Spatial patterns of mosquito communities and monkey malaria vectors in a tropical riverine forest in Sabah, Malaysian Borneo

Background Understanding how vector ecology intersects with host behaviour is essential for predicting zoonotic malaria risk in tropical ecosystems. In Malaysian Borneo, zoonotic transmission of simian malaria parasites such as Plasmodium knowlesi is shaped by interactions among mosquito vectors, non-human primate reservoir hosts, and human activities across heterogeneous landscapes. However, fine-scale spatial structuring of mosquito communities within tropical riverine forests remains poorly characterized. 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 simian malaria parasites in Sabah. Molecular screening detected simian Plasmodium DNA in two An. balabacensis individuals, including P. knowlesi , together with other simian Plasmodium species. Conclusions/Significance Our findings suggest that river-edge habitats may function as localized contact hotspots where mosquito vectors, 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. Author Summary Zoonotic malaria caused by monkey malaria parasites such as Plasmodium knowlesi has become an important public health concern in Malaysian Borneo. Transmission occurs when mosquito vectors feed on both infected non-human primates and humans, but the ecological conditions that increase opportunities for such contact remain poorly understood. In this study, we investigated mosquito communities in a tropical riverine forest along the Menanggul River in Sabah, Malaysia, an area where wildlife, human activities, and ecotourism frequently overlap. Over a two-year period, we repeatedly sampled mosquitoes across different distances from the river and at both ground and canopy levels. Although mosquitoes capable of transmitting simian malaria were relatively rare, they were detected more frequently near river edges and in ground-level traps. We also detected simian Plasmodium DNA in two Anopheles balabacensis mosquitoes, a major vector of zoonotic malaria in Sabah. Our findings suggest that river-edge habitats may act as localized “contact hotspots” where vectors, primate hosts, and humans repeatedly overlap in space and time, potentially increasing opportunities for zoonotic malaria transmission even when overall vector abundance is low. These results highlight the importance of considering wildlife ecology, human behaviour, and landscape structure together when assessing and managing zoonotic disease risk in tropical forest systems.