Individual variation in Plasmodium vivax malaria risk: Are repeatedly infected people just unlucky?

Subclinical malaria among pregnant women living in refugee settlements in Northern Uganda

Background

Malaria during pregnancy contributes to significant perinatal morbidity and mortality, accounting for almost 25% of global maternal mortality. However, the epidemiology and risk factors for subclinical malaria among pregnant women living in refugee settlements is poorly understood.

Objective

To determine the prevalence and predictors of subclinical malaria among pregnant women in refugee settlements in Northern Uganda.

Design

We conducted a multi-center, cross-sectional study.

Methods

The study was conducted between April and June 2023 and involved pregnant women aged 18-45 years attending routine antenatal care (ANC) at three health facilities serving refugee communities in Adjumani district, Uganda. We collected sociodemographic, environmental, maternal, and obstetric factors using a structured questionnaire. Both CareStart Malaria HRP-2/pLDH (Pf/Pan) combo rapid diagnostic test (RDT) and blood smear microscopy with 3% Giemsa staining were simultaneously performed on samples from each patient. Logistic regression analysis identified factors independently associated with subclinical malaria, reported as adjusted odds ratios (aORs) and 95% confidence intervals (CIs).

Results

We enrolled 304 pregnant women, with a mean age of 25 years. In total, 68.8% (n = 209) had lived in the settlement for over 12 months, 25.7% (n = 78) were primigravida, and 1.0% (n = 3) were living with HIV. Malaria prevalence was 5.3% (n = 16) by RDT and 3.2% (n = 10; seven Plasmodium falciparum and three P. malariae) by microscopy. Only 4 (25.0%) of the RDT-positive cases were also positive by microscopy (Cohen's kappa: 0.278-Fair agreement). All participants were asymptomatic. Factors associated with higher odds of subclinical malaria included primiparity (aOR: 2.79, 95% CI: 1.25-6.25, p = 0.013), ⩾4 ANC visits (aOR: 2.41, 95% CI: 1.34-4.34, p = 0.003), and residence in the settlement for less than 12 months (aOR: 2.54, 95% CI: 2.0-3.22, p < 0.001). Living in the settlement for over 3 years, being primigravida, and being married were associated with 68%, 50%, and 68% lower odds of subclinical malaria, respectively (aOR: 0.32, 95% CI: 0.13-0.79, p = 0.014; aOR: 0.50, 95% CI: 1.22-5.52, p = 0.016; aOR: 0.32, 95% CI: 0.13-0.78, p = 0.012).

Conclusion

Our study reveals the high prevalence of subclinical malaria among pregnant women in refugee settlements, particularly among primiparous women and recent arrivals. The poor agreement between RDT and microscopy suggests the need for dual screening in asymptomatic pregnant women.

Real-time PCR detection of mixed Plasmodium ovale curtisi and wallikeri infections in human and mosquito hosts

Plasmodium ovale curtisi (Poc) and Plasmodium ovale wallikeri (Pow) represent distinct non-recombining Plasmodium species that are increasing in prevalence in sub-Saharan Africa. Though they circulate sympatrically, co-infection within human and mosquito hosts has rarely been described. Separate 18S rRNA real-time PCR assays that detect Poc and Pow were modified to allow species determination in parallel under identical cycling conditions. The lower limit of detection was 0.6 plasmid copies/μL (95% CI 0.4-1.6) for Poc and 4.5 plasmid copies/μL (95% CI 2.7-18) for Pow, or 0.1 and 0.8 parasites/μL, respectively, assuming 6 copies of 18s rRNA per genome. However, the assays showed cross-reactivity at concentrations greater than 103 plasmid copies/μL (roughly 200 parasites/μL). Mock mixtures were used to establish criteria for classifying mixed Poc/Pow infections that prevented false-positive detection while maintaining sensitive detection of the minority ovale species down to 100 copies/μL (<1 parasite/μL). When the modified real-time PCR assays were applied to field-collected blood samples from Tanzania and Cameroon, species identification by real-time PCR was concordant with nested PCR in 19 samples, but additionally detected two mixed Poc/Pow infections where nested PCR detected a single Po species. When real-time PCR was applied to oocyst-positive Anopheles midguts saved from mosquitoes fed on P. ovale-infected persons, mixed Poc/Pow infections were detected in 11/14 (79%). Based on these results, 8/9 P. ovale carriers transmitted both P. ovale species to mosquitoes, though both Po species could only be detected in the blood of two carriers. The described real-time PCR approach can be used to identify the natural occurrence of mixed Poc/Pow infections in human and mosquito hosts and reveals that such co-infections and co-transmission are likely more common than appreciated.

Real-time PCR detection of mixed Plasmodium ovale curtisi and wallikeri species infections in human and mosquito hosts

Plasmodium ovale curtisi (Poc) and Plasmodium ovale wallikeri (Pow) represent distinct non-recombining malaria species that are increasing in prevalence in sub-Saharan Africa. Though they circulate sympatrically, co-infection within human and mosquito hosts has rarely been described. Separate 18S rRNA real-time PCR assays that detect Poc and Pow were modified to allow species determination in parallel under identical cycling conditions. The lower limit of detection was 0.6 plasmid copies/μL (95% CI 0.4-1.6) for Poc and 4.5 plasmid copies/μL (95% CI( 2.7- 18) for Pow, or 0.1 and 0.8 parasites/μL, respectively, assuming 6 copies of 18s rRNA per genome. However, the assays showed cross-reactivity at concentrations greater than 103 plasmid copies/μL (roughly 200 parasites/μL). Mock mixtures were used to establish criteria for classifying mixed Poc/Pow infections that prevented false-positive detection while maintaining sensitive detection of the minority ovale species down to 10° copies/μL (<1 parasite/μL). When the modified real-time PCR assays were applied to field-collected blood samples from Tanzania and Cameroon, species identification by real-time PCR was concordant with nested PCR, but additionally detected two mixed Poc/Pow infections where nested PCR detected a single Po species. When real-time PCR was applied to 14 oocyst-positive Anopheles midguts saved from mosquitoes fed on P. ovate-infected persons, mixed Poc/Pow infections were detected in 11 (79%). Based on these results, 8/9 P. ovate carriers transmitted both P. ovate species to mosquitoes, though both Po species could only be detected in the blood of two carriers. The described real-time PCR approach can be used to identify the natural occurrence of mixed Poc/Pow infections in human and mosquito hosts and reveals that such co-infections and co-transmission are likely more common than appreciated.