It was known for years that certain Ehrlichia species (some believe them to be variant strains of a single species) can cause disease in animals, for example E. canis and E. ewingii in dogs and E. phagocytophila in sheep and cattle. In the mid-1980s human ehrlichiosis was first recognized. The causative agent was found to be E. chaffeensis. This form is now known as human monocytic ehrlichiosis. In 1990 another form of the disease, human granulocytic ehrlichiosis, was identified. Althought the disease is now known as human granulocytic anaplasmosis (E. phagocytophilum, the causative agent, has been reclassified as Anaplasma phagocytophilum), the term ehrlichiosis continues to be used generically to describe the infection. Other species of Ehrlichia, such as E. ewingii, also cause ehrlichiosis in humans. The organisms invade various white blood cells (see blood; immunity). E. chaffeensis commonly invades monocytes; A. phagocytophilum and E. ewingii invade granulocytes.
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