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Wireless sensor-actuator networks (WSANs) have recently been suggested as an extension to conventional sensor networks. The powerful and mobile actuators can patrol along different routes and communicate with the static sensor nodes. Obviously, it is crucial to optimize the routes for the actuators to collect the sensor data in a timely fashion. Given the nonuniform and time-varying distribution of sensors and events in large networks, the route design has to be dynamic and scalable as well as balance the loads of the actuators. In this paper, we propose probabilistic route design (PROUD), which is an effective and adaptive algorithm for weight-differentiated route calculation. PROUD constructs an a priori route that covers the sensor locations, following which, the actuators probabilistically and cyclically visit the sensor locations according to their weights. We show that this probabilistic approach adapts well to network dynamics without frequent recalculation of the whole route. It works for both small-scale sensor-actuator networks and large-scale sensor-actuator networks with partitioning. We further develop a distributed implementation of PROUD and extend it to accommodate actuators with variable speeds. Finally, we devise a multiroute improvement and a task-exchange algorithm that enable load balancing. Our performance evaluation shows that PROUD effectively reduces the overall data-collection time and evenly distributes the energy consumption across the actuators, as compared with other state-of-the-art solutions.