Patho/Etiology
- Organism: A small, gram-negative coccobacillus that is facultatively anaerobic. It possesses a polysaccharide capsule, a key virulence factor, and exhibits bipolar staining ("safety pin" appearance).
- Source: It is a common commensal in the upper respiratory tracts of various animals, especially cats and dogs.
- Transmission: Primarily through animal bites or scratches, particularly from cats. Transmission can also occur via licking of open wounds or inhalation of respiratory droplets.
Clinical Presentation
- Classic Presentation: Rapidly progressive cellulitis at the site of an animal bite or scratch, typically developing within 24 hours. The area becomes erythematous, swollen, and markedly tender, often with purulent or serosanguinous drainage.
- Systemic Symptoms: Fever, lymphadenopathy, and signs of systemic illness can occur.
- Buzzwords: "Dog bite" or "cat bite" followed by a rapidly developing skin infection.
Diagnosis
- Clinical: Diagnosis is often based on the classic history of animal exposure followed by rapid onset of cellulitis.
- Lab: Gram stain of wound exudate shows gram-negative coccobacilli.
- Culture: Definitive diagnosis is made by culturing the organism from the wound or blood. It grows well on 5% sheep blood agar.
DDx
- Staphylococcus aureus / Streptococcus pyogenes: Common causes of cellulitis, but the onset is typically less rapid than with Pasteurella.
- Capnocytophaga canimorsus: Another organism found in dog oral flora, which can cause severe sepsis, particularly in asplenic or immunocompromised patients.
- Anaerobic infections: Often part of a polymicrobial wound infection following a bite.
Management/Treatment
- Wound Care: Crucial first step involves thorough irrigation and debridement of the wound.
- First-line Antibiotics: Amoxicillin-clavulanate is the empiric drug of choice as it covers Pasteurella and other common oral flora (aerobes and anaerobes) from animal mouths.
- Penicillin Allergy: Alternatives include doxycycline, trimethoprim-sulfamethoxazole, or a fluoroquinolone.
- Prophylaxis: Antibiotic prophylaxis is recommended for high-risk bites (e.g., deep puncture wounds from cat bites, bites to the hand, or in immunocompromised patients).
Key Associations/Complications
- Local Complications: The infection can spread to deeper tissues, leading to:
- Systemic/Invasive Disease (less common):
- Bacteremia and Sepsis
- Meningitis
- Pneumonia or lung abscesses, particularly in patients with pre-existing chronic lung disease.
- High-Risk Populations: Immunocompromised individuals (e.g., due to diabetes, liver cirrhosis, or asplenia) and the elderly are at increased risk for severe and disseminated infections.