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
- Very small on Gram stain, giving a "pink sand" appearance
- Do not stain well; fuchsine dye can help
- Needs cysteine to grow, so doesn't grow well on normal media
Weakly catalase positive, usually oxidase negative, and urease negative
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, water rat trappers disease, etc...

Epidemiology

Zoonotic infection whose main animal reservoirs are rodents and rabbits
- In North America, the most important reservoirs are Sylvilagus (especially Sylvilagus nuttalii, the cottontail rabbit) and Lepus lagomorphs (rabbits), and a number of rodents including voles, squirrels, muskrats (especially in Canada), 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
- In Canada, occurs mostly in the Prairies
Transmission through a number of mechanisms
- 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
  - Requires a minimum of 24 hours of tick attachment to transmit to the host
- 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)
- Potential for bioterrorism, especially with waterborne or aerosol transmission

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

Virulence Factors

Lipopolysaccharide is not well-recognized by TLR4
Capsule
Type IV pili binds to epithelial cells
Facultatively intracellular

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
Bloodwork may show leukocytosis and elevated ESR, as well as occasional thrombocytopenia, hyponatremia, elevated liver enzymes, elevated CK, myoglobinuria, and sterile pyuria

Ulceroglandular Tularemia

Most common
Ulcer develops at site of inoculation with tender lymphadenopathy and systemic symptoms
Ulcer can be red papule or vesicular (similar to HSV), then either heals or becomes necrotic

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
May involve endocarditis
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
Can also develop secondary to one of the other syndromes
Chest x-ray is non-specific

Complications and Prognosis

Suppurative lymphadenopathy is the most common complication despite antibiotics, and may require drainage
Severe disease may cause DIC, renal failure, rhabdomyolysis, jaundice, and hepatitis
Without treatment, complications include meningitis, encephalitis, pericarditis, peritonitis, osteomyelitis, splenic rupture, and thrombophlebitis
Rarely, Guillain-Barré syndrome following ulceroglandular tularemia
Symptoms, especially neuropsychiatric symptoms, may persist for weeks after treatment
Mortality is 60% without treatment, and decreases to 2 to 4% with antibiotics

Diagnosis

Diagnosis is primarily clinical, and treatment should be given while attempting to confirm with diagnostic testing

Culture

Notify lab that tularemia is suspected before sending samples
Gram stain is very rarely positive
May be isolated from blood, pleural fluid, lymph nodes, wounds, sputum, and gastric aspirates
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

The most common test used to diagnose
Can use tube agglutination (used in Ontario and the US), microagglutination, hemagglutination, enzyme-linked immunosorbent assay (used in Europe), or immunochromatographic assay
IgM and IgG appear together, usually after 2 weeks and peak at 4 to 5 weeks
Can persist for at least 10 years
Presumptive positive with a single titre ≥1:160 (tube agglutination) or ≥1:128 (microagglutination), but this can also be compatible with remote infection
Definitive diagnosis is made with a four-fold rise in acute and convalescent serology collected 2 to 3 weeks apart, with at least one test being above the threshold for presumptive positive
May cross-react with Brucella, Proteus OX19, Legionella, and Yersinia, but titres normally aren't particularly high, as well as with environmental, non-pathogenic Francisella

Other

DFA and PCR are available at reference labs
Slide agglutination of a colony as a rapid test to identify
Automated methods should not be used, due to the risk of aerosolization, and may also misindentify as Haemophilus or Aggregatibacter species

Management

Due to the presence of a β-lactamase and the fact that it's facultatively intracellular, it is not treated with any β-lactam antibiotics
For severe disease, streptomycin 10 mg/kg (max 1 g) IM q12h for 7 to 10 days
- Alternatives include gentamicin 2.5 mg/kg IM/IV q8h 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
Pregnant women or children are usually treated with ciprofloxacin

Prevention

Vaccination

Live attenuated vaccine derived from holarctica

Post-Exposure Prophylaxis

Only given after aerosolized exposure
Doxycycline or ciprofloxacin for 14 days

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