Francisella tularensis

• Zoonontic infection carried on rodents and rabbits and transmitted by biting insects
• Presentation depends on route of entry: (ulcero)glandular, oculoglandular, pharyngeal, typhoidal, or pneumonic
• Treatment is streptomycin

Background

Microbiology

• A fastidious Gram-negative coccobacillus
• Multiple subspecies, including tularensis, holarctica, novocida, mediasiatica, which are further subdivided into clades, and related species Francisella philomiragia and Francisella hispaniensis
• Subspecies of importance to humans include:
  • Subspecies tularensis (type A strains)
    • Found in North America and rarely Europe
    • Two major clades (AI and AII) and four subclades (AIa, AIb, AIIa, and AIIb)
    • Overall the subspecies is the most virulent subspecies, and specifically the AIb subclade is the most virulent strain
  • Subspecies holarctica (type B strains)
    • Found in the entire northern hemisphere as well as Australia
    • Four major clades (B4 in North America, B6 in Western Europe, B12 in Eastern Europe and Central Asia, and B16 in Japan and other areas in Eastern Asia) and a number of subclades
  • Subspecies novicida
    • Rare cause of disease in humans, usually in immunocompromised hosts, and presents with bacteremia rather than tularemia
  • Subspecies mediasiatica does not cause disease in humans

History

• Discovered in 1911 in Tulare county, California
• Many names: deer fly fever, rabbit fever, etc...

Epidemiology

• Zoonotic infection whose main animal reservoirs are rodents and rabbits
  • In North America, the most important reservoirs are Sylvilagus species (especially Sylvilagus nuttalii, the cottontail rabbit) and Lepus species lagomorphs (rabbits), and a number of rodents including voles, squirrels, muskrats, and beavers
  • In Europe, the reservoirs include voles, hamsters, mice, and hares
• Essentially worldwide Northern Hemisphere distribution, especially in the US, Japan, Russia, and Scandinavian countries
• Transmission:
  • Vector-borne
    • In North America, by Amblyomma americanum ticks, Dermacentor variabilis ticks, Dermacentor andersoni ticks, Chrysops discalis deer flies
    • In Europe, Aedes cinereus and Ochlerotatus exrucians mosquitoes are more important, as well as Chrysops relictus deer flies and Haematopota pluvialis horse flies
  • Direct contact with animal products, including skinning, dressing, and eating wild game
    • Viable in carcasses and dust for up to 136 days
  • Inhalation of aerosolized vectors (e.g. lawn-mowing), contact with contaminated water or mud, and animal bites of animals that have killed infected reservoir hosts (e.g. cats killing rodents)

Pathophysiology

• Infectious dose depends on route, but is as low as 10 to 50 organisms when injected intradermally or inhaled (or several orders of magnitude higher if ingested)
• TLR4 has less affinity for its LPS compared to other bacteria
• Capsule inhibits IgM and complement C3
  • Capsule-deficient strains are both less immunogenic and less virulent
• Facultative intracellular growth
  • Can infect and persist within erythrocytes, providing protection against aminoglycosides

Clinical Manifestations

• Incubation period of 3 to 5 days (range 1 to 21 days)
• The first symptom is usually a papule at the site of inoculation that develops into an ulcer over 1 to 2 days, but this may go unnoticed
• This is followed by fever which likely corresponds to initial lymphohematogenous dissemination
  • Other common symptoms include chills, headache, malaise, anorexia, and fatigue
  • May also have cough, myalgias, chest discomfort, vomiting, sore throat, abdominal pain, and diarrhea
  • May have a relative bradycardia (more common in US than Europe)
• These symptoms may remit and relapse, presenting as a subacute relapsing fever over weeks, with associated weight loss, deconditioning, and lymphadenopathy

Ulceroglandular Tularemia

• Ulcer develops at site of inoculation with tender lymphadenopathy and systemic symptoms

Glandular Tularemia

• Ulcer is undetectable or healed, only lymphadenopathy and systemic illness remains

Oculoglandular Tularemia

• Entry through the conjuctiva
• Rare

Pharyngeal Tularemia

• Entry through the oropharynx, with exudative pharyngitis/tonsillitis
• May be difficult to distinguish from other forms of tularemia that also may have sore throat

Typhoidal Tularemia

• Febrile illness without lymphadenopathy or ulcer, sometimes with diarrhea
• Patient often has an underlying chronic disease
• Often rapidly-progressive and fatal

Pneumonic Tularemia

• Acquired by direct inhalation, often from sheep shearing, landscaping, and microbiology laboratory work

Diagnosis

• Culture
  • Grows slowly on standard culture media, needs cystine-rich media (e.g. chocolate agar, BHI, or cystine media)
  • Looks bacillary in logarithmic growth phase (small Gram-negative rod), slow-growing only on chocolate agar
• Serology (EIA) can be used in the right context
• PCR

Management

• For severe disease, streptomycin 10 mg/kg (max 1 g) IM q12h for 7 to 10 days
  • Alternatives include gentamicin or amikacin
• For mild to moderate disease, alternatives include:
  • Doxycycline 100 mg po bid for 14 to 21 days
  • Ciprofloxacin 500 mg po bid for 10 to 14 days
• For meningitis, use an aminoglycoside (as above) plus either IV ciprofloxacin, doxycycline, or chloramphenicol for 14 to 21 days
  • Aminoglycosides have poor CNS penetration

Prevention

Vaccination

• Live attenuated vaccine derived from holarctica

Lab Safety

• This is a biosafety risk group 3 organism
• Should be suspected with any slowly-growing, small, and poorly-staining Gram-negative coccobacillus is isolated on chocolate agar but not blood agar
• Automated laboratory identification systems should not be used, because the risk of aerosol generation
• Commonly misidentified by automated methods as Haemophilus or Aggregatibacter species