Identification of the asymptomatic Plasmodium falciparum and Plasmodium vivax gametocyte reservoir under different transmission intensities

Abstract

Background: Understanding epidemiological variables affecting gametocyte carriage and density is essential to design interventions that most effectively reduce malaria human-to-mosquito transmission.

Methodology/principal findings: Plasmodium falciparum and P. vivax parasites and gametocytes were quantified by qPCR and RT-qPCR assays using the same methodologies in 5 cross-sectional surveys involving 16,493 individuals in Brazil, Thailand, Papua New Guinea, and Solomon Islands. The proportion of infections with detectable gametocytes per survey ranged from 44-94% for P. falciparum and from 23-72% for P. vivax. Blood-stage parasite density was the most important predictor of the probability to detect gametocytes. In moderate transmission settings (prevalence by qPCR>5%), parasite density decreased with age and the majority of gametocyte carriers were children. In low transmission settings (prevalence<5%), >65% of gametocyte carriers were adults. Per survey, 37-100% of all individuals positive for gametocytes by RT-qPCR were positive by light microscopy for asexual stages or gametocytes (overall: P. falciparum 178/348, P. vivax 235/398).

Conclusions/significance: Interventions to reduce human-to-mosquito malaria transmission in moderate-high endemicity settings will have the greatest impact when children are targeted. In contrast, all age groups need to be included in control activities in low endemicity settings to achieve elimination. Detection of infections by light microscopy is a valuable tool to identify asymptomatic blood stage infections that likely contribute most to ongoing transmission at the time of sampling.

Fig 1. Parasite prevalence and mean parasite density by qPCR in five cross-sectional surveys.

Error bars represent 95% confidence intervals.

Fig 2

Relationship of P. falciparum (A) and P. vivax (B) parasite density by qPCR and pfs25/pvs25 transcript numbers by RT-qPCR.

Fig 3. Proportion of gametocytes among all blood-stage parasites.

Proportion P. falciparum (A) and P. vivax (B) gametocytes among all blood-stage parasites by parasite density with LOWESS splines (red). Very low-density infections with <5 DNA copies/uL are excluded, as quantification (and thus the calculated proportions) might be inaccurate at low densities. Panels C and D show transcript vs. total parasite density stratified into those infections with and without a febrile episode in the 2 weeks prior to sampling (self-reported febrile illness or antimalarial treatment). Boxplots show median (solid line), 25th and 75th percentile (box), whiskers extend up to 1.5-fold the interquartile range.

Fig 4. pfs25 and pvs25 transcript densities for each survey stratified by age group.

The black line shows the median. The percentage shows the proportion of gametocyte carriers in an age group among all gametocyte carriers within this survey.

Fig 5. Infectivity of P. falciparum and P. vivax infections in membrane-feeding assays based on clinical status and ability of light microscopy to diagnose infections.

‘% Mosquitos infected’ shows the combined proportion of mosquitos infected including individuals that did not infect any mosquitos. n/N show the proportion of individuals that infected at least one mosquito. Infectivity varied widely, but with the exception of the study on P. vivax in Brazil, microscopy-positive individuals were 5-10-fold more infective that those with submicroscopic infection. Data from [, –50].