Ehrlichiosis: Human Monocytotropic Ehrlichiosis

Human Monocytotropic Ehrlichiosis (HME) is caused by Ehrlichia chaffeensis. In the year 2006 more than 2657 cases were reported to the Centers for Disease Control and Prevention (CDC), Atlanta, USA. But active prospective studies indicate an incidence of as high as 414 cases per 100,000 populations in some regions of the United States. White tailed deer is the major reservoir and Lone Star tick (A. americanum), the main vector feed on them (all life stages feed on white tailed deer).

Clinical manifestations: Illness develops after about 8 days (incubation period). The organisms spread through blood pool which is created by the feeding tick. Clinical manifestations are fever (97% of cases), headache (80% of cases), malaise (82% of cases) and myalgia (57% of cases). Symptoms like nausea, vomiting, and diarrhea, cough and rash are less frequent.

Severe complications include toxic shock like or septic shock like syndromes, adult respiratory distress syndrome (ARDS), cardiac failure, meningoencephalitis, hepatitis and hemorrhage. In immunocompromised patients an overwhelming infection may be seen.

Human Monocytotropic Ehrlichiosis can be severe. 62% of patients with documented cases are hospitalized, and about 3% die.

Diagnosis: HME can be fatal; so empirical antibiotic therapy based on clinical diagnosis is required. Diagnosis is suggested by fever with a known tick exposure during the preceding 3 weeks, thrombocytopenia and/or leukopenia, and increased serum aminotransferase activities. HME can be confirmed by PCR amplification of E. chaffeensis nucleic acids in blood which is obtained before the start of antibiotic therapy. It should be differentiated from Human Granulocytotropic Anaplasmosis (HGA).

Treatment: Treatment is with doxycycline 100 mg given orally or intravenously twice daily or tetracycline 250–500 mg given orally every 6 hourly and continued for 3-5 day after symptoms subside.  

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