Francisella tularensis

Francisella tularensis results tularaemia in man and certain small mammals, such as rabbits, hares, beavers and several rodent species.
Tularaemia was originally described in Tulare county, California.
It can be transmitted by direct contact, by biting flies, mosquitoes and ticks, by contaminated water or meat or aerosols.
Francisella tularensis is also know as Pasteurella tularensis or Brucella tulerensis)

**Morphology of Francisella tularensis:**

It is a very small, nonmotile, nonsporing, capsulate, gram-negative coccobacillus, about 0.3 to 0.7 μm × 0.2 μm in size.
In culture it tends to be pleomorphic to and larger, even filamentous, forms are present.
It stains poorly with methylene blue but dilute carbol fuchsin (10%) produces characteristic bipolar staining.

F. tularensis is strictly aerobic.
It will not grow on ordinary nutrient media but grows well on blood agar containing 2.5 percent glucose and 0.1 percent cysteine hydrochloride.
Minute droplet-like colonies develop in 72 hours.

Under suitable conditions acid is formed from glucose and maltose. Indole and urease tests are negative.
Two biovars are recognized. Strains of F. tularensis have been subdivided into biotypes based on their virulence and epidemiological behaviour.
Highly virulent strains are found only in N. America, while strains of low virulence are seen in Europe and Asia also.

The infection, which is a typical zoonosis, is mainly spread by insects or ticks among lagomorphs and rodents.
It is transmitted to man through handling of infected animals, e.g. rabbits or hares tick, mosquito or fly bites, inhalation of contaminated dust, ingestion of contaminated water or meat.
Laboratory workers are at higher risk while handling infected laboratory animals or cultures of the organism.
Man to man transmission of infection apparently does not occur.
In human beings, tularemia may present as a local ulceration with lymphadenitis, a typhoid like fever with glandular enlargement or an influenza like respiratory infection.
The severity of disease is much greater with type A strains and case fatality rates may exceed 5 percent.
Disease caused by type B strains is much less severe, with very low mortality.

F. tularensis is extremely dangerous to handle in the laboratory and Category 3 containment is required for all manipulations and animal work.
Diagnosis may be made by culture or by inoculation into guinea pigs or mice. A PCR has been described, but is not widely available.
Serology is most likely to be positive after 3 weeks.
Rising titres of agglutinins to F. tularensis or individual titres of 160 are diagnostic.
Serum from cases of F. tularensis may cross-react with brucellosis and vice versa, usually to relatively low titre.
An intradermal delayed hypersensitivity test has been used in the past but the antigen is not readily available.

Streptomycin or gentamicin are the antibiotics of choice in tularaemia and are usually curative.

Prophylaxis:

A vaccine based on the live-attenuated LVS strain confers some protection.
It can be administered by scarification to persons who are subject to high risk of infection.
F. tularensis has been developed as a biological warfare agent and has potential application in bioterrorism.