**3. Vector Competency and Oxidative Stress**

Vector competence (also termed vector potential) refers to the ability of arthropods to transmit pathogens, which is greatly influenced by the genetic and/or other intrinsic factors of arthropod vectors [46]. Additionally, it is also governed by the factors exerted by hosts themselves during pathogen inoculation, development, and propagation in particular hosts. During an infection, ROS have pivotal roles in the triangular relationship among vectors, pathogens, and hosts and may influence the triad either positively or negatively. A pluripotent molecule isolated from the salivary glands of *H. longicornis,* called longistatin [47], plays a central role in the feeding and development of ticks [3,4,9,48] and has been elegantly shown to ameliorate cellular ROS production in human endothelial cells [5], making it a key molecule in the survival of hard ticks. On the other hand, the acquisition of pathogens into a vector also induces modification of the normal ROS production resulting in oxidative stress to arthropod cells, which ultimately is being utilized by hematophagous arthropods to eliminate invading pathogens. Therefore, vector competence largely depends on a smart balance of the ROS that ensures the entrance, survival, and proliferation of pathogens into a vector. At the same time, maintenance of an optimum level of ROS is essential for the assurance of survival of the arthropods themselves to allow the feeding behavior that eventually ensures pathogen transmission [32,49–51].
