Parasitological correlates of Plasmodium ovale curtisi and Plasmodium ovale wallikeri infection

BACKGROUND: Malaria, 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. RESULTS: From 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.