Ehrlichia canis is an obligate, intracellular bacterium that acts as the causative agent of Ehrlichiosis, a disease most commonly affecting canine species. This pathogen is present throughout the United States (but is most prominent in the South), South America, Asia, and Africa. First defined in 1935, E. canis emerged in the United States in 1963 and its presence has since been found in all 48 contiguous United States. Reported primarily in dogs, E. canis has also been documented in felines and humans where it is transferred most commonly via Rhipicephalus sanguineus, the brown dog tick.