Ehrlichiosis (human monocytic ehrlichiosis)
Ehrlichiosis is a tick-borne febrile illness caused by the obligate intracellular bacterium Ehrlichia chaffeensis, transmitted by the Lone Star tick and infecting monocytes. It is treated empirically with doxycycline without awaiting confirmatory PCR or serology, since delayed treatment raises mortality.
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What this article covers
- Overview
Tick-borne monocytic infection: ehrlichiosis is a nonspecific acute febrile illness caused by Ehrlichia chaffeensis, a gram-negative obligate intracellular bacterium that infects monocytes…
- Epidemiology
Geographic distribution: most cases cluster in the southeastern, mid-Atlantic, and south-central United States, matching the range of the Lone Star tick.
- Risk factors
Outdoor exposure in an endemic region: hiking, camping, hunting, gardening, or occupational work in wooded or brushy areas of the southeastern, mid-Atlantic, or south-central United States during…
- Pathophysiology
Obligate intracellular replication in monocytes: Ehrlichia chaffeensis, a gram-negative bacterium that lacks a classic peptidoglycan cell wall, is inoculated into the skin at the tick bite site and…
- Clinical features
Nonspecific febrile illness: fever, headache, and myalgias begin 1 to 2 weeks after a tick bite; this triad is the most consistent presenting picture and is often mistaken for a viral syndrome.
- Differential diagnosis
Anaplasmosis: shares the fever-headache-myalgia picture but is transmitted by Ixodes ticks in Lyme-endemic regions, infects neutrophils rather than monocytes, and runs a milder course with lower fatality; see anaplasmosis.
- Diagnosis
No formal criteria; diagnosis rests on compatible exposure, clinical picture, and confirmatory microbiology: a tick-exposure history plus fever, headache, and myalgia with leukopenia…
- Management
First, start doxycycline empirically as soon as ehrlichiosis is suspected, without waiting for confirmatory testing: delayed treatment is associated with increased mortality, so treatment decisions…
- Complications
Central nervous system involvement: meningoencephalitis and altered mental status occur more often in ehrlichiosis than in anaplasmosis and mark a more severe course.
- Prognosis
Case fatality approximately 3%, higher than the under 1% fatality of anaplasmosis, reflecting the more severe overall clinical course of ehrlichiosis.
- Prevention
Tick avoidance in endemic areas: apply an EPA-registered repellent containing 20 to 30% DEET to exposed skin and treat clothing and gear with permethrin before outdoor activity in the southeastern…
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- Anaplasmosis (human granulocytic anaplasmosis)
Anaplasmosis is a tick-borne febrile illness caused by the obligate intracellular bacterium Anaplasma phagocytophilum, transmitted by Ixodes ticks and infecting neutrophils, often coinfecting with Lyme disease or babesiosis. It is treated empirically with doxycycline without awaiting confirmatory PCR or serology.
- Lyme disease
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Rocky Mountain spotted fever is the most common and most severe spotted fever rickettsiosis in the United States, caused by Rickettsia rickettsii and transmitted by the dog tick. The hallmark petechial rash typically lags days behind fever onset, so empiric doxycycline must be started on clinical suspicion alone, since treatment delay drives the disease's disproportionate mortality.