ehrlichiosis ârlĭkēōˈsĭs [key], any of several diseases caused by rickettsia of the genera Ehrlichia and Anaplasma. Ehrlichiosis is transmitted by ticks. Both human forms tend to develop about nine days after a tick bite. Symptoms include severe headache and chills and low white blood cell and platelet counts. The lack of a rash distinguishes them from Rocky Mountain spotted fever and Lyme disease; lack of upper respiratory and gastrointestinal symptoms distinguishes them from influenza. Many cases are mild, and all are treatable with antibiotics (tetracycline and doxycycline); however, ehrlichiosis can be fatal in some cases when diagnosis and treatment are delayed.

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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