Merozoite antigen-specific IgG levels increase with age.

<p>Box and whisker (Tukey) plots of IgG levels in each age group are shown. The levels of IgG to AMA1-3D7 (A), MSP1-3D7 (B), EBA175-3D7 (C) and MSP2-3D7 (D) were quantified. All responses below the limit of detection (44 ELISA units) were assigned a value of 22 ELISA units. IgG levels between the three age groups were compared using a Kruskal-Wallis test followed by Dunn's multiple comparison test (**, p<0.01; ***, p<0.001).</p

Increasing age and HbAS significantly protect Malian children against malaria.

<p>(A & B) The proportion of children who experienced at least one episode of malaria (malaria+) in the 2009 transmission season is shown. The children are categorized by age (A) or Hb type (B). (C & D) Survival analyses of the time to first malaria episode. Kaplan-Meier plots are shown for children grouped by age (C) or Hb type (D). Increasing age and HbAS were significantly associated with reduced malaria risk as measured by whether or not malaria was experienced (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0060182#pone-0060182-g001" target="_blank">Figure 1A</a>, p<0.0001; <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0060182#pone-0060182-g001" target="_blank">Figure 1B</a>, p = 0.0203; chi-squared tests), or time to first malaria episode (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0060182#pone-0060182-g001" target="_blank">Figure 1C</a>, p = 0.0002; <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0060182#pone-0060182-g001" target="_blank">Figure 1D</a>, p = 0.0013; by log-rank tests).</p

Increases in merozoite antigen-specific IgG levels during a transmission season do not differ by Hb type.

<p>For each child, the change in IgG level from May to December 2009 was calculated for each antigen (A, AMA1-3D7; B, MSP1-3D7; C, EBA175-3D7; D, MSP2-3D7). All responses below the limit of detection (44 ELISA units) were assigned a value of 22 ELISA units. Median and interquartile range are shown. Positive values represent increases in IgG levels at the end of the transmission season. Changes in IgG levels were not significantly different among the three Hb types for any antigen (p>0.05 by Kruskal-Wallis tests).</p

Merozoite antigen-specific IgG levels do not decay during a dry season, regardless of Hb type.

<p>For each child, the change in IgG level from December 2009 to May 2010 was calculated for each antigen (A, AMA1-3D7; B, MSP1-3D7; C, EBA175-3D7; D, MSP2-3D7). All responses below the limit of detection (44 ELISA units) were assigned a value of 22 ELISA units. Median and interquartile range are shown. Positive values represent increases in IgG levels at the end of the dry season. Changes in IgG levels were not significantly different among the three Hb types for any antigen (p>0.05 by Kruskal-Wallis tests).</p

High <i>Plasmodium falciparum</i> longitudinal prevalence is associated with high multiclonality and reduced clinical malaria risk in a seasonal transmission area of Mali

<div><p>The effects of persistent <i>Plasmodium falciparum</i> (Pf) infection and multiclonality on subsequent risk of clinical malaria have been reported, but the relationship between these 2 parameters and their relative impacts on the clinical outcome of infection are not understood. A longitudinal cohort study was conducted in a seasonal and high-transmission area of Mali, in which 500 subjects aged 1–65 years were followed for 1 year. Blood samples were collected every 2 weeks, and incident malaria cases were diagnosed and treated. Pf infection in each individual at each time point was assessed by species-specific nested-PCR, and Pf longitudinal prevalence per person (PfLP, proportion of Pf-positive samples over 1 year) was calculated. Multiclonality of Pf infection was measured using a 24-SNP DNA barcoding assay at 4 time-points (two in wet season, and two in dry season) over one year. PfLP was positively correlated with multiclonality at each time point (all r≥0.36; all <i>P</i>≤0.011). When host factors (e.g., age, gender), PfLP, and multiclonality (at the beginning of the transmission season) were analyzed together, only increasing age and high PfLP were associated with reduced clinical malaria occurrence or reduced number of malaria episodes (for both outcomes, <i>P</i><0.001 for age, and <i>P</i> = 0.005 for PfLP). When age, PfLP and baseline Pf positivity were analyzed together, the effect of high PfLP remained significant even after adjusting for the other two factors (<i>P</i> = 0.001 for malaria occurrence and <i>P</i><0.001 for number of episodes). In addition to host age and baseline Pf positivity, both of which have been reported as important modifiers of clinical malaria risk, our results demonstrate that persistent parasite carriage, but not baseline multiclonality, is associated with reduced risk of clinical disease in this population. Our study emphasizes the importance of considering repeated parasite exposure in future studies that evaluate clinical malaria risk.</p></div

Results of multivariate regression analysis adjusted for age and Hb type^a.

a<p>For these multivariate regression analyses, age (as a continuous variable), Hb type (HbAS or non-HbAS) and log₁₀-transformed ELISA units were used as variables.</p>b<p>OR, odds ratio</p>c<p>HR, hazard ratio</p>d<p>IRR, incident rate ratio</p

Merozoite antigen-specific IgG levels are lower in HbAS children.

<p>Box and whisker (Tukey) plots of IgG levels in each Hb type group are shown. The levels of IgG to AMA1-3D7 (A), MSP1-3D7 (B), EBA175-3D7 (C) and MSP2-3D7 (D) were quantified. All responses below the limit of detection (44 ELISA units) were assigned a value of 22 ELISA units. IgG levels between the three Hb type groups were compared using a Kruskal-Wallis test followed by Dunn's multiple comparison test (**, p<0.01; ***, p<0.001).</p

Correlation between polymorphic proportion (PmP) and <i>P</i>. <i>falciparum</i> longitudinal prevalence (PfLP).

<p>PmP positively correlates with PfLP at all 4 time-points: (A) Jun-V1 (r = 0.47, 95% confidence interval = 0.30–0.61, <i>P</i><0.001, Spearman rank correlation test); (B) Nov-V1 (r = 0.36, 0.18–0.52, <i>P</i><0.001); (C) Feb-V1 (r = 0.42, 0.15–0.63, <i>P</i> = 0.003); and (D) Apr-V1 (r = 0.39, 0.09–0.63, <i>P</i> = 0.011)</p

Cross-sectional prevalence of <i>P</i>. <i>falciparum</i> (Pf) infection and 2-week incidence of clinical malaria from June 2013 to May 2014.

<p>Left y-axis: Pf prevalence measured by nested-PCR; dark gray bars represent Pf prevalence. Right y-axis: number of clinical malaria cases recorded during each 2-week period preceding a blood sampling visit; light gray bars represent 2-week incidence. The time of visit is indicated on the x-axis, with V1 and V2 indicating the first and second blood sampling visits, respectively, for the months indicated.</p

Results of multivariate regression analysis of clinical malaria risk, including baseline Pf positivity^a.

<p>Results of multivariate regression analysis of clinical malaria risk, including baseline Pf positivity<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0170948#t004fn001" target="_blank">^a</a>.</p