An efficient microinjection method to generate human anaplasmosis agent Anaplasma phagocytophilum-infected ticks

Ticks are important vectors that transmit several pathogens including human anaplasmosis agent, Anaplasma phagocytophilum. This bacterium is an obligate intracellular rickettsial pathogen. An infected reservoir animal host is often required for maintenance of this bacterial colony and as a source for blood to perform needle inoculations in naïve animals for tick feeding studies. In this study, we report an efficient microinjection method to generate A. phagocytophilum-infected ticks in laboratory conditions. The dense-core (DC) form of A. phagocytophilum was isolated from in vitro cultures and injected into the anal pore of unfed uninfected Ixodes scapularis nymphal ticks. These ticks successfully transmitted A. phagocytophilum to the murine host. The bacterial loads were detected in murine blood, spleen, and liver tissues. In addition, larval ticks successfully acquired A. phagocytophilum from mice that were previously infected by feeding with DC-microinjected nymphal ticks. Transstadial transmission of A. phagocytophilum from larvae to nymphal stage was also evident in these ticks. Taken together, our study provides a timely, rapid, and an efficient method not only to generate A. phagocytophilum-infected ticks but also provides a tool to understand acquisition and transmission dynamics of this bacterium and perhaps other rickettsial pathogens from medically important vectors.