Anaplasma phagocytophilum. Anaplasma phagocytophilum (formerly Ehrlichia phagocytophilum) is a gramnegative bacterium that is unusual in its tropism to neutrophils. It causes anaplasmosis in sheep and cattle, also known as tickborne fever and pasture fever, and also causes the zoonotic disease human granulocytic anaplasmosis.