Francisella tularensis: Tularemia, Clinical Forms, Pathogenesis, Diagnosis, Treatment

A man who hunts and skins rabbits develops a sudden high fever, chills, and body aches a few days after his last trip. On one hand there is a small ulcer where he had a minor cut, and the lymph nodes in that armpit are swollen and tender. He did not think a scratch from cleaning a rabbit could matter. But it took only a tiny number of organisms entering that small break in his skin to infect him, and the site of entry is exactly why the ulcer and the swollen node appear together on the same side. This is tularemia, caused by Francisella tularensis, one of the most infectious bacteria known.

Introduction

Francisella tularensis causes tularemia, a serious zoonotic disease of animals (especially rabbits, hares, and rodents) and humans. It is also known as "rabbit fever" or "deer fly fever," names that point to how people catch it. One idea organizes the whole topic: F. tularensis is a facultative intracellular organism that survives and multiplies inside macrophages, and the route by which it enters the body decides which clinical form of tularemia develops. Almost everything about the disease follows from these two facts.

Figure: Francisella tularensis basic feature

Properties of Francisella tularensis

• Extremely infectious. As few as about 10 organisms can cause disease, and it can enter through the skin, eyes, mouth, or lungs. This extraordinarily low infectious dose is its defining feature and the reason it is both a serious occupational hazard and a bioterrorism concern. It is, however, treatable with antibiotics.
• Bioterrorism agent. Because of its low infectious dose and ability to spread by aerosol, F. tularensis is classified as a CDC Category A agent.
• Laboratory hazard. It causes laboratory-acquired infection easily, so suspected cultures must be handled in a biosafety level 3 (BSL-3) laboratory. Always alert the laboratory when tularemia is suspected.
• Gram-negative coccobacillus, often with a bipolar (safety-pin) appearance.
• Non-motile, non-spore-forming, and capsulated.
• Facultative intracellular and a strict aerobe. (These are not in conflict: "aerobic" describes how it uses oxygen for metabolism, while "facultative intracellular" describes that it can live both inside and outside host cells.)
• Fastidious: it needs cysteine for growth, so it is grown on cysteine-enriched media such as cysteine-enriched chocolate agar, buffered charcoal yeast extract (BCYE), and cysteine heart agar with blood (CHAB). The cysteine requirement is a key identifying feature.

Figure: Francisella tularensis identification flow chart Image source: Laboratory Response Network (LRN)

Transmission routes

F. tularensis persists in the environment, in insects, and in animal reservoirs. Humans are infected by several routes, and the route matters because it determines the clinical form (detailed below).

• Bite of blood-sucking arthropods such as ticks and deer flies.
• Direct skin contact with infected animals (for example, hunting, skinning, or handling rabbits).
• Ingestion of contaminated water or undercooked meat.
• Inhalation of contaminated aerosols or dust (for example, from farming, landscaping, or mowing over an infected animal carcass).
• Laboratory exposure: because the infectious dose is so low, laboratory staff can be infected while handling cultures, doing Gram stains, or examining plates, which is why suspected tularemia work is done at BSL-3.

There is no person-to-person spread

## Prevalence

F. tularensis has four subspecies: tularensis (type A), holarctica (type B), novicida, and mediasiatica. Subspecies tularensis (type A) is the most virulent and is found mainly in North America. Subspecies holarctica (type B) is generally milder and is found across the Northern Hemisphere, including Europe and Asia. This difference in virulence between subspecies is worth knowing, because type A causes more severe disease.

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

Virulence factors and pathogenesis of Francisella tularensis

The central event in tularemia is that F. tularensis not only survives inside the macrophage, the cell meant to destroy it, but turns it into a place to multiply. The mechanism runs in a sequence.

1. Uptake into the macrophage. The organism is engulfed by the macrophage and ends up inside a phagosome.
2. Escape from the phagosome. This is the key step. Instead of staying trapped, F. tularensis first delays and disrupts the fusion of the phagosome with the lysosome (helped by enzymes such as acid phosphatase), and then breaks out of the phagosome altogether.
3. Replication in the cytosol. Once free in the cytosol (the main body of the cell), it multiplies to large numbers, safe from the killing machinery of the phagosome.
4. Cell death and spread. The infected macrophage eventually dies, releasing organisms that infect new cells and spread through the lymphatics and blood.

Because the organism hides and grows inside cells, antibodies cannot reach it, and clearing the infection depends on cell-mediated immunity. This also explains why the effective antibiotics are ones that penetrate cells.

Supporting virulence factors:

• Capsule: antiphagocytic; helps resist being killed and helps the organism survive in the blood.
• Unusual lipopolysaccharide (LPS): its outer membrane LPS is structurally atypical and poorly recognized by the host's innate immune sensors (Toll-like receptors), so it dampens the early immune alarm and delays the response.
• Type IV pili: help the organism attach to host cells.
• Acid phosphatase: interferes with phagosome-lysosome fusion, aiding intracellular survival.
• Siderophores: capture iron, which the organism needs to grow inside the host.

Putting it together: a tiny inoculum enters, the organism is taken up by macrophages, escapes into the cytosol and multiplies while its atypical LPS keeps the immune alarm quiet, then spreads through the lymphatics to regional lymph nodes and beyond. This is why even a very small number of organisms can cause disease, and why regional lymphadenopathy is such a consistent feature.

## Clinical Manifestations

Tularemia symptoms usually begin within about 3 to 5 days of infection, but can appear up to about 2 weeks later. It always starts with sudden, non-specific features: fever, chills, headache, and body aches. What differs is the local picture, and that depends entirely on where the organism entered the body.

The single most useful way to learn the clinical forms is to map each route of entry to the form it produces:

Once you see this pattern, the forms are not a list to memorize but a logical consequence of the route. The ulcer-plus-same-side-node of ulceroglandular disease, seen in the hook, is simply the local reaction where the organism entered plus the lymph node draining that site.

Ulceroglandular tularemia is the most common form. An ulcer forms at the site where the organism entered the skin, and the lymph nodes draining that area (for example, in the armpit or groin) become swollen and tender.

Glandular tularemia

It presents with swollen lymph nodes (without ulcer)

Pneumonic tularemia is the most serious form. It follows inhalation of the organism (a particular risk for laboratory workers and in a bioterrorism scenario) or can develop when other forms spread to the lungs through the blood. Features include cough, chest pain, and difficulty breathing.

Oropharyngeal tularemia

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

Oculo-glandular 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:

• BCG agar (blood cysteine glucose agar)
• CHAB agar (cysteine heart agar supplemented with 9% heated sheep blood).

Figure: F. tularensis in cysteine enriched chocolate agar

Specimens are inoculated onto cysteine-enriched media and incubated aerobically at 35 to 37°C for about 2 to 4 days (it grows slowly). Cultures should only be set up when the laboratory has been warned, because of the infection risk.

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

• F. turalensis is a small gram-negative coccobacillus with bipolar appearance, non-motile and capsulated.
• It is weakly catalase positive and oxidase negative.
• It produces acid but no gas from glucose, maltose and mannose.
• Direct fluorescent antibody tests can be done with commercially available antisera, directly from the culture colonies for subsequent identification.

Antibody detection

Antibody detection (serology) is the mainstay of diagnosis in practice, because culture is difficult and dangerous. Agglutination tests (tube and latex) and ELISA are used. A limitation to remember: antibodies take time to rise, so a single early sample may be negative, and paired samples showing a rising titer are more reliable.

Molecular Diagnosis

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

Treatment of tularemia

Treatment follows from the intracellular biology: the drugs used are ones that reach the organism, not the beta-lactams, which are not reliable against F. tularensis.

• Aminoglycosides (streptomycin or gentamicin) are the traditional drugs of choice, especially for severe disease.
• Doxycycline and fluoroquinolones (ciprofloxacin) are alternatives, often used for milder disease or as oral options.

Two points worth knowing: relapse can occur if treatment is stopped too early or if a bacteriostatic drug such as doxycycline is used, so an adequate course matters; and treatment should be started on clinical and epidemiological suspicion rather than waiting for culture, because culture is slow and hazardous.

Prevention

Steps to prevent tularemia include:

• Using insect repellent
• Wearing gloves when handling sick or dead animals
• Avoiding mowing over dead animals (which can create infectious aerosols). There is no widely available licensed vaccine for general use; prevention relies on avoiding exposure.

How to Remember

Key exam facts in one table

Where Students Get Confused

References

1. Tille, P. M. (2022). Bailey & Scott's Diagnostic Microbiology (15th ed.). Elsevier.
2. Procop, G. W., & Koneman, E. W. (2017). Koneman's Color Atlas and Textbook of Diagnostic Microbiology (7th ed.). Wolters Kluwer.
3. Carroll, K. C., Pfaller, M. A., et al. (2020). Murray's Medical Microbiology (9th ed.). Elsevier.
4. Madigan, M. T., Bender, K. S., Buckley, D. H., et al. (2021). Brock Biology of Microorganisms (16th ed.). Pearson.
5. Centers for Disease Control and Prevention. Tularemia: information for healthcare providers. CDC (current version).