Ehrlichia chaffeensis, an intracellular gram-negative zoonotic bacterium, is the 
causative agent of human monocytotropic ehrlichiosis (HME). In humans, the 
disease can range from a mild, non-specific illness with few to no clinical 
signs to a moderately severe to fatal disease, especially those with compromised 
immune systems. E. chaffeensis is maintained in a complex cycle involving 
white-tailed deer (WTD; Odocoileus virginianus) as a primary reservoir and the 
lone star tick (LST; Amblyomma americanum) as a primary vector. Numerous other 
species are naturally exposed to E. chaffeensis and disease has been documented 
in some domestic animals and wildlife including domestic dogs and ring-tailed 
lemurs. The organism has been found throughout the natural range of the LST and 
as the tick continues to expand its range, the geographic range of risk for E. 
chaffeensis infections will likely continue to expand. Recent data have 
indicated that E. chaffeensis, or a closely related organism, has been found in 
many species of ticks and vertebrate hosts in numerous countries.