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By Alan R Walker – Own work, CC BY-SA 3.0,

OpenStax Microbiology

The zoonotic tickborne disease human granulocytic anaplasmosis (HGA) is caused by the obligate intracellular pathogen Anaplasma phagocytophilum. HGA is endemic primarily in the central and northeastern US and in countries in Europe and Asia.

HGA is usually a mild febrile disease that causes flu-like symptoms in immunocompetent patients; however, symptoms are severe enough to require hospitalization in at least 50% of infections and, of those patients, less than 1% will die of HGA. Small mammals such as white-footed mice, chipmunks, and voles have been identified as reservoirs of A. phagocytophilum, which is transmitted by the bite of an Ixodes tick. Five major virulence factors have been reported in Anaplasma; three are adherence factors and two are factors that allow the pathogen to avoid the human immune response. Diagnostic approaches include locating intracellular microcolonies of Anaplasma through microscopic examination of neutrophils or eosinophils stained with Giemsa or Wright stain, PCR for detection of A. phagocytophilum, and serological tests to detect antibody titers against the pathogens. The primary antibiotic used for treatment is doxycycline.


Parker, N., Schneegurt, M., Thi Tu, A.-H., Forster, B. M., & Lister, P. (n.d.). Microbiology. Houston, Texas: OpenStax. Access for free at: