A disease of mammals caused by a specialized group of gram-negative bacteria in the order Rickettsiales, family Anaplasmataceae, genus Anaplasma. Anaplasma is an obligate intracellular parasite that infects erythrocytes (red blood cells) of cattle, sheep, goats, and wild ruminants in much of the tropical and subtropical world. A newly reclassified species that infects the white blood cells of mammals was recently added to this genus. However, the most important species and the one traditionally associated with the disease anaplasmosis is A. marginale. Anaplasmosis is one of the most important diseases of cattle, causing anemia and sometimes death, resulting in significant economic losses worldwide.