Amblyomma americanum map/CDC
Reported cases ehrlichiosis in US 2013/CDC
Ehrlichiosis is caused by at least three different ehrlicial species in the United States: Ehrlichia chaffeensis, E. ewingii, and a species provisionally called Ehrlichia muris-like (EML). E. chaffeensis and E. ewingii are difficult to distinguish serologically, but may be distinguished if they are cultured or specific molecular testing is performed. E. ewingii is found in granulocytes.
Ehrlichia are intracellular bacterial parasites that form microcolonies in the cytoplasm of white blood cells (WBCs) called morulae. The disease caused by E. chaffeensis is called human monocytotropic ehrlichiosis. It is mostly seen in the Southeastern and South Central United States. 35% of cases are acquired in Oklahoma, Arkansas, and Missouri. About a thousand cases were reported to the CDC in 2014.
Ehrlichiosis is transmitted by the Amblyomma americanum (Lone Star) tick.