Hidden Infections, Emerging Resistance: Underdiagnosed <i>Plasmodium vivax</i> and Multidrug Resistance-1 ( <i>pvmdr1</i> ) Gene Amplification in the Guiana Shield

BACKGROUND: Plasmodium vivax is the most widespread malaria parasite, and its co-circulation with Plasmodium falciparum often leads to underdiagnosis and underestimated transmission. We investigated whether underdiagnosis contributes to the selection and spread of drug-resistant parasites in co-endemic areas of the Brazilian Amazon. METHODS: We enrolled 727 confirmed malaria cases from three Brazilian states, primarily from Roraima in the Guiana Shield, a region characterized by co-circulation of P. vivax and P. falciparum, intense cross-border mobility, illegal mining activities, and informal antimalarial drug use. To examine indirect P. vivax exposure to antimalarial targeting P. falciparum, we evaluated pvmdr1 copy number variation (CNV). RESULTS: In Roraima, molecular diagnosis revealed frequent misclassification of mixed infections, with P. vivax often missed by microscopy due to higher P. falciparum densities, demonstrating the limitations of microscopy in accurately detecting mixed infections. The pvmdr1 CNV exhibited significant geographic variation. While nearly all isolates from the western Amazon carried a single copy, pvmdr1 amplification was detected in 60% of Roraima isolates. In addition to this spatial heterogeneity, temporal analysis revealed significant shifts in amplification patterns over the years, rising to over 70% after 2020. CONCLUSIONS: These findings emphasize the critical importance of accurate diagnosis and rational use of antimalarials, as inadequate or unintended drug exposure may inadvertently drive the selection and spread of parasite strains with reduced drug susceptibility.