Ehrlichia, Anaplasma, and Related Intracellular Bacteria

Members of the genera Ehrlichia and Anaplasma are now recognized to be important human pathogens. Ehrlichia and Anaplasma species infect bone marrow-derived cells, such as granulocytes, monocytes, erythrocytes, and platelets, of humans and other mammals. Ehrlichia and Anaplasma spp. are Gram-negative obligate intracellular bacteria that reside and propagate within membrane-lined vacuoles in the cytoplasm of the bone marrow-derived cells. They are zoonotic agents transmitted to animals and humans by ticks. Recognized natural reservoirs for Ehrlichia chaffeensis include deer, domestic dogs, and perhaps other animals that host Amblyomma ticks. The causative agent of human monocytic ehrlichiosis (HME) is E. chaffeensis, a monocytotropic ehrlichia first identified as a human pathogen in a patient with a severe febrile illness after tick bites in 1986. The causative agent of human granulocytic anaplasmosis (HGA) is Anaplasma phagocytophilum. Other human ehrlichioses include Venezuelan human ehrlichiosis, E. ewingii ehrlichiosis, Ehrlichia muris-like agent ehrlichiosis, “Candidatus Neoehrlichia mikurensis” ehrlichiosis, and neorickettsiosis. Currently, there are 3 methods for diagnosis of acute HME or HGA: (i) PCR amplification of nucleic acids from Ehrlichia or Anaplasma species in blood, (ii) detection of morulae in the cytoplasm of infected leukocytes by Romanowsky stains (e.g., Giemsa or Wright) or by specific immunostains using E. chaffeensis or A. phagocytophilum antibodies, and (iii) culture of Ehrlichia or Anaplasma from blood or cerebrospinal fluid. Although disease can be severe or fatal, in retrospective studies and routine clinical practice, patients with either HME or HGA defervesce within 48 h of therapy with doxycycline, the drug of choice.