Ehrlichiosis is an emerging disease caused by a rickettsia: a type of bacteria that infects dogs, people, and less commonly cats. For dogs in North America and world-wide most cases of ehrlichiosis are caused by Ehrlichia canis (canine monocytotropic ehrlichiosis).1 Transmission is always from tick bites. Although people and dogs often live in close relationship to, and may come in contact with, the same species of tick, E. canis infections in people are thought to be rare.

E canis is most often transmitted by the brown dog tick (Rhipicephalus sanguineus). Infection likely requires several hours of tick attachment to the dog, and clinical signs may not appear for 1—3 weeks or longer.