Francisella tularensis

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