Anaplasma phagocytophilum is a Gram-negative obligate intracellular bacterium of the order Rickettsiales (Goodman et al. 1996; Dumler and Bakken 1998; Demma et al. 2005). It is the causative agent of tick-borne fever (TBF), an emerging zoono-sis in the United States and in other regions of the world. This infection is characterized by fever, headache, myalgias, thrombocytopenia, and leukopenia. The principal reservoirs are sheep, cattle, and goats. In humans, this pathogen causes granulocytic anaplasmosis. Bacterial invasion of neutrophil granulocytes is the hallmark of the disease, but other cells such as endothelial cells are also infected by this pathogen (Goodman et al. 1996; Munderloh et al. 2004; Herron et al. 2005).
One of the principal characteristics of this obligate intracellular bacterium is its ability to survive inside the harsh environment of the neutrophil by seizing host cell functions and regulating critical antimicrobial activities. For example, it has been demonstrated that neutrophils of sheep infected with TBF have a reduced phago-cytic capacity, a situation that may further alter neutrophil function and predispose the host to opportunistic infections and dysregulation of inflammation.
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