Tick-borne ehrlichioses

Tick-transmitted ehrlichiae are small obligately intracellular bacteria tha t are maintained in zoonotic cycles involving persistently ehrlichaemic rod ents, ruminants, or canids. Ehrlichiae grow as clusters within a cytoplasti c vacuole in monocytes, macrophages, and neutrophils, and in animal in plat elets, erythrocytes, and endothelial cells. Ongoing reclassification of ehr lichiae will place the agent of human granulocytic ehrlichiosis (HGE), Ehrl ichia phagocytophila, in the genus Anaplasma, whereas E chaffeensis, the ag ent of human monocytotropic ehrlichiosis (HME), and E ewingii which also gr ows in neutrophils, remain in the genus Ehrlichia. HGE is transmitted by Ix odes species ticks in the upper midwestern and northeastern USA, northern C alifornia, and northwestern and eastern Europe. Ranging in severity from mi ld or symptomless to fatal (0.7%), HGE presents nonspecifically except for frequent thrombocytopenia, leucopenia, and elevated hepatic transaminases. HME presents similarly in the southwestern and south-central USA, but more often manifests as meningoencephalitis, adult respiratory distress syndrome , acute renal failure, rash, and death (2.7%). Diagnosis of human ehrlichio ses is usually achieved by a four-fold rise in antibody immunofluorescence titre or by species-specific PCR. Detection of ehrlichiae in leucocytes in peripheral blood smears is achieved more often in HGE than HME. Doxycycline is the drug of choice for human ehrlichioses in most clinical situations. Higher recognised incidences, additional clinical manifestations, wider wor ldwide geographic distribution, and further discoveries of new human ehrlic hioses are likely.