Several species in the genus Ehrlichia can cause disease in humans. Human illness caused by the pathogen Ehrlichia chaffeensis is referred to as Human Monocytic Ehrlichiosis (HME). It is transmitted by Amblyomma americanum, also known as the lone star tick, which is one of the most commonly encountered ticks in the southeastern United States. Ticks become infected by feeding on white-tailed deer or other animals and can transmit the disease to humans or dogs at a subsequent feeding. What was originally thought to be a second species of Ehrlichia was recently reclassified as Anaplasma phagocytophilum, which Anaplasmosis or Human Granulocytotropic Anaplasmosis (HGA) in humans and is transmitted by Ixodes species ticks, such as I. scapularis, the black-legged tick that transmits Lyme Disease.
HME caused by E. chaffeensis is found primarily in the southeastern part of the U.S. Cases are commonly reported from Missouri, Oklahoma, and Arkansas. Areas with reported HGA cases correspond with the geographic distribution of Lyme disease. In Florida, 89 cases of HME were reported from 2002 through 2011. 33 cases of HGA were reported. The majority of HME cases (73%) are reported as being acquired in Florida, primarily in the north and central parts of the state. HGA is more likely to be acquired outside Florida and is more prevalent in the northeast United States. Like Lyme disease HGA has less than half (45%) of cases classified as Florida-acquired. The number of cases increased during the spring and summer months, though cases are reported in Florida year-round.
McQuiston JH, McCall CL, Nicholson WL. Zoonosis Update: Ehrlichiosis and related infections. JAVMA 2003.