Ehrlichia

Snapshot

A 36-year-old woman presents to her primary care physician after a tick bite. She reports having hiked in Alabama last week and found a tick on her arm after the hike. She denies having any rashes. One day ago, she started having subjective fevers, headache, nausea, and vomiting. Her physical exam is unremarkable and does not have any maculopapular rashes or targetoid lesions. A peripheral blood smear shows monocytes with morulae in the cytoplasm, confirming the diagnosis. (Ehrlichia infection)

Introduction

Classification
- Ehrlichia chaffeensis/Anaplasma phagocytophilum (formerly Ehrlichia phagocytophila)
  - obligate intracellular, gram-negative bacteria
  - members of the family Rickettsiae
  - transmission
    - lone star tick (Amblyomma americanum)
    - Ixodes species
  - clinical syndrome
    - ehrlichiosis (can be caused by Ehrlichia or Anaplasma)
Epidemiology
- incidence
  - E. chaffeensis
    - southeast and south-central US
  - A. phagocytophilum
    - midwest, California, and Texas
- risk factors
  - asplenia
  - immunocompromised status
  - outdoor activities
  - travel or residence in endemic areas
Pathogenesis
- the bacteria infects monocytes (E. chaffeensis) or granulocytes (A. phagocytophilum)
Associated conditions
- often co-infected with Lyme disease
Prognosis
good prognosis with treatment

Presentation

Studies

Peripheral blood smear with Wright or Giemsa stain
- E. chaffeensis
  - monocytes with morulae, or inclusions, in the cytoplasm
- A. phagocytophilum
  - granulocytes with morulae in the cytoplasm
Labs
- may have neutropenia
- detection of immunoglobulin on enzyme-linked immunosorbent assay
Making the diagnosis
based on clinical presentation and laboratory studies

Differential

Rocky Mountain spotted fever
- distinguishing factor
maculopapular rash and physical exam findings such as bilateral periorbital edema

Treatment

Complications