Tularemia (Rabbit Fever) (Francisella tularensis)

Overview

Plain-Language Overview

Tularemia (Rabbit Fever) is an infectious disease caused by the bacterium Francisella tularensis. It primarily affects the skin, lymph nodes, and sometimes the lungs. People usually get infected through contact with infected animals like rabbits or ticks. The disease can cause symptoms such as fever, chills, and painful swollen lymph nodes. It can also lead to skin ulcers or respiratory problems depending on how the infection enters the body. Early diagnosis is important because the infection can become severe if untreated. The disease is rare but can be serious without proper medical care.

Clinical Definition

Tularemia is a zoonotic infection caused by the facultative intracellular gram-negative bacterium Francisella tularensis. It is transmitted to humans primarily through arthropod vectors such as ticks or direct contact with infected animals, especially rabbits. The core pathology involves macrophage infection leading to granulomatous inflammation and necrosis in affected tissues. Clinically, it presents with ulceroglandular, glandular, oculoglandular, or pneumonic forms depending on the route of entry. The disease is characterized by fever, regional lymphadenopathy, and sometimes systemic symptoms like malaise and weight loss. It is highly infectious with a low infectious dose and is considered a potential bioterrorism agent. Diagnosis and treatment are critical to prevent complications such as sepsis or respiratory failure.

Inciting Event

Direct contact with infected animal tissues or fluids during hunting or skinning.
Bites from infected ticks or deer flies transmitting the bacteria.
Inhalation of aerosolized bacteria from contaminated soil or animal carcasses.
Ingestion of contaminated water or undercooked meat from infected animals.

Latency Period

Incubation period ranges from 3 to 5 days, typically up to 14 days post-exposure.
Symptom onset is usually abrupt following the incubation period.

Diagnostic Delay

Nonspecific early symptoms such as fever and malaise mimic viral illnesses, delaying suspicion.
Low clinical awareness due to rarity and similarity to other infections like plague or cat scratch disease.
Difficulty culturing Francisella tularensis requires specialized media and biosafety precautions.
Serologic tests may be negative early in disease, delaying confirmation.

Clinical Presentation

Signs & Symptoms

Fever, chills, and malaise are common systemic symptoms
Painful skin ulcer with a central necrotic eschar at the site of inoculation
Tender regional lymphadenopathy often ipsilateral to the skin lesion
Headache and myalgias frequently accompany systemic illness
Respiratory symptoms such as cough and chest pain in pneumonic tularemia

History of Present Illness

Sudden onset of high fever, chills, and headache following exposure.
Painful ulcer or papule at inoculation site often develops within days.
Regional lymphadenopathy adjacent to the lesion is common and tender.
Respiratory symptoms such as cough and chest pain occur in pneumonic forms.
Systemic symptoms include fatigue, myalgia, and anorexia progressing over 1 to 2 weeks.

Past Medical History

No specific prior conditions are required but immunosuppression may worsen disease severity.
History of outdoor activities or animal exposure increases pretest probability.
Previous tick bites or insect exposures may be relevant.

Family History

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Physical Exam Findings

Ulceroglandular lesions characterized by a painful papule that ulcerates with a central eschar
Tender regional lymphadenopathy often ipsilateral to the skin lesion
Fever and signs of systemic illness such as tachycardia and diaphoresis
Hepatosplenomegaly may be present in systemic or typhoidal forms
Conjunctival injection in cases of oculoglandular tularemia

Diagnostic Workup

Diagnostic Criteria

Diagnosis of tularemia is established by isolation of Francisella tularensis from clinical specimens such as blood, lymph node aspirate, or sputum. Serologic testing demonstrating a significant rise in specific antibody titers is also diagnostic. PCR assays can detect bacterial DNA and provide rapid confirmation. Clinical presentation with ulceroglandular lesions and regional lymphadenopathy in an appropriate epidemiologic context supports the diagnosis. Culture requires biosafety level 3 precautions due to the organism's high infectivity.

Pathophysiology

Key Mechanisms

Intracellular survival of Francisella tularensis within macrophages leads to evasion of host immune response.
Phagosome escape allows bacterial replication in the cytoplasm, causing cellular necrosis and tissue damage.
Granulomatous inflammation develops at the site of infection and regional lymph nodes, contributing to lymphadenopathy.
Endotoxin-mediated systemic inflammation triggers fever, malaise, and sepsis-like symptoms in severe cases.

Involvement	Details
Organs	Lymph nodes commonly enlarge and become tender due to bacterial infection and immune response.
	Lungs are involved in inhalational tularemia, causing pneumonia and systemic illness.
	Spleen may become enlarged as part of systemic immune activation and bacterial dissemination.
Tissues	Lymphoid tissue is involved due to regional lymphadenopathy caused by bacterial dissemination and immune activation.
	Skin is the primary site of entry and local ulceration in cutaneous tularemia.
	Lung tissue can be affected in pneumonic tularemia, leading to pneumonia and respiratory symptoms.
Cells	Macrophages are the primary host cells infected by Francisella tularensis, where the bacteria replicate intracellularly.
	Neutrophils are recruited to sites of infection and contribute to bacterial clearance and inflammation.
	Dendritic cells present bacterial antigens to activate adaptive immune responses.
Chemical Mediators	Interferon-gamma (IFN-γ) is critical for activating macrophages to kill intracellular Francisella tularensis.
	Tumor necrosis factor-alpha (TNF-α) promotes inflammation and granuloma formation in infected tissues.
	Interleukin-12 (IL-12) enhances the Th1 immune response necessary for controlling tularemia.

Treatments

Pharmacological Treatments

Streptomycin
- Mechanism:
  - Inhibits bacterial protein synthesis by binding the 30S ribosomal subunit.
- Side effects:
  - Ototoxicity
  - Nephrotoxicity
  - Neuromuscular blockade
- Clinical role:
  - First-line
Gentamicin
- Mechanism:
  - Binds the 30S ribosomal subunit to inhibit bacterial protein synthesis.
- Side effects:
  - Nephrotoxicity
  - Ototoxicity
  - Neuromuscular blockade
- Clinical role:
  - First-line
Doxycycline
- Mechanism:
  - Binds the 30S ribosomal subunit to inhibit bacterial protein synthesis.
- Side effects:
  - Photosensitivity
  - Gastrointestinal upset
  - Tooth discoloration in children
- Clinical role:
  - Alternative
Ciprofloxacin
- Mechanism:
  - Inhibits bacterial DNA gyrase and topoisomerase IV, preventing DNA replication.
- Side effects:
  - Tendonitis
  - Gastrointestinal upset
  - QT prolongation
- Clinical role:
  - Alternative

Non-pharmacological Treatments

Supportive care including hydration and fever management is essential in tularemia treatment.
Avoidance of tick and insect bites through protective clothing and repellents reduces risk of infection.
Proper handling and cooking of wild game meat prevents transmission of Francisella tularensis.

Prevention

Pharmacological Prevention

Aminoglycoside prophylaxis (e.g., streptomycin or gentamicin) for high-risk exposures
Doxycycline prophylaxis may be used in endemic areas after tick bites
No licensed vaccine is currently widely available for tularemia prevention
Post-exposure prophylaxis with ciprofloxacin in laboratory or occupational exposures

Non-pharmacological Prevention

Avoidance of tick and insect bites using protective clothing and repellents
Proper handling and cooking of wild game to prevent transmission
Use of gloves and protective equipment when handling potentially infected animals
Control of rodent populations in endemic areas
Education on safe outdoor activities in endemic regions to reduce exposure risk

Outcome & Complications

Complications

Sepsis and systemic inflammatory response syndrome in severe cases
Pneumonia with respiratory failure in pneumonic tularemia
Meningitis or meningoencephalitis in rare disseminated infections
Abscess formation in lymph nodes requiring drainage
Chronic ulceration or scarring at the site of skin lesions

Short-term Sequelae	Long-term Sequelae
Persistent lymphadenopathy lasting weeks after acute infection Local tissue necrosis and ulcer healing with scarring Secondary bacterial superinfection of skin lesions Transient hepatosplenomegaly during systemic illness Prolonged fever and malaise during recovery phase	Fibrotic scarring at sites of ulceration Chronic lymphadenitis with possible calcification Pulmonary fibrosis following severe pneumonic tularemia Rare chronic fatigue syndrome post-infection No known chronic carrier state or latent infection

Differential Diagnoses

Tularemia (Rabbit Fever) (Francisella tularensis) versus Cat Scratch Disease (Bartonella henselae)

Tularemia (Rabbit Fever) (Francisella tularensis)	Cat Scratch Disease (Bartonella henselae)
Exposure to rabbits, ticks, or deer flies	History of cat scratch or bite
Infection with Francisella tularensis	Infection with Bartonella henselae
Ulceroglandular disease with systemic febrile illness	Localized lymphadenopathy with mild systemic symptoms

Tularemia (Rabbit Fever) (Francisella tularensis) versus Plague (Yersinia pestis)

Tularemia (Rabbit Fever) (Francisella tularensis)	Plague (Yersinia pestis)
Contact with rabbits or tick bites	Contact with rodents or fleas in endemic areas
Infection with Francisella tularensis	Infection with Yersinia pestis
Slower onset ulceroglandular or pneumonic tularemia	Rapidly progressive bubonic or pneumonic illness

Tularemia (Rabbit Fever) (Francisella tularensis) versus Leptospirosis

Tularemia (Rabbit Fever) (Francisella tularensis)	Leptospirosis
Exposure to rabbits or tick bites	Exposure to water contaminated with animal urine
Ulceroglandular disease with lymphadenopathy	Biphasic illness with jaundice and renal failure
Positive serology or culture for Francisella tularensis	Positive microscopic agglutination test for Leptospira

Tularemia (Rabbit Fever) (Francisella tularensis) versus Tularemia Pneumonia (Differential from other pneumonias)

Tularemia (Rabbit Fever) (Francisella tularensis)	Tularemia Pneumonia (Differential from other pneumonias)
Patchy infiltrates with hilar lymphadenopathy in tularemia pneumonia	Lobar consolidation typical of community-acquired pneumonia
Intracellular gram-negative coccobacillus Francisella tularensis	Common bacterial pathogens like Streptococcus pneumoniae
Requires aminoglycosides or fluoroquinolones for effective treatment	Improvement with beta-lactam antibiotics

Tularemia (Rabbit Fever) (Francisella tularensis) versus Brucellosis

Tularemia (Rabbit Fever) (Francisella tularensis)	Brucellosis
Exposure to wild animals like rabbits or tick bites	Contact with livestock or unpasteurized dairy products
Acute febrile illness with ulceroglandular lesions	Chronic undulating fever with arthralgia
Positive serology or culture for Francisella tularensis	Positive blood culture or serology for Brucella species