Ehrlichia

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Ehrlichia / (Redirected from Ehrlichia chaffeensis)


Background

  • Causes human monocytotropic ehrlichiosis, aka. "Rocky Mountain spotless fever"

Microbiology

  • Small, obligately intracellular Gram-negative bacteria
  • Related to Anaplasma and Rickettsia genera
  • Includes the following species:
    • E. chaffeensis is the most common cause of human infection
    • Other species that can cause disease in humans includes E. ewingii, E. muris, E. muris-like agent, E. canis, and E. ruminantium, with slightly different tick vectors and mammalian hosts
  • Cells have two ultrastructural forms: larger reticulate cells (RC) and small, dense core cells (DC)
  • Intracellularly, they form aggregates called morulae (mulberries)

Epidemiology

  • E. chaffeensis is primarily found in south-central and eastern North America
    • Rejected by NML for patients from Ontario without travel
  • Vectors are a variety of ticks:
  • Reservoirs
    • The main reservoir is Odocoileus virginianus (white-tailed deer)
    • For E. chaffeensis, others include humans, dogs, coyotes, and marsh deer
    • For E. ewingii, dogs, humans, and deer
  • Rick factors
    • Incidence peaks in May to August
    • Exposure is rural or suburban, and usually involves recreational, peridomestic, occupational, and military activities
      • Therefore, 60% male

Pathophysiology

  • After inoculation, spreads lymphangitically and hematogenously to reach and invade macrophages and granulocytes
    • Intracellular morulae also found in blood, bone marrow, liver, lymph nodes, spleen, and CSF macrophages
    • Affects almost any organ except for endothelium
  • Surface porin proteins are responsible for antigenic variation and host cell adhesion
  • The dense core cells also secrete TRP120, which is involved adhesion but also has effects on host cell DNA and DNA transcription
  • DC cells predominate soon after infection, then give way to RC cells
  • Most of the pathology is caused by host responses, including toxic shock
    • Increased TNF-alpha, IL 1-alpha and 1-beta, IL-6, IL-10, and defective Th1 cytokines (IFN-gamma and IL-2)

Clinical Manifestations

E. chaffeensis (human monocytotropic ehrlichiosis)

E. ewingii and E. muris

  • Mostly affects immunocompromised patients
  • Presents similarly to HME, but less severe

Differential Diagnosis

Diagnosis

  • Should be treated empirically without waiting for diagnosis
  • Major method of diagnosis is IFA serology, looking for a fourfold rise in titres over the course of disease, with a minimum peak of 1:64
    • Usually peaks at 6 weeks
  • Diagnosis can be suggested by morulae on blood film (in monocytes), but insensitive unless overwhelming infection
  • Culture not used outside of research
  • PCR is possible

Management