Introducing mobile data sinks into wireless sensor networks (WSNs) improves the energy efficiency and the network lifetime, and is demanded for many application scenarios, such as battlefield vehicle security, mobile data acquisition, and cellular phone based sensor networks. However, highly mobile sink nodes cause frequent topology changes, resulting in high packet loss rate and poor energy efficiency of traditional reactive WSN routing algorithms. A directional-antenna-assisted reactive routing protocol for WSNs, IDDA (Interest Dissemination with Directional Antenna) is introduced to resolve this problem. Different from traditional interest diffusion routing protocols, IDDA exploits the antenna directivity to prearrange interest dissemination along the direction of motion. IDDA enhances important performance metrics in a target detection application scenario, namely, energy efficiency, packet delivery ratio, and target detection ratio. An analytical model is established to calculate the optimal width of the antenna beam pattern and optimal transmitting power. Extensive simulation results show that IDDA outperforms the traditional directed diffusion protocol in all three aforementioned metrics, which guarantees that IDDA can be applied to WSNs with highly mobile data sink nodes.