Under the Radar: Epidemiology of Plasmodium ovale in the Democratic Republic of the Congo

Abstract

Background: Plasmodium ovale is an understudied malaria species prevalent throughout much of sub-Saharan Africa. Little is known about the distribution of ovale malaria and risk factors for infection in areas of high malaria endemicity.

Methods: Using the 2013 Democratic Republic of the Congo (DRC) Demographic and Health Survey, we conducted a risk factor analysis for P. ovale infections. We evaluated geographic clustering of infections and speciated to P. ovale curtisi and P. ovale wallikeri through deep sequencing.

Results: Of 18 149 adults tested, we detected 143 prevalent P. ovale infections (prevalence estimate 0.8%; 95% confidence interval [CI], .59%-.98%). Prevalence ratios (PR) for significant risk factors were: male sex PR = 2.12 (95% CI, 1.38-3.26), coprevalent P. falciparum PR = 3.52 (95% CI, 2.06-5.99), and rural residence PR = 2.19 (95% CI, 1.31-3.66). P. ovale was broadly distributed throughout the DRC; an elevated cluster of infections was detected in the south-central region. Speciation revealed P. ovale curtisi and P. ovale wallikeri circulating throughout the country.

Conclusions: P. ovale persists broadly in the DRC, a high malaria burden country. For successful elimination of all malaria species, P. ovale needs to be on the radar of malaria control programs.

Keywords: Plasmodium ovale; amplicon sequencing; epidemiology; nonfalciparum malaria.

Figure 1.

Point prevalence estimates of Plasmodium ovale infections by Democratic Republic of the Congo province (A) for the 2013 Demographic and Health Survey (DHS) of 18 149 adults and (B) for the 2007 DHS survey of 8793 adults. The number of positive ovale samples and the province sample size (weighted) is listed below each province name.

Figure 2.

A, Spatial clustering of ovale malaria infections detected by SaTScan. Large cities with a population estimate of greater than 250 000 in 2000 and Demographic and Health Survey sampling cluster locations are included for reference. Sample clusters represented by a closed circle indicate the presence of 1 or more individuals positive for Plasmodium ovale, clusters represented with an “X” indicate that no P. ovale was detected. B, Sampling locations corresponding to P. ovale curtisi and P. ovale wallikeri-positive samples. For 12 samples across 5 provinces, global positioning system (GPS) sampling cluster coordinates were missing; these samples were plotted at the centroid coordinates of their respective province.

Figure 3.

Neighbor-joining tree of genetic relatedness across the 35 haplotypes identified from pooled amplicon deep sequencing results of 62 Plasmodium ovale polymerase chain reaction (PCR)-positive samples. Each unique haplotype is numbered and branches of the tree for each malaria species are laid over a color (brown, P. falciparum; purple, P. malariae; blue, P. ovale wallikeri; and green, P. ovale curtisi). In parentheses after the haplotype number is the number of samples that contained the haplotype, the GenBank accession name for closest match, and the number of single nucleotide polymorphisms/number of indels difference in the haplotype from the closest GenBank reference. Twenty-three samples (37%) were positive for P. ovale curtisi, 16 (26%) were positive for P. ovale wallikeri, and 3 (5%) contained both P. ovale curtisi and P. ovale wallikeri. However, given that the malaria genome contains multiple copies of the 18S gene, samples may contain more than 1 haplotype of a species. The scale bar represents the number of base substitutions per site.