Antibiotic of Choice for Animal Bites
Amoxicillin-clavulanate 875/125 mg twice daily orally is the first-line antibiotic for animal bites, providing coverage against both aerobic and anaerobic bacteria including the critical pathogen Pasteurella multocida. 1
Rationale for Amoxicillin-Clavulanate
The Infectious Diseases Society of America (IDSA) provides a strong recommendation (strong, moderate evidence) that antimicrobial agents active against both aerobic and anaerobic bacteria should be used for animal bites, specifically naming amoxicillin-clavulanate as the preferred agent 1. This recommendation is based on the polymicrobial nature of bite wounds, which typically contain an average of 5 different bacterial species 1, 2.
Key Pathogens Covered
- Pasteurella multocida: Isolated from 50% of dog bites and 75% of cat bites; amoxicillin-clavulanate provides excellent coverage 1, 3
- Staphylococci and streptococci: Found in approximately 40% of bites from both dogs and cats 1
- Anaerobes: Including Bacteroides, Fusobacterium, Porphyromonas, and Peptostreptococcus species, present in 60% of wounds 1
- Capnocytophaga canimorsus: A fastidious organism that can cause fatal sepsis, especially in asplenic or immunocompromised patients 1, 2
When to Initiate Antibiotic Therapy
Preemptive antimicrobial therapy for 3-5 days is strongly recommended for high-risk patients and wounds 1:
High-Risk Patient Factors
- Immunocompromised status 1
- Asplenia 1
- Advanced liver disease 1
- Preexisting or resultant edema of the affected area 1
High-Risk Wound Characteristics
- Moderate to severe injuries, especially to the hand or face 1
- Injuries that may have penetrated the periosteum or joint capsule 1
- Puncture wounds 1
- Cat bites (infection risk is double that of dog bites) 2, 4
- Wounds presenting >9 hours after injury 1
Alternative Antibiotic Options
For Oral Therapy (if amoxicillin-clavulanate contraindicated)
Doxycycline 100 mg twice daily provides excellent activity against P. multocida, though some streptococci may be resistant 1. This is a reasonable alternative for penicillin-allergic patients.
Combination therapy options 1:
- Penicillin VK 500 mg four times daily PLUS dicloxacillin 500 mg four times daily
- Fluoroquinolone (ciprofloxacin 500-750 mg twice daily, levofloxacin 750 mg daily, or moxifloxacin 400 mg daily) PLUS metronidazole 250-500 mg three times daily for anaerobic coverage
For Intravenous Therapy (severe infections)
Ampicillin-sulbactam 1.5-3.0 g every 6-8 hours is the preferred IV option 1. Alternative IV agents include piperacillin-tazobactam, carbapenems (ertapenem, imipenem, meropenem), or second/third-generation cephalosporins with metronidazole 1.
Critical Pitfalls to Avoid
Do NOT use the following antibiotics as monotherapy for animal bites 1:
- First-generation cephalosporins (e.g., cephalexin) - poor activity against P. multocida 1
- Penicillinase-resistant penicillins alone (e.g., dicloxacillin) - miss P. multocida 1
- Macrolides (e.g., erythromycin) - inadequate P. multocida coverage 1
- Clindamycin alone - misses P. multocida despite good activity against staphylococci, streptococci, and anaerobes 1
Additional Essential Management
Tetanus prophylaxis: Administer tetanus toxoid to patients without vaccination within 10 years; Tdap is preferred over Td if not previously given 1
Rabies evaluation: Postexposure prophylaxis may be indicated; consult local health officials to determine if vaccination should be initiated 1
Wound care: Copious irrigation with normal saline using a 20-mL or larger syringe is the most important therapy to prevent infection 3, 4
Evidence Quality Note
While the IDSA guideline provides strong recommendations, the evidence base is acknowledged as moderate quality due to the absence of large, well-controlled trials definitively establishing optimal antibiotic regimens 1. However, the consistent recommendation for amoxicillin-clavulanate across multiple guidelines and its broad-spectrum coverage of the polymicrobial flora in bite wounds makes it the clear first-line choice 1, 3, 4.