Francisella tularensis - Tularemia

 

Clinical Background

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

Epidemiology

  • Incidence – <1/100,000 in U.S.
    • Most cases occur in the west central and mountain regions (56% of total cases from 1990 – 2000 occurred in Arkansas, Oklahoma, Missouri, and South Dakota)
  • Transmission – blood-sucking arthropods or contact with infected animals via inapparent abrasions or consumption of contaminated water
    • Primarily a disease of wild animals, especially rabbits
    • In U.S., human vectors are primarily ticks and deer flies
      • Tick vectors include Amblyomma americanum, Dermacentor andersoni, Dermacentor variabilis
    • Peak seasons – spring and summer
      • Historical peaks during fall and early winter (hunting season)

Organism

  • Gram-negative, aerobic, non-spore forming, fastidious coccobacillus
    • Two main biovars are F. tularensis subsp. tularensis (type A) and F. tularensis subsp. holarctica (formerly subsp. palearctica, type B)
      • Type A is found predominantly in the U.S.
      • Type B is found predominantly in Europe and Asia
    • F. tularensis subsp. novicida (type C) is a low-virulence strain in North America

Clinical Presentation

  • Incubation is generally 2-10 days
  • The disease often begins with the sudden onset of flu-like symptoms, including chills, fever, headache and generalized aches
  • Forms of tularemia
    • Ulceroglandular (21-85%)
      • Direct contact with animal or insect bite
      • Most common form of tularemia
      • Starts as small, painful papule that becomes an ulcer at entry portal with associated lymphadenopathy (glandular tularemia [3-20%] similar presentation but lacking ulcer)
    • Oculoglandular (2-5%)
      • Direct contact with animal 
      • Conjunctival (entry via contaminated fingers, splashes, aerosols)
      • Unilateral intense conjunctivitis, preauricular, submandibular, cervical lymphadenopathy
    • Oropharyngeal and gastrointestinal (2-12%)
      • Follows ingestion of contaminated food or water
      • Oral – exudative pharyngitis; deep, cervical lymphadenopathy
      • Gastrointestinal – ulcerative GI lesions, diarrhea
    • Pneumonic (8-20%)
      • Inhalational exposure or extension from systemic disease
      • Pneumonia with cough, pleuritic chest pain
      • Often occupational exposure (sheep shearing, animal husbandry, farming, lawn/brush cutting, laboratory workers)
    • Typhoidal (5-30%)
      • Rare in U.S.
      • Usually associated with bacteremic gastrointestinal disease and consumption of poorly cooked wild game
      • High fever, headache, diarrhea shock
  • Complications – septicemia, meningitis, endocarditis, hepatitis, renal failure

Treatment and Prevention

  • Antibiotic treatment is necessary, but relapses may occur with certain therapies
  • Vaccination available for at-risk individuals