Pathogenesis of Human Ehrlichioses

Humans have been reported to be infected by five Ehrlichia species: E. chaffeensis, E. ewingii, E. canis, E. ruminantium, and E. muris-like agent. Developmental forms of Ehrlichiae have been observed in ixodid ticks and are considered part of the bacterial life cycle in the vector. Patients with fever, leukopenia and /or thrombocytopenia and increase in hepatic transaminases in serum and potential tick exposure should have ehrlichiosis in the differential diagnoses. Human patients with acute infection with E. chaffeensis expand activated T cells expressing the T-cell receptor (TCR) gamma/delta, CD45RO, and HLA-DR antigens; however, these cells (primed and activated) are removed from the body by programmed or apoptotic cell death. Ehrlichioses as tick-borne diseases can be controlled by avoidance of these arthropods in endemic areas by use of repellents, protective clothing and careful search of the body, and immediate removal of ticks.