What first‑line antibiotic(s) should be prescribed for a healthy adult bitten by a rat, and what alternatives are recommended if the patient is allergic to penicillins?

Antibiotic Treatment for Rat Bite

For a healthy adult with a rat bite, prescribe amoxicillin-clavulanate 875/125 mg orally twice daily for 3–5 days as first-line prophylaxis; for penicillin-allergic patients, use doxycycline 100 mg orally twice daily or a fluoroquinolone (levofloxacin 750 mg daily or ciprofloxacin 500–750 mg twice daily) plus clindamycin 300 mg three times daily. 1

Rationale for Antibiotic Selection

Rat bites carry similar microbiological risks to cat bites, with polymicrobial flora including Pasteurella species, streptococci, staphylococci, and anaerobes. 1 The wound typically harbors an average of 5 organisms, making broad-spectrum coverage essential. 1

First-Line Therapy: Amoxicillin-Clavulanate

• Amoxicillin-clavulanate 875/125 mg orally twice daily provides comprehensive coverage against the typical polymicrobial mix found in animal bite wounds, including Pasteurella multocida, staphylococci, streptococci, and anaerobes. 1

• The clavulanate component protects against beta-lactamase-producing organisms commonly found in oral flora. 1, 2

• This formulation has demonstrated high bacteriological efficacy in respiratory and soft tissue infections caused by similar pathogens. 3, 4

Duration of Prophylaxis

• Prescribe 3–5 days for uncomplicated wounds without signs of infection. 1

• Extend to 7–14 days if early signs of infection are present (erythema, warmth, purulent drainage, or systemic symptoms). 1

Indications for Prophylactic Antibiotics

Prophylactic antibiotics are specifically indicated when the rat bite involves:

• Deep puncture wounds that penetrate beyond the dermis. 1

• Wounds on high-risk anatomical sites: hands, feet, face, or near joints. 1

• Immunocompromised patients (diabetes, chronic steroid use, HIV, chemotherapy). 1

• Delayed presentation (>8–12 hours after injury) with early signs of infection. 1

Alternative Regimens for Penicillin Allergy

For Mild/Non-Severe Penicillin Allergy

• Doxycycline 100 mg orally twice daily for 3–5 days provides excellent activity against P. multocida and reliable coverage of staphylococci and anaerobes. 1

• Doxycycline is contraindicated in children under 8 years and pregnant women. 5

For Severe Penicillin Allergy (History of Anaphylaxis)

• Fluoroquinolone plus clindamycin: Ciprofloxacin 500–750 mg orally twice daily (or levofloxacin 750 mg daily) PLUS clindamycin 300 mg orally three times daily. 1

• The fluoroquinolone covers P. multocida and gram-negative organisms, while clindamycin provides anaerobic and additional gram-positive coverage. 1

• Alternative: Trimethoprim-sulfamethoxazole 160/800 mg (one double-strength tablet) orally twice daily PLUS metronidazole 250–500 mg orally four times daily. 1

Critical Pitfalls to Avoid

Do NOT use the following as monotherapy for rat bites, as they have inadequate coverage:

• First-generation cephalosporins (cephalexin, cefazolin) have poor activity against P. multocida. 1

• Penicillinase-resistant penicillins (dicloxacillin) lack adequate P. multocida coverage. 1

• Clindamycin alone fails to cover P. multocida. 1

• Erythromycin shows limited P. multocida activity. 1

• Fluoroquinolones alone (without clindamycin) miss anaerobic coverage. 1

When to Escalate to Intravenous Therapy

Transition to IV antibiotics if:

• Systemic signs develop: fever, tachycardia, hypotension, or altered mental status. 1

• Deep tissue involvement is suspected: tenosynovitis, septic arthritis, or osteomyelitis. 1

• Immunocompromised patient with moderate-to-severe injury. 1

• Rapidly spreading cellulitis or lymphangitis develops. 1

Recommended IV Regimens

• First-line IV: Ampicillin-sulbactam 1.5–3.0 g IV every 6–8 hours. 1

• For penicillin-allergic patients: Carbapenem (ertapenem 1 g IV daily, meropenem 1 g IV every 8 hours, or imipenem 1 g IV every 6–8 hours). 1

Essential Adjunctive Wound Care

• Thorough irrigation with copious sterile normal saline under pressure to remove debris and reduce bacterial load. 1

• Evaluate tetanus immunization status and administer tetanus toxoid or tetanus immune globulin as indicated. 1

• Assess rabies risk based on local epidemiology and animal behavior; consult public health authorities if indicated. 1

• Elevate the affected extremity to reduce swelling and improve venous drainage. 1

• Avoid primary closure of puncture wounds, as this increases infection risk by creating an anaerobic environment. 1

Special Consideration: Hand Wounds

• Hand bites carry the highest risk of infection and complications due to limited soft tissue coverage, proximity to tendons and joints, and poor vascular supply. 1

• Prophylactic antibiotics can significantly reduce infection risk in hand wounds. 1

• Consider early orthopedic or hand surgery consultation for deep hand bites, especially those involving the dorsum of the hand over the metacarpophalangeal joints. 1

Follow-Up Instructions

Instruct the patient to return immediately if they experience:

• Increasing pain, redness, or swelling beyond the initial wound margins. 1

• Purulent drainage from the wound. 1

• Fever (temperature >38°C or 100.4°F). 1

• Decreased range of motion in the affected extremity or joint. 1

• Red streaking (lymphangitis) extending proximally from the wound. 1