Francisella tularensis: Properties, Pathogenesis, Lab Diagnosis

By Acharya Tankeshwar •  Updated: 04/22/22 •  4 min read

Francisella tularensis causes life-threatening infections of animals (rabbits, hares, and rodents) and people (zoonotic disease). It is also known as ‘Rabbit Fever’ or ‘Deer Fly fever’.

Francisella tularensis basic feature

Properties of Francisella tularensis

Francisella tularensis identification flowchart
Image source: Laboratory Response Network (LRN)

Pathogenesis

Mode of Transmission

Source: It persists in contaminated environments, insects, and animal carries.

Transmission routes:

Prevalence

F. tularensis has four subspecies: tularensis, holarctica, novicida, and mediasiatica. The first three species are found in North America whereas subspecies mediasiatica is found in central Asia. Subspecies tularensis is the most common and the most virulent among all. It has been isolated only from North America, where it accounts for more than 70% of cases.

Increasing number of cases due to other subspecies have been reported from the Scandinavian countries, Eastern Europe and Siberia.

Virulence Factors

Once F. tularensis is engulfed by immune cells (macrophages) that routinely kill bacteria, it replicates and evades parts of the immune system using an array of virulence factors:

Clinical Manifestations  

Tularemia symptoms usually appear within 3-5 days of infection, but may also appear up to 15 days later. Tularemia is characterized by various clinical syndromes depending on how the person was infected.
Main forms of the disease are:

Ulceroglandular tularemia

It is the most common form, (75-85% of total cases), characterized by an ulcerative lesion at  the site of inoculation, with regional lymphadenopathy (armpit or groin.)

Glandular tularemia

It presents with swollen lymph nodes (without ulcer)

Pulmonary tularemia

Oropharyngeal tularemia

It occurs following the ingestion of contaminated undercooked meat. It is characterized by membranous pharyngitis with cervical lymphadenopathy.

Oculoglandular tularemia

This form occurs when the bacteria enter through the eye. It is characterized by purulent conjunctivitis with preauricular lymphadenopathy.

Typhoid-like illness

It presents with the standard, nonspecific febrile (fever) symptoms, but without a known route of infection (via the skin, eye, ingestion, or inhalation)

Laboratory Diagnosis

Difficult to diagnose because it’s a rare disease with nonspecific signs and symptoms. Diagnosis may rely on epidemiologic evidence (e.g., history of mowing the lawn in an endemic area)

Specimen

Ulcer scrapings, lymph node biopsy, gastric washings, sputum, and blood.

Culture

Isolation is very difficult as F. tularensis is highly fastidious

It needs special media such as:

F. tularensis in cysteine enriched chocolate agar

Specimens are inoculated onto the media and incubated at 37oC for 2-4 days aerobically as F. tularensis is an obligate aerobe.

Colonies are blue-gray, round, smooth, and slightly mucoid with small zone of alpha-hemolysis.

Safety precautions such as biosafety level III must be used to handle clinical specimens to avoid the risk of laboratory-acquired infections.

Identification

Antibody detection

It is the mainstay of diagnosis as isolation is difficult. Agglutination test (latex and tube agglutination) and ELISA formats are available.

Molecular Diagnosis

PCR assay has been used to detect F. tularensis specific genes encoding the outer-membrane proteins. It can also differentiate subspecies.

Prevention & Treatment

Steps to prevent tularemia include:

Treatment
Streptomycin, gentamicin, doxycycline, and ciprofloxacin are used to treat tularemia. Treatment usually lasts 10 to 21 days depending on the stage of illness and the medication used.

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please email at microbeonline@gmail.com

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