Equine granulocytic anaplasmosis.

Anaplasma phagocytophilum is an emerging pathogen of horses that is transmitted by Ixodid ticks. Recent studies suggest that multiple strains of A. phagocytophilum may be circulating in wild and domestic animal populations, and these strains may have differential host tropisms and pathogenicity. The organism infects and survives within neutrophils. Co-infections with other tick-borne pathogens may occur, especially Borrelia burgdorferi. A. phagocytophilum causes an acute febrile illness in horses with lethargy, inappetence, lameness and hemorrhages. Diagnosis is based on finding morulae within granulocytes in the peripheral blood, and detection of the DNA of A. phagocytophilum using specific polymerase chain reaction assays. Most reports suggesting that anaplasmosis is a self-limiting disease that responds well to a tetracycline therapy.