Canine monocytic ehrlichiosis (CME) due to Ehrlichia canis is a widespread disease in tropical and temperate areas of the world, like in the USA, Europe (Mediterranean) and Africa. The geographical distribution of E. canis has expanded with the distribution of the primary tick vector, the Brown Dog tick, Rhipicephalus sanguineus, which spends all three stages of its life cycle on dogs.
E. chaffeensis and E. ewingii have been diagnosed in dogs only within the USA. Both species are encountered most frequently in dogs living in the southern states. Recently they have also been reported from Africa and Korea.
Natural reservoirs of Ehrlichiae
The Brown Dog tick is the known main vector of E. canis. In the USA the American Dog tick (Dermacentor variabilis) is also suspected as an additional vector, but epidemiological importance is unclear. Canines, both wild and domestic, are susceptible and represent reservoirs for the organism.
The Lone Star tick, Amblyomma americanum, is the most important vector for E. chaffeensis and E. ewingii in the USA. Deer serve as a major reservoir for both of these organisms and transmission to dogs and humans is more likely in areas that support a large deer population. For E. chaffeensis, the American Dog tick (Dermacentor variabilis) is also reported as vector.
As transovarial transmission in ticks is not confirmed for E. canis so far, ticks are not considered as true reservoir, whereas mammals are representing a reservoir. Especially in infected dogs, E. canis might persist for years.
The zoonotic role of dogs as a reservoir for human infection has not been clearly established for any Ehrlichia species. In South America, E. canis has been reported to cause human monocytic ehrlichiosis (HME) with dogs as the probable reservoir host. Besides deer, rodents and other small mammals may serve as major reservoirs for the other Ehrlichia species, with dogs playing only a minor role in the maintenance of the organism in a given geographic location.