Ehrlichiae are small, gram-negative cocci that stain dark blue to purple with Romanowsky stain. Ehrlichiae are generally round but sometimes highly pleomorphic. The organisms are found in host membrane-lined vacuoles within the cytoplasm of infected eukaryotic host cells, primarily leukocytes. Ehrlichiae have distinct ribosomes and a fine meshwork of DNA strands. Clumps of ribosomes are found in the cytoplasm as well as beneath the cytoplasmic membrane. Small (0.2 to 0.4 µm), dense forms resembling chlamydial elementary bodies and relatively large, light forms (0.8 to 1.5 µm) resembling reticular bodies have been noted in several species of Ehrlichia; however, a chlamydia-like life cycle has not been demonstrated.

    The relatedness of 16S rRNA gene sequences of the genus Rickettsia to those of Ehrlichia, Cowdria, Anaplasma, and Wolbachia spp. is 83-84%. Ehrlichia spp. are as divergent among themselves as between Erhlichia spp. and Rickettsia spp. 16S rRNA gene sequences revealed that Ehrlichia spp. along with several related genera can be divided into three distinct genetic groups, with about 7-15% sequence divergence between the groups. Group 1 is renamed as genus Ehrlichia consisting of former Ehrlichia canis, E. chaffeensis, E. muris, E. ewingii, and Cowdria ruminantium (renamed to Ehrlichia ruminantium). Group 2 is renamed as genus Anaplasma which consists of former E. equi, E. phagocytophilum, human granulocytic ehrlichiosis (HGE) agent, and E. platys, which infects platelets. Since a maximum of only three nucleotides are different among E. equi, E. phagocytophilum, and HGE agent, these three belong to the same species. Anaplasma marginale, which has a tropism for erythrocytes, is closest to Group 2 in 16S rRNA gene sequence comparison (97% similarity). Group 3 consists of E. sennetsu, E. risticii, Stellantchasmus falcatus agent (SF agent) isolated from the fluke S. falcatus (a fish parasite) in Japan, and Neorickettsia helminthoeca. Group 3 is now renamed as genus Neorickettsia. Members of the genus Wolbachia and several insect symbionts, which are not known to cause a disease in vertebrates, fit among Ehrlichia spp. According to 16S rDNA sequence comparison, genera Haemobartonella and Eperythrozoon of the family Anaplasmataceae, are not related to the order Rickettsiales at all and now reclassified in Mycoplasma sp. Full nucleotide sequences of HSP60 gene have been determined for several ehrlichial species and these sequences show a relatedness similar to that observed among the 16S rRNA gene sequences, though sequence variation is greater (please read next section (Phylogram of the Family Anaplasmataceae) for more details).

    New genus Ehrlichia, except for E. ewingii and E. ruminantium, infects monocytes and macrophages. E. ewingii infects granulocytes and C. ruminantium infects vascular endothelial cells and neutrophils. The genus Anaplasma group infect granulocytes, platelets, red blood cells. The Neorickettsia group infects monocytes and macrophages. N. risticii in addition infects intestinal epithelial cells and mast cells.