Human monocytic ehrlichiosis is a tick-borne infectious disease transmitted by several tick species, especially Amblyomma spp caused by Ehrlichia chaffeensis. E. chaffeensis is an obligatory intracellular, tick-transmitted bacterium that is maintained in nature in a cycle involving at least one and perhaps several vertebrate reservoir hosts. Two additional Ehrlichia spp, Anaplasma (formerly Ehrlichia) phagocytophila (the agent of human granulocytic ehrlichiosis [HGE]) and E. ewingii (a cause of granulocytic ehrlichiosis in dogs) act as human pathogens. Human E. chaffeensis infections have generally been reported in North America, Asia and Europe, but recently human cases have been reported in Brazil only. Human monocytic ehrlichiosis is diagnosed by demonstration of a four-fold or greater change in antibody titer to E. chaffeensis antigen by IFA in paired serum samples, or a positive PCR assay and confirmation of E. chaffeensis DNA, or identification of morulae in leukocytes and a positive IFA titer to E. chaffeensis antigen, or immunostaining of E. chaffeensis antigen in a biopsy or autopsy sample, or culture of E. chaffeensis from a clinical specimen.