METHODOLOGY/PRINCIPAL FINDINGS: We conducted a two-year mosquito survey (November 2016-October 2018) in a lowland riverine forest along the Menanggul River, Sabah, Malaysian Borneo. Mosquitoes were sampled on 44 nights using CO2-baited light traps repeatedly deployed across horizontal (0 m, 250 m, and 500 m from the river) and vertical (ground and canopy) gradients, yielding 244 trap collections. A total of 9,928 mosquitoes were collected, dominated by Culex spp. (91.4%), whereas Anopheles spp. were rare (153 individuals; 1.5%). Female Anopheles mosquitoes (n = 57) were more frequently detected near the river and tended to occur more often in ground traps. Zero-inflated negative binomial generalized linear mixed models based on the female mosquito dataset revealed significant effects of river distance and trap height on mosquito abundance, while short-term microclimatic variation showed little explanatory power. The Anopheles assemblage was dominated by Anopheles balabacensis, a recognized vector of monkey malaria parasites in Sabah. Molecular screening detected monkey Plasmodium DNA in two An. balabacensis individuals, including P. knowlesi, together with other monkey Plasmodium species.
CONCLUSIONS/SIGNIFICANCE: Our findings suggest that river-edge habitats may function as localized contact hotspots where mosquito vectors, non-human primate reservoir hosts, and human activities repeatedly overlap in space and time, potentially facilitating zoonotic malaria transmission even when vector densities are low. These results support the growing view that zoonotic malaria risk in tropical landscapes is shaped less by overall vector abundance than by fine-scale ecological overlap among vectors, wildlife, and humans.