Anaplasma phagocytophilum (formerly Ehrlichia phagocytophilum) is a Gram-negative bacterium. It causes anaplasmosis in sheep and cattle, also known as tick-borne fever and pasture fever, and also causes the zoonotic disease human granulocytic anaplasmosis.
In animals, the disease is multisystemic, but the most severe changes are anaemia and leukopenia. This organism causes lameness, which can be confused with symptoms of Lyme disease, another tick-borne illness. It is a vector-borne zoonotic disease whose morula can be visualized within neutrophils (a type of white blood cell) from the peripheral blood and synovial fluid. It can cause lethargy, ataxia, loss of appetite, and weak or painful limbs.
In Humans, A. phagocytophilum causes human granulocytic anaplasmosis (HGA). Since 1990, incidence of HGA has increased, and it is now recognized in Europe. During the last stage of infection, groups of small bacteria can be seen within neutrophils . Other symptoms include fever, headache, absence of skin rash, leucopenia, thrombocytopenia, and mild injury to the liver.