Ehrlichiosis is caused by several bacterial species of the genus Ehrlichia. Ticks like the Brown Dog tick (Rhipicephalus sanguineus), the Lone Star tick (Amblyomma americanum) and the American Dog tick (Dermacentor variabilis) are the main transmitters of these pathogens. Although the clinicopathologic course of the disease will vary depending upon the infecting Ehrlichia species, illness is typically characterised by an acute reduction in cellular blood elements, most often thrombocytopenia.

The canine monocytic ehrlichiosis (CME) is caused by Ehrlichia canis and characterised by three clinical phases. The infectious agent is widely distributed in warm climates and causes severe clinical symptoms in infected dogs. The life cycle of E. canis implicates ticks as vectors and mammals as host. Generally all members of the family Canidae can serve as host whereas ticks like the Brown Dog tick and in the USA presumably also the American Dog tick, are the main transmitters of the pathogen.

Historical synonyms for canine ehrlichiosis include canine rickettsiosis, canine typhus, tropical canine pancytopenia, idiopathic hemorrhagic syndrome, canine hemorrhagic fever and tracker dog disease.

In humans, human monocytic ehrlichiosis (HME) is caused by Ehrlichia chaffeensis. The disease possesses zoonotic character. Furthermore E. ewingii has also been isolated from a human patient and thus is as well of zoonotic potential. In the USA, the disease occurs primarily in the south-eastern and south-central regions of the country, however due to chronic infection and a prolonged incubation period, infection is frequently documented in dogs that move from endemic to non-endemic regions.

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