Infectious Disease and Microbiology - Tularemia

Basics

Description

Tularemia is a zoonotic bacterial infection caused by Francisella tularensis. It can produce several distinct clinical syndromes depending on how the organism enters the body.

The most common presentation is ulceroglandular tularemia, characterized by a skin ulcer at the inoculation site with painful regional lymphadenopathy. Other forms include:

• Glandular
• Oculoglandular
• Oropharyngeal
• Pneumonic
• Typhoidal/systemic disease

The organism is highly infectious and is also considered a potential bioterrorism agent.

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Epidemiology

Tularemia is uncommon.

In the United States, only a few hundred cases are reported annually.

The disease occurs primarily in the Northern Hemisphere, especially in:

• North America
• Europe
• Parts of Asia

In the US, cases are concentrated particularly in the south-central states, including:

• Arkansas
• Oklahoma
• Missouri

Tick-associated cases are more common during the warmer months, while hunting-associated cases may occur during colder months when people handle infected animals.

A wide range of animals can harbor F. tularensis, including:

• Rabbits
• Rodents
• Other small mammals
• Birds

In the US, rabbits are an important reservoir.

Vectors include:

• Ticks
• Deer flies
• Mosquitoes in some geographic regions

Important tick vectors in the US include:

• American dog tick
• Wood tick
• Lone star tick

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Risk Factors

People at increased risk include:

• Hunters
• Trappers
• Farmers
• Veterinarians
• Wildlife workers
• Laboratory personnel
• People with frequent tick exposure

Risk is increased by:

• Handling infected animal carcasses
• Skinning rabbits or other wild animals
• Arthropod bites
• Drinking contaminated water
• Inhaling contaminated aerosols

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General Prevention

Prevention focuses on minimizing exposure to infected animals, arthropods, and contaminated material.

Important precautions include:

• Use insect repellent in tick-endemic areas.
• Wear long sleeves and trousers when outdoors.
• Check the body carefully for attached ticks.
• Wear gloves while skinning or handling wild animals.
• Cook game meat thoroughly.
• Avoid untreated surface water in endemic regions.
• Follow strict biosafety procedures when handling suspected laboratory specimens.

There is no routinely available vaccine for the general public.

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Pathophysiology

Humans may acquire tularemia through several routes:

• Tick or other arthropod bite
• Direct skin contact with infected animals
• Inhalation
• Ingestion
• Animal bites or scratches
• Laboratory exposure

After entering the body, F. tularensis spreads to regional lymph nodes.

The organism survives and replicates within macrophages and can subsequently disseminate through lymphatic and bloodstream routes.

Transient bacteremia may occur early in the disease.

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Etiology

Francisella tularensis is a:

• Small
• Gram-negative
• Aerobic
• Facultative intracellular coccobacillus

It is highly virulent, and only a very small inoculum is required to produce infection.

The organism:

• Requires enriched media for growth
• Grows slowly
• Can survive for prolonged periods in the environment
• May remain viable in animal tissues and water
• Poses an important laboratory-acquired infection risk

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Incubation Period

The incubation period is usually:

3–5 days

but can range from approximately:

1–14 days

Symptoms usually begin abruptly.

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Clinical Presentation

Common systemic manifestations include:

• Fever
• Chills
• Severe fatigue
• Headache
• Myalgias
• Sore throat
• Dry cough
• Malaise

Some patients experience relapsing or prolonged fever.

The clinical syndrome depends strongly on the route of infection.

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Ulceroglandular Tularemia

This is the most common form, accounting for the majority of cases.

It typically follows:

• Tick bite
• Deer fly bite
• Direct contact with an infected animal

A papule develops at the inoculation site and progresses to an ulcer.

The lesion may become:

• Painful
• Necrotic
• Crusted

A scar may remain after healing.

The characteristic accompanying finding is:

Markedly enlarged and tender regional lymph nodes

The lymphadenopathy may persist for weeks or even months.

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Glandular Tularemia

Glandular tularemia resembles the ulceroglandular form but no obvious skin ulcer is identified.

The major findings are:

• Fever
• Painful regional lymphadenopathy
• Constitutional symptoms

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Pneumonic Tularemia

Pulmonary involvement may occur by:

1. Direct inhalation of contaminated aerosols
2. Hematogenous spread from another site

Symptoms include:

• Fever
• Dry cough
• Dyspnea
• Pleuritic chest pain
• Myalgias
• Fatigue

Pneumonic tularemia can be severe and may progress to:

• Respiratory failure
• Lung abscess
• Acute respiratory distress syndrome

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Typhoidal Tularemia

Typhoidal tularemia refers to a systemic febrile illness without an obvious inoculation lesion or prominent lymphadenopathy.

Patients may develop:

• High fever
• Severe constitutional symptoms
• Headache
• Abdominal symptoms
• Diarrhea
• Cough

Pneumonic findings are common.

This form can resemble:

• Sepsis
• Enteric fever
• Other severe systemic infections

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Oculoglandular Tularemia

This occurs when the organism is inoculated into the eye.

Features include:

• Painful conjunctivitis
• Conjunctival injection
• Yellowish conjunctival ulcers
• Preauricular lymphadenopathy
• Cervical lymphadenopathy

The combination of conjunctivitis plus regional lymphadenopathy is an important clue.

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Oropharyngeal Tularemia

This form usually results from ingestion of contaminated food or water.

Manifestations may include:

• Severe sore throat
• Tonsillitis
• Pharyngitis
• Oral or pharyngeal ulcers
• Cervical lymphadenopathy

A pharyngeal membrane may occasionally resemble diphtheria.

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Physical Examination

Possible findings include:

• Fever
• Relative bradycardia in some patients
• Skin ulcer
• Regional lymphadenopathy
• Rash
• Conjunctivitis
• Pharyngeal inflammation
• Pulmonary findings

A rash may begin as:

• Macular
• Maculopapular

and occasionally evolve into pustular lesions.

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Diagnosis

Diagnosis requires a strong epidemiologic and clinical suspicion.

Important clues include:

• Tick exposure
• Rabbit or wildlife exposure
• Hunting
• Occupational exposure
• Unexplained ulcer with lymphadenopathy
• Severe atypical pneumonia
• Exposure to contaminated aerosols or water

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Laboratory Findings

Routine laboratory abnormalities are nonspecific.

Possible findings include:

• Mild leukocytosis
• Mild elevation of liver enzymes
• Sterile pyuria

More severe disease may produce:

• Elevated inflammatory markers
• Renal dysfunction
• Hepatic dysfunction
• Rhabdomyolysis

Rhabdomyolysis may indicate more severe disease.

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Serology

Serology is commonly used to confirm tularemia.

Antibodies usually do not become reliably detectable during the first several days of illness.

Diagnostic evidence may include:

• A significant rise in antibody titer between acute and convalescent samples
• A high single titer in an appropriate clinical setting

Because early serology may be negative, repeat testing is often necessary.

Cross-reactivity may occur with other organisms, so results should be interpreted in conjunction with the clinical picture.

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Culture

Culture of F. tularensis is possible but requires specialized laboratory handling.

The organism grows slowly and requires enriched media.

Because tularemia poses a substantial laboratory-acquired infection risk, clinicians must notify microbiology personnel before specimens are processed whenever the diagnosis is suspected.

This is a critical practical point.

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Molecular Testing

PCR and other molecular methods may be available through specialized or public-health laboratories.

These tests can be particularly helpful when:

• Early diagnosis is needed
• Culture is hazardous
• Serology is still negative

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Imaging

Chest radiographs in pneumonic tularemia may show:

• Patchy pulmonary infiltrates
• Nodular infiltrates
• Hilar lymphadenopathy
• Pleural effusions

Findings are nonspecific and may resemble other bacterial or atypical pneumonias.

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Pathology

Early lesions may show:

• Focal tissue necrosis
• Neutrophilic inflammation
• Macrophages

Later disease may produce:

• Granulomatous inflammation
• Necrotizing granulomas

These histologic findings can resemble tuberculosis and some fungal infections.

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Differential Diagnosis

Important differential diagnoses include:

• Plague
• Cat-scratch disease
• Staphylococcal or streptococcal lymphadenitis
• Typhoid fever
• Atypical pneumonia
• Q fever
• Psittacosis
• Rickettsial infections
• Tuberculosis
• Brucellosis

The ulceroglandular form may particularly resemble:

• Plague
• Anthrax
• Cat-scratch disease

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Treatment

Prompt antimicrobial therapy is important because untreated tularemia can become severe or disseminated.

Modern treatment depends on:

• Disease severity
• Clinical syndrome
• Age
• Pregnancy
• Drug availability
• Ability to tolerate oral treatment

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First-Line Therapy

Gentamicin

Gentamicin is a traditional first-line agent for severe tularemia.

It is given intravenously or intramuscularly.

Typical treatment duration is approximately:

7–10 days, sometimes longer depending on severity and clinical response.

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Streptomycin

Streptomycin has historically been considered a highly effective treatment.

It is usually administered intramuscularly.

Its use may be limited by:

• Availability
• Ototoxicity
• Nephrotoxicity
• Need for parenteral therapy

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Oral Alternatives

For mild or moderate disease, depending on the patient and current recommendations, oral therapy may include:

• Doxycycline
• Ciprofloxacin

These agents can be effective, although shorter tetracycline courses have historically been associated with relapse.

Adequate duration is therefore important.

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Doxycycline

Doxycycline may be used in uncomplicated disease.

Typical therapy is usually continued for:

14–21 days

to reduce the risk of relapse.

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Fluoroquinolones

Fluoroquinolones, particularly ciprofloxacin, have substantial activity against F. tularensis and are increasingly used in appropriate cases.

They can be useful for:

• Mild-to-moderate disease
• Oral step-down therapy
• Selected severe cases after initial stabilization

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Antibiotics That Should Not Be Relied Upon

F. tularensis is intrinsically resistant to many commonly used β-lactam antibiotics.

Therefore, agents such as:

• Penicillin
• Amoxicillin
• Many cephalosporins

should not be relied upon for treatment.

This is an important examination point.

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CNS Tularemia

Meningitis is uncommon but serious.

Management requires antimicrobial agents selected for activity against F. tularensis and adequate CNS treatment, usually under infectious-disease specialist guidance.

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Additional Treatment

Supportive care may include:

• Intravenous fluids
• Antipyretics
• Analgesia
• Oxygen
• Respiratory support when necessary

Severe disease may require ICU care.

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Surgical Management

Most enlarged lymph nodes do not require immediate surgery.

However, lymph nodes that become:

• Fluctuant
• Suppurative
• Persistently painful

may require aspiration or drainage.

Necrotic or secondarily infected lesions may occasionally require debridement.

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Inpatient Considerations

Hospitalization is appropriate for patients with:

• Septic shock
• Severe pneumonia
• Respiratory failure
• ARDS
• Meningitis
• Severe disseminated disease
• Inability to tolerate oral medications

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Infection Control

Routine person-to-person transmission of tularemia is essentially not recognized.

Therefore, standard precautions are generally sufficient for hospitalized patients.

The major occupational risk is to laboratory personnel, particularly during manipulation of cultures or aerosol-generating procedures.

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Bioterrorism Consideration

Francisella tularensis is considered a potential bioterrorism agent because:

• It is highly infectious.
• Only a small inoculum is required.
• It can potentially be aerosolized.
• Inhalational disease can be severe.

An unusual cluster of severe febrile pneumonia without an obvious natural exposure should prompt consideration of deliberate aerosol exposure and notification of public-health authorities.

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Prognosis

With appropriate antimicrobial therapy, prognosis is generally excellent.

Mortality is low in treated disease.

Untreated severe tularemia can cause:

• Sepsis
• Respiratory failure
• Multiorgan dysfunction
• Death

Pneumonic and typhoidal forms tend to be more severe than uncomplicated ulceroglandular disease.

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Complications

Possible complications include:

• Persistent suppurative lymphadenitis
• Pneumonia
• Lung abscess
• Acute respiratory distress syndrome
• Meningitis
• Pericarditis
• Endocarditis
• Osteomyelitis
• Peritonitis
• Hepatic dysfunction
• Renal failure
• Disseminated intravascular coagulation
• Septic shock

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High-Yield Clinical Approach

Tick bite + painful ulcer + tender regional lymph nodes

→ Think ulceroglandular tularemia

Tender regional lymphadenopathy without a skin lesion

→ Think glandular tularemia

Rabbit exposure + ulcer + lymphadenopathy

→ Strongly consider Francisella tularensis

Conjunctivitis + preauricular lymphadenopathy after animal exposure

→ Think oculoglandular tularemia

Contaminated water + severe pharyngitis + cervical lymphadenopathy

→ Think oropharyngeal tularemia

Aerosol exposure + fever + atypical pneumonia

→ Think pneumonic tularemia

Systemic fever without ulcer or prominent lymphadenopathy

→ Consider typhoidal tularemia

Suspected tularemia specimen

→ Notify the microbiology laboratory before processing

Severe tularemia

→ Aminoglycoside therapy such as gentamicin or streptomycin

Mild/moderate disease

→ Doxycycline or ciprofloxacin may be appropriate

β-lactam antibiotics

→ Generally unreliable against F. tularensis

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Exam Essentials

Causative organism:

→ Francisella tularensis

Type of organism:

→ Small gram-negative intracellular coccobacillus

Main reservoirs in the US:

→ Rabbits and other small mammals

Important vectors:

→ Ticks and deer flies

Most common form:

→ Ulceroglandular tularemia

Classic presentation:

→ Skin ulcer + painful regional lymphadenopathy

Glandular form:

→ Lymphadenopathy without a visible ulcer

Eye involvement:

→ Oculoglandular tularemia

Ingestion-associated form:

→ Oropharyngeal tularemia

Inhalation-associated form:

→ Pneumonic tularemia

Average incubation:

→ Approximately 3–5 days

Common diagnostic confirmation:

→ Serology, often with paired acute and convalescent titers

Important laboratory safety rule:

→ Warn the laboratory when tularemia is suspected

Traditional drugs of choice for severe disease:

→ Gentamicin or streptomycin

Useful oral drugs:

→ Doxycycline or ciprofloxacin

Important resistance clue:

→ β-lactams are generally ineffective

Person-to-person spread:

→ Essentially absent

Bioterrorism significance:

→ Highly infectious organism capable of causing severe aerosol-associated pneumonia