Tularemia

Epidemiology

  • Sporadic, worldwide, endemic zoonosis
  • Incidence: <200 cases annually in US
  • Geography: mostly in south central and western states
  • Any case of pneumonic tularemia should be considered a possible bioterrorism event

Etiology

  • Francisella tularensis
  • As few as 10 oorganisms are capable of infection (by either cutaneous or aerosol route)

Physiology

  • Incubation Period: 3-5 days
  • Route of Infection:
    • Direct Contact with Tissues of Infected Small Animals: associated with skinning or eating (usually of rabbits or squirrels)
    • Bite of Infected Tick or Deerfly:
    • Inhalation of Contaminated Aerosols: landscapers or agricultural workers who generate aerosols in highly endemic areas are predisposed to pneumonic tularemia
      • Efficiency of aerosol transmission makes F. tularensis a potential biowarfare agent
  • Pneumonia: occurs due to inhalational exposure or bacteremia

Diagnosis

  • Sputum GS/Cult+Sens:
    • Fastidious, pleomorphic Gram-negative rod
    • Growth requires cysteine-enriched culture media at BSL-3 (preferably forwarded to a Level B lab, due to hazards to lab personnel)
    • Fluorescent Ab or Immunochemical Stains of Sputum or Tissue: may be positive
  • CXR/Chest CT Pattern: variable pattern, but usually normal at onset of the symptoms (typically 3-5 days after the exposure)
    • Diffuse Bronchopneumonia: often with hilar adenopathy
    • Pleural Involvement (Relatively Common): may occur without parenchymal involvement
    • Lobar Consolidation/Apical Infiltrates/Lung Abscess: less common patterns
  • Serology: acute and convalescent titers demonstrates 4-fold rise
    • Single titer of >1:160: compatible with past or current infection
  • Blood c/s: positive blood cultures are rare

Clinical Presentations

  • Ulceroglandular Tularemia: could potentially represent a bioterrorism event
  • Pneumonic Tularemia (very uncommon)
    • First Phase: abrupt onset of non-specific febrile illness (CXR usually normal during this phase)
      • Fever/Chills
      • Fatigue
      • Headache
      • Malaise
    • Second Phase: pneumonic symptoms (CXR abnormal during this phase)
      • Cough:
      • Chest Pain:
      • Dyspnea:

Treatment

  • Streptomycin (Aminoglycosides are the Preferred Agents): x 10 days
    • Gentamicin (x 10 days): acceptable alternative
    • Ciprofloxacin: acceptable alternative
    • Doxycycline or Chloramphenicol (x 14 days): acceptable alternatives (however, these agents require longer courses of therapy and have higher relapse rates)
    • Ceftriaxone: inadequate (despite its demonstrated in vitro activity)
  • Live-Attenuated Vaccine (Derived from Avirulent Strain): available for laboratory workers
    • Immunity develops over 2 wks
    • Provides incomplete protection against inhalational exposure (must also use antibiotic post-exposure prophylaxis)
  • Post-Exposure Prophylaxis: Doxycycline or Ciprofloxacin x 14 days
    • Patient should follow temperature for signs of fever
    • Vaccine is not effective alone for this purpose
  • Infection control: standard precautions (no human-to-human spread)
    • Treatment of contacts is not required

Prognosis

  • Case-Fatality Rate if Untreated: 30-60%
  • Case-Fatality Rate if Treated: 2%

References

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