Ehrlichiosis

Basics

Tick-borne human infection presenting as a nonspecific febrile illness
Several forms of ehrlichiosis exist; 2 predominate in North America
- Human monocytic ehrlichiosis (HME), first described in late 1980s:
  - Vector tick: Amblyomma americanum (lone star tick)
  - Geographic range: Central, southern, and mid-Atlantic states, with range expanding to parts of New England
- Human granulocytic ehrlichiosis or human granulocytic anaplasmosis (HGE or HGA), first described in 1994:
  - Vector tick: Ixodes scapularis (deer tick)
  - Geographic range: East Coast, mid-Central States, and Pacific Northwest (same areas as Lyme disease which is more common in the U.S. than HME)
- A third type caused by Ehrlichia ewingii was discovered in 1999
All are tick borne but have different vectors and geographic ranges. Other species have been reported, but at present HME and HGE are the important ehrlichial human pathogens

2 distinct species of obligate intracellular organisms
The taxonomy of these pathogens has changed in recent years as more DNA and ribosomal RNA data become available
HME is caused by the organism Ehrlichia chaffeensis
HGE/HGA is caused by Anaplasma phagocytophila (a new name as of 2002)
The vasculitis found in Rocky Mountain spotted fever (RMSF) is usually not present
A third type may also be encountered, caused by Ehrlichia ewingii, which has the tick vector of the lone star tick. Clinically similar to HME
Compared with RMSF, older individuals are usually affected, commonly >40 yr of age

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