Factors determining the occurrence of submicroscopic malaria infections and their relevance for control

Abstract

Malaria parasite prevalence in endemic populations is an essential indicator for monitoring the progress of malaria control, and has traditionally been assessed by microscopy. However, surveys increasingly use sensitive molecular methods that detect higher numbers of infected individuals, questioning our understanding of the true infection burden and resources required to reduce it. Here we analyse a series of data sets to characterize the distribution and epidemiological factors associated with low-density, submicroscopic infections. We show that submicroscopic parasite carriage is common in adults, in low-endemic settings and in chronic infections. We find a strong, non-linear relationship between microscopy and PCR prevalence in population surveys (n=106), and provide a tool to relate these measures. When transmission reaches very low levels, submicroscopic carriers are estimated to be the source of 20-50% of all human-to-mosquito transmissions. Our findings challenge the idea that individuals with little previous malaria exposure have insufficient immunity to control parasitaemia and suggest a role for molecular screening.

Figure 1. Prevalence of infection by PCR versus microscopy in 106 prevalence surveys and model fits.

Bayesian Markov chain Monte Carlo methods were used in model fitting to allow for measurement error in both microscopy and PCR prevalence measures. Posterior medians are shown. (a) Prevalence data with 95% CI from 86 surveys containing both adults and children, and fitted model (blue line) with 95% credible interval of the mean (light blue area). The model fitted was a linear relationship on the log odds scale. (b) Age-specific prevalence with 95% CI for survey data containing children only (<16 years, red dots, n=28) or adults only (≥16 years, orange dots, n=13). Fitted models (lines) with 95% CI (shaded areas) were obtained using the same methods as in a. (c) Correlation between observed and model-predicted PCR prevalence in the all-age surveys (a), obtained using leave-one-out cross-validation. Correlation coefficient=0.941. (d) Estimated average sensitivity of microscopy and 95% credible interval of the mean in the all-age surveys according to underlying PCR prevalence.

Figure 2. Trends in parasite densities within-host and across different transmission settings.

(a) Average probability of microscopic detection over the course of a single P. falciparum infection in 89 individuals, among those who had not yet had their last episode of patent parasitaemia (data from malaria therapy studies 2122). None were treated nor had a record of previous infection. Results are shown up to 300 days (some patients had longer infections, but there were few data points after this time). Points and CIs were calculated in 10-day time bands. (b) Relationship between slide prevalence and parasite density among infected individuals in 24 Tanzanian villages (data from Drakeley et al. red points=data with 95% CI, red line=linear regression fit on log scale), and the estimated percentage of infections which are submicroscopic in all age groups based on the model in Fig. 1a (blue dashed line).

Figure 3. Age-specific microscopy and PCR prevalence.

Eleven studies reported prevalence for three or more age groups (references a–b, c–d, e, f, g, h, i, j, k59). Microscopy prevalence in the whole study population is shown in the top-right corner of each panel.

Figure 4. Estimated contribution of submicroscopic infections to the infectious reservoir.

Percentage of mosquito infections originating from submicroscopic carriers, according to the observed slide prevalence of infection. Estimates are based on results from Fig. 1a and measures of infectiousness from three studies ( and ALO, unpublished data). The estimated % of all infections which are submicroscopic from Fig. 1a is shown as a black dashed line.

Figure 5. Hypothesized within-host dynamics of parasitaemia.

Schematic showing hypothesized patterns of parasitaemia in a single host in low- and high-transmission settings, showing infection times (arrows) and periods of submicroscopic parasitaemia (grey areas). Grey lines indicate detection limits of microscopy and PCR.