What is the appropriate antibiotic prophylaxis for a dog bite wound in a patient with a cephalosporin (cephalexin) allergy?

Dog Bite Antibiotic Prophylaxis in Cephalexin-Allergic Patients

For a patient with cephalexin allergy requiring dog bite prophylaxis, use doxycycline 100 mg twice daily or a fluoroquinolone (moxifloxacin 400 mg daily preferred for anaerobic coverage, or levofloxacin 750 mg daily) for 3-5 days. 1

Understanding the Allergy Context

The key issue is that cephalexin shares R1 side chains with common penicillins (amoxicillin, ampicillin), creating a 12.9% cross-reactivity risk 2, 3. This means if you're allergic to cephalexin, you should also avoid:

• Amoxicillin-clavulanate (the first-line agent for dog bites) 1
• Other penicillins with similar side chains 2
• Cefaclor (14.5% cross-reactivity) and cefamandole (5.3% cross-reactivity) 2

Recommended Antibiotic Options

First-Line Alternatives (Oral)

Doxycycline 100 mg twice daily is the most straightforward choice because it:

• Has excellent activity against Pasteurella multocida (the primary dog bite pathogen) 1
• Provides reasonable coverage for both aerobic and anaerobic organisms 1
• Has zero cross-reactivity with beta-lactams 2
• Is explicitly listed in IDSA guidelines for animal bites 1

Fluoroquinolones are excellent alternatives:

• Moxifloxacin 400 mg daily provides the best single-agent coverage, including anaerobes 1
• Levofloxacin 750 mg daily has good Pasteurella coverage but weaker anaerobic activity 1
• Ciprofloxacin 500-750 mg twice daily covers Pasteurella well but misses some anaerobes and MRSA 1

Combination Therapy Options

If you need broader coverage (high-risk wounds), consider:

• Trimethoprim-sulfamethoxazole (160-800 mg twice daily) PLUS metronidazole (500 mg three times daily) 1

  • TMP-SMX covers aerobes but misses anaerobes 1
  • Metronidazole covers anaerobes but misses aerobes 1
  • Together they provide comprehensive coverage 1

• Clindamycin (300 mg three times daily) PLUS a fluoroquinolone 1, 4

  • Clindamycin covers staphylococci, streptococci, and anaerobes but misses Pasteurella 1
  • Adding a fluoroquinolone fills the Pasteurella gap 1, 4

Safe Beta-Lactam Alternatives (If Needed)

If you determine the cephalexin allergy was mild and non-immediate, you have additional options:

Cephalosporins with dissimilar side chains are safe 2, 3:

• Cefuroxime 500 mg twice daily has good Pasteurella activity but misses anaerobes 1
• Ceftriaxone 1 g every 12 hours IV (if parenteral therapy needed) 1, 2
• These carry only 1-2% cross-reactivity risk with cephalexin 2

Carbapenems (for severe infections requiring IV therapy):

• Can be used without testing, with only 0.87% cross-reactivity 2, 5
• Ertapenem or meropenem provide excellent coverage 2

Aztreonam has zero cross-reactivity with all beta-lactams 2, 5

Clinical Decision Algorithm

Step 1: Assess Risk Factors for Prophylaxis

Antibiotics are indicated if the patient has ANY of these 1:

• Immunocompromised state
• Asplenia
• Advanced liver disease
• Edema of affected area
• Moderate to severe injury, especially hand or face
• Penetration of periosteum or joint capsule

Step 2: Choose Antibiotic Based on Severity

For standard prophylaxis (low-moderate risk wounds):

• Doxycycline 100 mg twice daily × 3-5 days 1

For high-risk wounds or established infection:

• Moxifloxacin 400 mg daily × 3-5 days (best single agent) 1
• OR TMP-SMX 160-800 mg twice daily PLUS metronidazole 500 mg three times daily 1

For severe infections requiring IV therapy:

• Carbapenem (meropenem 1 g every 8 hours) 2
• OR Fluoroquinolone IV PLUS metronidazole IV 1

Important Caveats

• Low-risk wounds (superficial, not on hand/face, presenting <12 hours, immunocompetent patient) may not require antibiotics at all 1, 6
• The infection rate for dog bites is only 16% without antibiotics, and prophylaxis reduces this modestly 1
• Wound care is paramount: copious irrigation with povidone-iodine solution and debridement are more important than antibiotic choice 4, 6
• Hand wounds have higher infection rates (up to 13.5% even with antibiotics) and warrant more aggressive treatment 6, 7
• Consider rabies prophylaxis in consultation with local health officials 1

Common Pitfall to Avoid

Do not assume all cephalosporins are contraindicated in cephalexin allergy. Cephalosporins with dissimilar side chains (cefuroxime, ceftriaxone, cefazolin) carry only 1-2% cross-reactivity risk and can be used if the original reaction was not severe 2, 3. However, in the acute setting without formal allergy testing, the safest approach is to use a non-beta-lactam alternative like doxycycline or a fluoroquinolone 2.