Infectious Disease and Microbiology – Pasteurella Species

Overview

Pasteurella species are Gram-negative bacilli/coccobacilli commonly carried in the oropharyngeal flora of animals, particularly cats and dogs. Pasteurella multocida is the species most strongly associated with human infection.

Human disease most commonly follows an animal bite, scratch, or other exposure to animal oral secretions. A characteristic feature is the rapid onset of painful cellulitis, often developing within 24 hours of a cat or dog bite. Infection can extend into deeper structures and cause tenosynovitis, septic arthritis, or osteomyelitis.

Classification

Genus: Pasteurella

Species listed in the source include:

• P. aerogenes

• P. canis

• P. gallinarum

• P. haemolytica

• P. multocida

• P. pneumotropica

• Other species

The most clinically important human pathogen is:

Pasteurella multocida

Microbiologic Characteristics

Pasteurella species are:

• Gram-negative

• Small bacilli or coccobacilli

• Facultatively anaerobic

• Nonmotile

• Usually oxidase positive

• Usually catalase positive

On Gram stain, P. multocida may demonstrate characteristic:

Bipolar staining

giving the organism a “safety-pin” appearance, although this finding is not always present.

High-Yield Microbiology Pattern

Small Gram-negative coccobacillus

• ●

Cat or dog oral flora

• ●

Animal bite

• ●

Rapid cellulitis

→ Think Pasteurella multocida

Incubation Period

The incubation period is usually:

Less than 24 hours

This rapid development of infection following an animal bite is an important diagnostic clue.

Epidemiology

Pasteurella species commonly colonize the mouths and upper respiratory tracts of animals.

The source reports colonization in approximately:

70–90% of cats

and

20–50% of dogs

Therefore, bites or scratches contaminated by animal saliva can readily introduce the organism into human tissue.

Transmission

The most important route of transmission is:

Animal bite

particularly from:

Cats and dogs

Other Routes of Transmission

Infection may also occur after:

• Animal scratches

• Licking of broken skin or wounds

• Contact with animal oral secretions

• Respiratory exposure to animals in some circumstances

The source notes that:

Human-to-human transmission has not been documented.

Why Cat Bites Are Important

Cat bites can be particularly problematic because their:

Long, narrow teeth

can produce deep puncture wounds.

This can inoculate bacteria into:

• Tendon sheaths

• Joints

• Deep soft tissues

• Periosteal structures

Therefore, a small external wound can sometimes conceal a significant deep infection.

High-Yield Exposure Pattern

Cat bite

↓

Within hours

↓

Rapid pain, erythema, and swelling

↓

Possible extension into tendon, joint, or bone

→ Think Pasteurella multocida

Cellulitis

The most characteristic infection is:

Cellulitis at the site of an animal bite or scratch

The infection typically develops rapidly.

Clinical Manifestations

Patients may develop:

• Pain

• Erythema

• Swelling

• Warmth

• Tenderness

• Purulent drainage in some cases

The rapid onset after animal exposure is particularly characteristic.

Regional Lymphadenopathy

The source notes that:

Regional lymphadenopathy is common

because infection at the bite or scratch site may stimulate drainage to nearby lymph nodes.

Deep Local Infection

Infection may spread from the original wound into deeper structures.

Important complications include:

• Septic arthritis

• Tenosynovitis

• Osteomyelitis

These complications are particularly important after bites involving the:

Hand

because tendons, joints, and bones are located close to the skin surface.

Tenosynovitis

Penetrating animal bites involving the hand can inoculate organisms into:

Tendon sheaths

resulting in:

Infectious tenosynovitis

This requires prompt recognition because progressive infection can impair hand function.

Septic Arthritis

If a bite penetrates or spreads into a joint:

Septic arthritis

may develop.

Patients may experience:

• Severe joint pain

• Swelling

• Restricted range of motion

• Fever

Joint aspiration and culture may be necessary.

Osteomyelitis

Deep bite wounds may occasionally extend into bone and cause:

Osteomyelitis

This complication generally requires longer antimicrobial treatment than uncomplicated cellulitis.

Respiratory Tract Infection

Pasteurella species can also cause:

Respiratory tract infections

particularly in individuals with substantial animal exposure or underlying respiratory disease.

Possible manifestations include:

• Pneumonia

• Bronchitis

• Other lower respiratory infections

Invasive and Extrapulmonary Infections

Although bite-associated cellulitis is the classic presentation, Pasteurella can occasionally cause infection at other sites.

The source lists:

• Meningitis

• Peritonitis

• Urinary tract infection

• Appendicitis

• Hepatic abscess

• Ocular infections

Ocular Infection

Reported ocular manifestations include:

• Conjunctivitis

• Keratitis

• Endophthalmitis

These infections are uncommon but demonstrate the organism’s ability to produce disease outside the classic bite-associated setting.

Bacteremia and Severe Infection

Invasive Pasteurella infection can occasionally lead to:

Bacteremia and sepsis

particularly in patients with serious underlying illness or impaired host defenses.

Isolation of Pasteurella from blood should therefore be treated as clinically significant.

Diagnosis

The primary diagnostic method is:

Culture

Depending on the clinical syndrome, specimens may include:

• Wound material

• Abscess material

• Blood

• Synovial fluid

• CSF

• Respiratory specimens

• Other normally sterile fluids

Important Clinical History

When Pasteurella infection is suspected, ask specifically about:

Animal exposure

including:

• Cat bites

• Dog bites

• Scratches

• Wound licking

• Other close animal contact

Patients may not initially consider a minor scratch or lick medically important.

Treatment

For uncomplicated bite-associated cellulitis, the source lists:

Penicillin G

or

Amoxicillin–clavulanate

The source describes approximately:

7–10 days

of therapy for cellulitis.

Duration should ultimately depend on the clinical syndrome and response.

Why Amoxicillin–Clavulanate Is Important

Animal-bite wounds are often:

Polymicrobial

They may contain:

• Pasteurella

• Streptococci

• Staphylococci

• Anaerobic organisms

• Other animal oral flora

Therefore:

Amoxicillin–clavulanate

is particularly useful because it provides broader coverage appropriate for many infected animal-bite wounds.

Additional Treatment Options

The source lists:

• Doxycycline

• Third-generation cephalosporins

• Ofloxacin

• Trimethoprim–sulfamethoxazole

Antibiotic selection should account for the entire expected flora of an animal bite rather than Pasteurella alone.

Invasive Infection

The source recommends treatment for at least:

14 days

for invasive infection.

However, the required duration varies substantially according to the site.

For example:

Bacteremia

→ Longer than uncomplicated cellulitis may be required

Septic arthritis

→ Requires appropriate joint management plus prolonged antimicrobial therapy

Osteomyelitis

→ Usually requires substantially longer treatment

Meningitis

→ Requires systemic therapy with adequate CNS penetration

Wound Management

Antimicrobial therapy is only one component of animal-bite management.

The wound should be:

Promptly cleaned

↓

Copiously irrigated

↓

Evaluated for:

• Foreign material

• Tendon injury

• Joint penetration

• Bone involvement

• Neurovascular injury

↓

Debrided when necessary

Antibiotic Prophylaxis

Preventive antibiotics may be considered after certain animal bites, particularly when the risk of infection is high.

Important higher-risk situations include:

• Cat bites

• Bites involving the hand

• Deep puncture wounds

• Wounds near joints or tendons

• Significant immunocompromise

• Delayed presentation

Amoxicillin–clavulanate is commonly used when antibiotic prophylaxis is indicated.

Other Important Animal-Bite Considerations

Management of an animal bite should not focus exclusively on Pasteurella.

The clinician should also assess:

Tetanus immunization status

and

Rabies exposure risk

according to the animal, circumstances of the bite, vaccination status, and local epidemiology.

Pasteurella vs. Capnocytophaga

Pasteurella multocida

→ Cat and dog bites

→ Rapid cellulitis, often within 24 hours

→ Septic arthritis/tenosynovitis/osteomyelitis possible

→ Amoxicillin–clavulanate commonly used

Capnocytophaga canimorsus

→ Dog exposure particularly important

→ Can cause fulminant sepsis

→ Classically severe in patients with asplenia or other major risk factors

Pasteurella vs. Bartonella henselae

Pasteurella multocida

→ Bite or scratch

→ Rapid cellulitis at inoculation site

→ Symptoms often begin within hours

Bartonella henselae

→ Cat scratch disease

→ Papule at inoculation site may occur

→ Regional lymphadenopathy is prominent

→ Usually develops over a longer interval

High-Yield Distinction

Cat bite + cellulitis within 24 hours

→ Pasteurella multocida

Cat scratch + delayed prominent regional lymphadenopathy

→ Bartonella henselae

Prevention

The primary preventive strategy is:

Avoid animal bites and scratches

When an exposure occurs:

• Immediately wash the wound

• Copiously irrigate the area

• Remove contamination or foreign material

• Debride devitalized tissue when necessary

• Seek evaluation for deep or high-risk wounds

• Assess tetanus status

• Assess rabies risk when appropriate

High-Yield Clinical Pattern

Cat or dog bite

• ●

Symptoms develop in <24 hours

• ●

Rapidly progressive painful cellulitis

• ●

Gram-negative coccobacillus

→ Think Pasteurella multocida

High-Yield Complication Pattern

Animal bite to hand

• ●

Rapid cellulitis

• ●

Pain with joint or tendon movement

→ Consider deeper Pasteurella infection:

Tenosynovitis / septic arthritis / osteomyelitis

Exam Essentials

Genus: Pasteurella

Major human pathogen: Pasteurella multocida

Type: Gram-negative bacillus/coccobacillus

Reservoir: Oropharyngeal flora of animals

Classic animals: Cats and dogs

Cat carriage in source: 70–90%

Dog carriage in source: 20–50%

Major transmission: Animal bite or scratch

Incubation: Usually <24 hours

Classic infection: Rapid cellulitis at bite/scratch site

Regional lymphadenopathy: Common

Local complications: Tenosynovitis, septic arthritis, osteomyelitis

Other infections: Respiratory disease, meningitis, peritonitis, UTI, hepatic abscess, and ocular infection

Diagnosis: Culture

Classic treatment: Penicillin-active therapy

Animal-bite infection: Amoxicillin–clavulanate is particularly useful because wounds may be polymicrobial

Source duration for cellulitis: 7–10 days

Source duration for invasive disease: At least 14 days, with longer therapy depending on the site

Prevention: Avoid bites/scratches and promptly clean and irrigate wounds

Additional bite management: Consider tetanus, rabies risk, and antibiotic prophylaxis when indicated

Key clinical pearl: A cat or dog bite followed within hours by rapidly developing painful cellulitis is classic for Pasteurella multocida. Cat bites are particularly concerning because deep puncture wounds can inoculate tendon sheaths, joints, and bone, producing tenosynovitis, septic arthritis, or osteomyelitis; prompt wound irrigation and appropriate antimicrobial therapy, commonly amoxicillin–clavulanate for infected bite wounds, are central to management.