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, while E. chaffeensis and EML are typically in monocytes and macrophages.
Ehrlichiosis is transmitted by the lone star tick, Amblyomma americanum.
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. There was a decrease in cases in 2020 most likely due to the pandemic with cases remaining lower than normal in 2021 as seen in the chart on the right.
