Intermediate forest cover and malaria risk in an Amazon deforestation frontier.

Malaria exposure risk, measured by anopheline infection rates, appears highest in Amazonian landscapes with approximately 50 % remaining forest cover, a common condition across the region. However, the effects of landscape configuration on the density of infected vectors and human malaria incidence remain poorly understood. A pilot natural experiment was carried out across 40 sites in a deforestation frontier in Acre state, Brazilian Amazon, where malaria is endemic. Sites were selected to represent a gradient of forest cover and deforestation levels under controlled conditions. We collected data on mosquito vectors (abundance and infection rates) and human malaria infections to assess how landscape structure influences transmission. Sites with intermediate forest cover, often associated with high fragmentation, exhibited higher Nyssorhynchus mosquito abundance and greater anopheline infection rates, leading to increased risk of Plasmodium vivax and P. falciparum infections. All human infections detected were asymptomatic (n = 16), representing a persistent reservoir for transmission and a major obstacle to malaria elimination. Our findings suggest that malaria transmission may persist where intermediate forest cover dominates, even with antimalarial drug-based interventions. Elimination strategies should integrate radical cure treatments with alternative vector control approaches, especially for outdoor-biting Nyssorhynchus anophelines. Depending on the amount of existing forest cover, landscape-based strategies (forest conservation/restoration payments) may help naturally regulate vector populations and support both malaria elimination and Amazon rainforest preservation.