Clinically silent Plasmodium vivax infections in native Amazonians of northwestern Brazil: acquired immunity or low parasite virulence?

Abstract

Background: Malaria remains common among native Amazonians, challenging Brazil's elimination efforts.

Objectives: We examined the epidemiology of malaria in riverine populations of the country's main hotspot - the upper Juruá Valley in Acre state, close to the Brazil-Peru border, where Plasmodium vivax accounts for > 80% of cases.

Methods: Participants (n = 262) from 10 villages along the Azul River were screened for malaria parasites by microscopy and genus-specific, cytochrome b (cytb) gene-based polymerase chain reaction. Positive samples were further tested with quantitative TaqMan assays targeting P. vivax- and P. falciparum-specific cytb domains. We used multiple logistic regression analysis to identify independent correlates of P. vivax infection.

Findings: Microscopy detected only one P. vivax and two P. falciparum infections. TaqMan assays detected 33 P. vivax infections (prevalence, 11.1%), 78.1% of which asymptomatic, with a median parasitaemia of 34/mL. Increasing age, male sex and use of insecticide-treated bed nets were significant predictors of elevated P. vivax malaria risk. Children and adults were similarly likely to remain asymptomatic once infected.

Main conclusions: Our findings are at odds with the hypothesis of age-related clinical immunity in native Amazonians. The low virulence of local parasites is suggested as an alternative explanation for subclinical infections in isolated populations.

Study site. The upper left panel shows Brazil in South America and the location of Acre state, while the lower left panel shows the municipality of Mâncio Lima (green) in the upper Juruá Valley region. The rural area of Mâncio Lima is shown in greater detail in the right panel. Rivers are represented in blue. Figure prepared by Igor C. Johansen with QGIS software version 3.14. Publicly available shapefiles were obtained from Instituto Brasileiro de Geografia e Estatística (IBGE) website (https://bit.ly/34gMq0S) and river locations from the OpenStreetMap Foundation website (https://bit.ly/3pzh4xp).