Rickettsiales

Members of the order Rickettsiales of the class Alphaproteobacteria are Gram-negative, obligately intracellular bacteria transmitted primarily by arthropod vectors. The order Rickettsiales is composed of two families, Anaplasmataceae and Rickettsiaceae. These families have major genera Anaplasma, Ehrlichia and Neorickettsia and Rickettsia that contain multiple pathogens of importance to veterinary medicine. This chapter summarizes the current knowledge of major rickettsial pathogens and associated diseases in livestock and companion animals, emphasizing disease pathogenesis, immunity, and immunopathology. The immunopathologic mechanisms involved in monocytic ehrlichiosis have been studied most extensively with the natural murine ehrlichial pathogen, E. japonica, which causes an aggressive and lethal infection. Rickettsiae that escape innate immune mechanisms are controlled by the adaptive immune response with cytotoxic CD8 T cells being the primary effector cell.