Members of two genera can cause clinical ehrlichiosis in the United States. Ehrlichia and Anaplasma species belong to the family Anaplasmat-aceae. Ehrlichia and Anaplasma are gram negative, obligate intracellular bacteria that replicate in the vacuoles of eukaryotic cells. In humans, the ehrlichial infections are named based on the type of white blood cell that is infected, i.e., monocytic or granulocytic. Anaplasmaphagocytophilum, known prior to 2001 simply as the HGE agent, is the causative organism of HGE. Also, several cases of human ehrlichiosis in Missouri have been attributed to infection with E. ewingii, the causative agent of granulocytic ehrlichiosis in dogs (85,86).

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