Francisella tularensis
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Overview
- Causes tularaemia in wildlife, domestic animals and humans
- Wildlife reservoir of infection, including rabbits, rodents, galliform birds and deer
- Survives in the soil for up to 4 months
- Type A strains associated with terrestrial animal reservoirs; type B strains with water-borne infections and aquatic animals
- Ticks and deerfly act as vectors
- Fatal infection in humans, with ulers and lymphadenopathy; vaccine available
Characteristics
- Gram negative rod with coccobacillary appearance
- Non-moltile, obligate aerobe
- Oxidase negative, weakly catalase positive
- Fastidious, requiring cysteine for growth on blood agar
- No growth on MacConkey
- Facultative intracellular pathogen
Pathogenesis and pathogenicity
- High lipid content; virulent isolates produce capsules
- Highly virulent type A stains, F. tularensis subspecies tularensis in North America; less virulent type B strains, F. tularensis subspecies holarctica in Eurasia and North America
- Infection through skin abrasions or arthropod bites, inhalation or ingestion
- Organism survives and replicates in acidic environment of macrophages by inhibiting phagosome/lysosome fusion
- Acidic environment required for release of iron, required for growth, from tranferin
- Lymphadenitis and septicaemia
- Enlarged lymph nodes with pale necrotic foci
- Miliary lesions in spleen and liver
- Pulmonary consolidation; primary pulmonary lesions due to aerosol inhalation in dogs
Clinical infections
- Fulminant disease in immunosuppressed
- Septicaemia with fever, depression, inappetence, stiffness
- Chronic granulomatous or subclinical infections
- Type A strains cause most infections in domestic animals; type B strains cause mild disease
- Rare outbreaks of tularaemia in sheep, horses and young pigs
- Adult pigs and cattle resistant
- Dogs and cats seroconvert without clinical signs
Diagnosis
- Heavy tick infestation in severely infected animals in an endemic area indicate tularaemia
- Blood serology - high agglutinating antibody titres or rising antibody titre
- Scraping from ulcers, lymph node aspirates and biopsy material
- Post mortem
- Fluorescent antibody technique on tissues, exudates or culture
- Glucose-cysteine-blood agar used for culture
- Small, grey, mucoid colonies, surrounded by narrow zone of incomplete haemolysis, after 3-4 days incubation
- Immunofluorescence confirms presence in smears
- Slide agglutination test on cultures
- Biochemical tests to distinguish type A from type B strains
- PCR for detection in blood
- Isolation in embryonated eggs or lab animals
Treatment
- Antibiotics including amikacin, streptomycin, fluoroquinolones
Control
- Ectoparasite control
- Daily removal of ticks from dogs and cats
- Prevent dogs and cats hunting wildlife