Ehrlichiosis is caused by at least three different ehrlichial 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 cultured or specific molecular testing is performed. E. ewingii is found in granulocytes.
Ehrlichiosis is transmitted by the Amblyomma americanum (Lone Star) tick, whose geographic distribution is provided in the map on the top right.
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. More than 50% of cases are acquired in Arkansas, Missouri, North Carolina, and New York. Two thousand ninety-three cases were reported to the CDC in 2019, displayed in the chart on the bottom right.