with High Resolution Melting revelation

“…In fact, some differences in the latency period of the hypnozoites were highlighted 11 , higher thrombocytopenia and nausea with Pow was reported by another study 12 . Yet, microscopic examination of blood smear does not allow differentiation of the two species, even if a recent study described a lack of discernible Schüffner’s stippling in Pow samples 13 . Moreover, RDT efficiency was low and no differences in RDT positivity were found between Poc and Pow 13 .…”

“…Yet, microscopic examination of blood smear does not allow differentiation of the two species, even if a recent study described a lack of discernible Schüffner’s stippling in Pow samples 13 . Moreover, RDT efficiency was low and no differences in RDT positivity were found between Poc and Pow 13 . Therefore, differentiation of Poc and Pow remains challenging, and the development of convenient tools to differentiate Poc and Pow is crucial to investigate the burden on malaria infections of each species, as well as their clinical implication.…”