Anaplasmosis is caused by several bacterial species of the genus Anaplasma. From their reservoir hosts (e.g. mice, deer, possibly birds) the bacteria are transmitted by ixodid ticks like the Castor Bean tick (Ixodes ricinus), the Deer tick (Ixodes scapularis), the Western black-legged tick (Ixodes pacificus) and the Brown Dog tick (Rhipicephalus sanguineus). In general, anaplasmosis leads to milder disease than monocytic ehrlichiosis caused by Ehrlichia canis and appears to be largely a self-limiting infection in dogs.
Anaplasma phagocytophilum is the most important representative and has been detected in blood samples from a wide range of wild and domestic animals. It can cause an acute febrile illness e.g., granulocytic anaplasmosis in dogs, cats and horses. In humans, the pathogen is responsible for human granulocytic anaplasmosis (HGA), formerly known as human granulocytic ehrlichiosis (HGE).