Chapter 109 – Ehrlichia

Human ehrlichioses are emerging, often life-threating, tick-borne diseases caused by obligately intracellular Gram-negative bacteria belonging to the genus Ehrlichia. Ehrlichiae lack cell wall components, lipopolysaccharide and peptidoglycan. Ehrlichiae are maintained in the enzootic cycle involving persistently infected vertebrate hosts such as deer and the vector tick. Human monocytotropic ehrlichiosis (HME), caused by Ehrlichia chaffeensis, is highly prevalent in the south-central, southeastern and mid-Atlantic regions of the United States. Molecular and serological evidence suggest a worldwide distribution of ehrlichiae. Ehrlichia chaffeensis resides in monocytes and causes moderate-to-severe flu-like illness characterised by non-specific symptoms such as fever, headache, myalgia, etc. The disease can progress to a severe toxic-shock-like syndrome with multiorgan failure, liver injury, meningoencephalitis, acute respiratory distress syndrome and myocarditis. Aberrant host inflammatory responses are implicated in the disease pathogenesis. Frequent laboratory findings in HME patients include leukopenia, thrombocytopenia and elevated hepatic transaminases. Clinical diagnosis frequently requires application of appropriate molecular diagnostics. Ehrlichia ewingii and newly discovered Ehrlichia muris-like agent (EMLA) also cause human disease. Currently no vaccine is available against ehrlichiosis. Doxycycline is the drug of choice, and initiation of antibiotic therapy early during the infection is critical for its effectiveness.