What is it about?
Our pooled analysis of 34 population-based cross-sectional surveys in riverine villages and farming settlements in Brazil and Peru showed that 70.9% of P vivax infections are asymptomatic and 69.7% are sub-patent. Importantly, young children remain asymptomatic at P vivax density levels that usually elicit clinical symptoms in adults. This is the first evidence of age-specific parasite density thresholds associated with clinical illness in populations naturally exposed to P vivax. An darlingi infection rates are very low at sub-patent parasite densities but reach 50% at 2,300 parasites/μL. Asymptomatic carriers of P vivax are estimated to be the source of 28.2% to 79.2% mosquito infections across study sites, while sub-patent infections are responsible for smaller proportions of the community-wide transmission, estimated at 12.7% to 24.9% across study sites.
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Why is it important?
Asymptomatic carriers constitute a critical yet neglected infectious reservoir that fuels residual P vivax transmission in the Amazon. Malaria elimination policies in the region require active case detection strategies. Importantly, parasite densities that contribute significantly to P vivax transmission are typically above the microscopy detection threshold and may not require more complex and expensive molecular methods to be routinely detected.
Perspectives
We investigated the relative contribution of asymptomatic infections to Plasmodium vivax transmission in the Amazon using data from previous cohort studies. To this end, we combined rich datasets from epidemiological studies with results from experiments conducted in laboratory with local mosquitoes. Our goal was to quantify an unmeasurable quantity using only local data.
Rodrigo M. Corder
Universidade de Sao Paulo Campus da Capital
Read the Original
This page is a summary of: 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, The Lancet Regional Health - Americas, May 2022, Elsevier,
DOI: 10.1016/j.lana.2021.100169.
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Resources
Modelling the epidemiology of residual Plasmodium vivax malaria in a heterogeneous host population: A case study in the Amazon Basin
Malaria transmission models that disregard risk heterogeneity at the community level, classifying individuals as uniformly susceptible or infected, may not properly recapitulate the epidemiology of malaria in real-life settings. Here we fit a compartmental susceptible-infected-susceptible model to malaria morbidity data from Mâncio Lima, the main urban transmission hotspot of Brazil, and estimate that 20% of the urban residents contribute 86% of the overall vivax malaria burden in the town. Despite the low average force of infection, one order of magnitude lower that in rural Africa, high-risk individuals experience enough repeated infections to develop clinical immunity and constitute an asymptomatic reservoir that fuels onwards malaria transmission. Therefore, these high-risk subjects account for the paradoxical finding of clinical immunity and frequent asymptomatic parasite carriage in low-endemicity Amazonian communities. We argue that mathematical models accounting for risk heterogeneity are crucial to plan and evaluate malaria control and elimination interventions targeted to high-risk groups in communities, municipalities, and regions.
Individual variation in Plasmodium vivax malaria risk: Are repeatedly infected people just unlucky?
Extensive research has examined why some people have frequent Plasmodium falciparum malaria episodes in sub-Saharan Africa while others remain free of disease most of the time. In contrast, malaria risk heterogeneity remains little studied in regions where P. vivax is the dominant species. Are repeatedly infected people in vivax malaria settings such as the Amazon just unlucky? Here, we briefly review evidence that human genetic polymorphism and acquired immunity after repeated exposure to parasites can modulate the risk of P. vivax infection and disease in predictable ways. One-fifth of the hosts account for 80% or more of the community-wide vivax malaria burden and contribute disproportionally to onward transmission, representing a priority target of more intensive interventions to achieve malaria elimination. Importantly, high-risk individuals eventually develop clinical immunity, even in areas with very low or residual malaria transmission, and may constitute a large but silent parasite reservoir.
Contributors
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