Parasitological correlates of Plasmodium ovale curtisi and Plasmodium ovale wallikeri infection

Abstract

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.

Keywords: Malaria diagnosis; Microscopy; PCR; Plasmodium ovale; Rapid antigen detection test.

Fig. 1

Plasmodium ovale isolates confirmed by Public Health Ontario Laboratories by year of import. 2006 and 2015 represent partial calendar years (October–December 2006, and January–June 2015). Emergence of P. ovale was equally distributed among the two sub-species, with six (27.3%) isolates of P. ovale curtisi identified prior to 2011, and 16 (72.7%) in or after 2011, compared to four (14.8%) isolates of P. ovale wallikeri identified prior to 2011, and 23 (85.2%) in or after 2011 (p = 0.31)

Fig. 2

Giemsa-stained microscopy of Plasmodium ovale. Note prominent Schüffner’s stippling in a, compared to the absence of Schüffner’s stippling in b