Anaplasma phagocytophilum is a gram-negative intracellular bacterium that transmitted by hard ticks. A. phagocytophilum infect and multiply in the organs of ticks, in particular the salivary glands which enable the transmission to vertebrate hosts during feeding. The tick becomes infected by feeding on an infected host and there is transstadial but not transovarial passage of the organism. The majority of ticks are infected with the organism in enzootic areas. There are strains of A. phagocytophilum that have biological and ecological difference, including variations in host pathogenicity, vectors and geographical distribution. The organism has an interesting feature to grow in neutrophils by stopping the antibacterial activity of neutrophils. The bacterium is able to survive in the immune host, using complex mechanisms of antigenic variation. A. phagocytophilum infects humans and various animal species including dogs, sheep, cows, horses, wild deer and rodents. The disease is known as human granulocytic anaplasmosis in humans, canine granulocytic anaplasmosis in dogs, equine granulocytic anaplasmosis in horse and tick borne fever in ruminants. Cattle tick borne fever caused by A. phagocytophilum is characterized by high fever, reduced milk yield, inclusions in circulating neutrophils, leukopenia, abortions, reduced fertility, coughing, respiratory signs and swelling of the hind limbs. Clinical signs of human occur a week after the tick bites, the disease usually presents as an acute, sometimes fatal febrile syndrome, illness characterized by headache, chills, myalgias, arthralgia, malaise, and hematological abnormalities, such as neutropenia, lymphocytopenia, thrombocytopenia, leukopenia, and elevated hepatic aminotransferase levels and may lead to death. In this review article the history, bacteriology, epidemiology, pathogenesis, diagnosis, treatment and prevention of the disease caused by A. phagocytophilum is written based on the latest scientific findings. Several hard tick species are distributed in Iran and they are the most important ectoparasites of animals. A. phagocytophilum has been detected not only in Ixodes ricinus but also in cattle and sheep of Iran using molecular techniques. However, despite the zoonotic potential of the agent, there is no evidence in the identification of A. phagocytophilum in humans, and it seems necessary to research on the prevalence and epidemiology of the disease in the human population.
