Relative contribution of low-density and asymptomatic infections to Plasmodium vivax transmission in the Amazon: pooled analysis of individual participant data from population-based cross-sectional surveys

Abstract

Background: Low-density and asymptomatic Plasmodium vivax infections remain largely undetected and untreated and may contribute significantly to malaria transmission in the Amazon.

Methods: We analysed individual participant data from population-based surveys that measured P vivax prevalence by microscopy and polymerase chain reaction (PCR) between 2002 and 2015 and modelled the relationship between parasite density and infectiousness to vectors using membrane feeding assay data. We estimated the proportion of sub-patent (i.e., missed by microscopy) and asymptomatic P vivax infections and examined how parasite density relates to clinical manifestations and mosquito infection in Amazonian settings.

Findings: We pooled 24,986 observations from six sites in Brazil and Peru. P vivax was detected in 6·8% and 2·1% of them by PCR and microscopy, respectively. 58·5% to 92·6% of P vivax infections were asymptomatic and 61·2% to 96·3% were sub-patent across study sites. P vivax density thresholds associated with clinical symptoms were one order of magnitude higher in children than in adults. We estimate that sub-patent parasite carriers are minimally infectious and contribute 12·7% to 24·9% of the community-wide P vivax transmission, while asymptomatic carriers are the source of 28·2% to 79·2% of mosquito infections.

Interpretation: Asymptomatic P vivax carriers constitute a vast infectious reservoir that, if targeted by malaria elimination strategies, could substantially reduce malaria transmission in the Amazon. Infected children may remain asymptomatic despite high parasite densities that elicit clinical manifestations in adults.

Funding: US National Institutes of Health, Fundação de Amparo à Pesquisa do Estado de São Paulo, and Belgium Development Cooperation.

Keywords: Amazon; Plasmodium vivax; asymptomatic infections; fever threshold; malaria; sub-patent infections.

Figure 1

Map showing the Amazon Basin in South America and the study sites in Brazil and Peru that provided data for the present analyses. The locations of the six study sites (CAH and LUP in Peru; and ACR, GRA, JAU, and REM in Brazil) are indicated.

Figure 2

Age-specific relative frequency of patent vs. sub-patent and symptomatic vs. asymptomatic Plasmodium vivax infections diagnosed by polymerase chain reaction across six studies in the Amazon. Patent (red symbols) and sub-patent (blue symbols) infections are shown in A; symptomatic (red symbols) and asymptomatic (blue symbols) P vivax infections are shown in B. Error bars indicate 95% confidence intervals of proportions We pooled data from cross-sectional surveys carried out in six sites. CAH: villages of Cahuide, La Habana, and Doce de Abril, all in Loreto, Peru.^, LUP: villages of Lupuna, Santa Rita, and San Pedro, all in Loreto, Peru.^, ACR: 7 farming settlements in Acrelândia, Acre, Brazil. GRA: Granada farming settlement, Acre, Brazil. JAU: 14 riverine villages in Jaú National Park, Amazonas, Brazil. REM: Remansinho farming settlement, Amazonas. The number of observations in each age group was as follows: CAH, 0–5 years, 1,421; 6–15 years, 3,017; 16–40 years, 3,082; 41 years and older, 2,261; total, 9,781. LUP, 0-5 years, 986; 6–15 years, 2,006; 16–40 years, 3,021; 41 years and older, 2,443; total, 8,456. ACR, 0–5 years, 261; 6–15 years, 715; 16–40 years, 856; 41 years and older, 737; total, 2,569. GRA, 6–15 years, 427; 16–40 years, 627; 41 years and older, 304; total, 1,358. (Under-five children from GRA were removed, as this age group has not been systematically sampled during the community-wide surveys.) JAU, 0–5 years, 238; 6–15 years, 280; 16–40 years, 373; 41 years and older, 152; total, 1,043. REM, 0–5 years, 205; 6–15 years, 454; 16–40 years, 605; 41 years and older, 515; total, 1,779. The symptom-free period used to define asymptomatic parasite carriage was at least 7 days before sample collection in CAH, LUP, ACR and REM, at least 30 days before and 30 days after blood collection in GRA, and at least 30 days before and 15 days after blood collection in JAU. Error bars indicate 95% confidence intervals of proportions.

Figure 3

Distribution of total Plasmodium vivax densities (parasites/μl) estimated by quantitative polymerase chain reaction in four Amazonian settings. Kernel density function estimates are shown for (A) patent (red) vs sub-patent (blue) P vivax infections and (B) symptomatic (red) vs asymptomatic (blue) P vivax infections in the CAH (Cahuide; n=491) and LUP (Lupuna; n=736) study sites in Peru,^, and in the ACR (Acrelândia; n=216) and REM (Remansinho; n=128) study sites in Brazil. Individuals reporting no fever or headache within at least 7 days prior to sample collection were defined as asymptomatic.

Figure 4

Proportion of individuals with clinical manifestations of vivax malaria by parasite density measured by quantitative polymerase chain reaction. Separate univariate logistic regression models were fitted to empirical data from Cahuide (CAH; n=491) and Lupuna (LUP; n=736) using the stats R package: (A) total population of CAH and LUP; (B) age-specific model fits for CAH; (C) age-specific model fits for LUP. Age groups considered were: 0–5 years, 6–15 years, 16–40 years, and 41 years and older. The shaded area indicates the 95% confidence intervals.

Figure 5

Proportion of Anopheles darlingi mosquitoes infected with Plasmodium vivax in membrane feeding experiments in relation to parasite density measured by microscopy. Data are from 87 independent membrane feeding assays with 10 to 64 (mean, 37•3) laboratory-reared An darlingi mosquitoes examined for oocysts per experiment. Parasite density estimates are presented separately for sexual (A) and asexual (B) blood stages. Data from symptomatic and asymptomatic blood donors are represented by red and blue dots, respectively. The continuous line shows the Hill function (appendix p 2) fitted to the data and the grey shading indicates the 95% confidence intervals.