Ehrlichiosis and Anaplasmosis

GENERAL PRINCIPLES

Ehrlichiosis and anaplasmosis are systemic tick-borne infections caused by obligate intracellular bacteria of the Anaplasmataceae family. Two similar syndromes are recognized:

• Human monocytic ehrlichiosis (HME), caused by Ehrlichia chaffeensis and transmitted by the lone star tick, is endemic to the Southern and South Central US.

• Human granulocytic anaplasmosis (HGA), caused by Anaplasma phagocytophilum, is found in the same regions as Lyme disease and shares the same tick vector (Ixodes spp.).

DIAGNOSIS

Clinical Presentation

Clinical onset of illness usually occurs 1 week after tick exposure with fever, headache, myalgias, and arthralgias. Rash is uncommon in adults. Leukopenia, thrombocytopenia, and elevated liver transaminases are important clues to the diagnosis. Severe disease can result in respiratory failure, renal insufficiency, and meningoencephalitis. CNS involvement is uncommon in HGA.

Diagnostic Testing

• Identification of morulae in circulating monocytes (HME) or granulocytes (HGA) on a blood smear is uncommonly seen but diagnostic.

• Acute and convalescent serologies obtained 3-6 weeks apart remain the diagnostic gold standard, but cross-reactivity among Ehrlichia spp. and reduced antibody response due to early treatment are common.

• PCR of the blood has high specificity (60%-85%) and sensitivity (60%-90%).

TREATMENT

Treatment should be started promptly based on clinical suspicion. Doxycycline 100 mg PO/IV q12h for 7-10 days is the drug of choice. Rifampin 300 mg PO q12h for 7-10 days is an option for patients with contraindications to doxycycline therapy. Lack of defervescence after 72 hours of treatment suggests an alternative diagnosis.⁴³