Introduction to Ehrlichia spp.

    Ehrlichiae are small gram-negative pleomorphic cocci that are obligatory intracellular bacteria. Ehrlichiae replicate in the membrane-bound vacuoles (parasitophorous vacuoles) in the cytoplasm of a specific type of host cells, chiefly granulocytes or monocytes. Ehrlichial organisms are vector-borne; they replicate in the tick or the trematode and are horizontally transmitted from infected cells in vectors to the blood cells of animals or humans.
    Once thought only veterinary pathogens, recently several Ehrlichia spp. have been recognized as emerging human pathogens in the U.S. Human monocytic ehrlichiosis (HME), caused by Ehrlichia chaffeensis a new monocytotropic ehrlichia, was discovered in 1986 and human granulocytic ehrlichiosis (HGE), caused by the HGE agent (newly named Anaplasma phagocytophilum), a new granulocytotropic ehrlichia, was discovered in 1994. In 1999 another granulocytotropic ehrlichia, E. ewingii which was previously known as a canine pathogen has been recognized as human pathogen. E. sennetsu (renamed to Neorickettsia sennetsu) is another monocytotropic Ehrlichia sp. and the first human pathogen discovered in Japan in the 1950’s and recently found in Malaysia.