Emerging Human Ehrlichioses: Recently Recognized, Widely Distributed, Life-Threatening Tick-Borne Diseases

Phylogenetic analysis based on 16S rRNA gene homology places organisms currently designated Ehrlichia spp. within three different clusters. The theme of evolution for survival as obligately intracellular organisms within insects and ticks seems to characterize most of those bacteria well, except for those in the group containing Ehrlichia sennetsu and Neorickettsia helminthoeca. The etiologic organisms of human monocytotropic ehrlichiosis (HME) and human granulocytotropic ehrlichiosis (HGE) have surely been present on Earth for eons. The body of knowledge provided by veterinary scientists, including techniques to cultivate some of the Ehrlichia spp. as antigens for immunofluorescence serology and descriptions of their microscopic and ultrastructural appearance, facilitated the identification of the etiologic agent of HME as an Ehrlichia sp. Increasing numbers of commercial laboratories are offering serologic and PCR diagnostic assays for the diagnosis of the human ehrlichioses. A commercial laboratory with the most experience in the diagnosis of HME, demonstrated the presence of antibodies to E. chaffeensis in 1, 136 serum specimens between October 1991 and May 1996. Cell culture recovery of the HGE agent in HL60 cells (a human myeloid cell line) has yielded more isolates than recovery of E. chaffeensis has in DH82 cells. HGE is treated successfully with doxycycline with the same doses as for HME. Based on cell culture antimicrobial susceptibility studies, rifampin, rifabutin, and selected fluoroquinolones show promise as alternative drugs that might prove useful in situations such as pregnancy.