Ehrlichiosis is a disease caused by a group of rickettsiae, known as ehrlichiae, which parasitize circulating leukocytes of man and a variety of domestic and wild animals. A characteristic morphologic feature of ehrlichiae is their occurrence in membrane-bound vacuoles in the cytoplasm of leukocytes, forming inclusions that contain variable numbers of organisms. The tick is the apparent vector of ehrlichiae, but such evidence is not available for all species.