Francisella tularensis

Francisella tularensis

 

Francisella tularensis is a cause of potentially severe zoonotic disease in humans.

Epidemiology

  • Incidence – <1/100,000 in United States
  • Transmission – blood-sucking insects or contact with infected animals via inapparent abrasions
    • Primarily a disease of wild animals
    • Human vectors in U.S. are ticks and rabbits

Organism

  • Gram-negative, non-sporeforming bacillus
    • Two main biovars are F. tularensis-tularensis (type A) and F. tularensis-palearctica (type B)

Clinical Presentation

  • Incubation is generally 2-10 days
  • The disease often begins with the sudden onset of flu-like symptoms, chills, fever, headache and generalized aches
  • Forms of tularemia
    • Ulceroglandular (75-85%)
      • Most common form of tularemia
      • Direct contact or insect bite
      • Small ulcer at portal of entry with associated lymphadenopathy
    • Oculoglandular
      • Conjunctival (entry via contaminated fingers) – 1%
      • Unilateral intense conjunctivitis
    • Oropharyngeal and gastrointestinal
      • Follows ingestion of contaminated food
      • Oral – exudative pharyngitis, cervical, preauricular adenopathy
      • Gastrointestinal – ulcerative GI lesions, diarrhea
    • Respiratory 
      • Inhalational exposure or extension from systemic disease
      • Presents as infiltrates on chest X-ray  
      • Pneumonia with cough, pleuritic chest pain
    • Typhoidal
      • Rare in U.S.
      • Usually associated with bacteremic gastrointestinal disease and consumption of poorly cooked wild game
      • High fever, headache and shock
  • Complications – septicemia, meningitis, endocarditis, hepatitis and renal failure

Diagnosis

  • Indications for testing
    • A history of contact with rabbits, ticks, dogs, cats or skunks is suggestive, but a negative history of animal contact does not rule out the diagnosis
    • Laboratory testing
      • Serology – may require acute and convalescent phase titers

Differential Diagnosis

  • Ulceroglandular – streptococcal/staphylococcal skin infection, anthrax, pasteurellosis, atypical mycobacteria
  • Respiratory – hantavirus, influenza, mycoplasma, brucellosis, legionella plague, anthrax, SARS
  • Oropharyngeal – Group A streptococcus, mononucleosis

Treatment and Prevention

  • Antibiotic treatment is necessary and curative for most cases
  • Vaccination available for at-risk individuals

See Also