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ehrlichiosis[ArlikEO´sis] Pronunciation Key, any of several diseases caused by rickettsia of the genus Ehrlichia. 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 species (some believe them to be variant strains of a single species) can cause disease in animals, for example E. canis 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. The Ehrlichia organisms invade various white blood cells (see blood; immunity). E. chaffeensis invades monocytes; granulocytic Ehrlichia invades granulocytes.
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