Human and Animal Bites
Clinical Snapshot
Bite wounds are polymicrobial inoculation injuries, not ordinary cellulitis. Human bites contain oral streptococci, Eikenella corrodens, and anaerobes; cat and dog bites frequently involve Pasteurella multocida and can involve Capnocytophaga canimorsus.[1] Good wound management, tetanus/rabies assessment, and recognition of deep structural injury are as important as antibiotics. Amoxicillin/clavulanate is 1st-line oral therapy because it reliably covers the expected mixed flora — narrower agents (cephalexin, cloxacillin, clindamycin, or macrolide monotherapy) do not.
Recognize the Condition
Bite patterns by source
Human bite
An open bite wound, or a small laceration over a knuckle after a punch. A closed-fist injury can penetrate the joint capsule or extensor tendon even when the skin wound looks minor.
Cat bite
Deep, narrow puncture wounds, often on the hand, with potentially rapid cellulitis or tenosynovitis.
Dog bite
Crush injury, laceration, or abrasion; tissue devitalization may be greater than the surface wound suggests.
Risk Factors
- Hand, face, foot, or genital location.
- Deep puncture or crush injury.
- Delayed presentation.
- Proximity to joint, tendon sheath, bone, or prosthetic joint.
- Diabetes, immunosuppression, advanced liver disease, or asplenia.
- Significant edema or vascular compromise.
Red Flags & Refer
Urgent/surgical assessment for any of the following:
- A closed-fist human bite.
- Hand tendon/joint involvement.
- Neurovascular deficit.
- Exposed bone.
- Septic arthritis/tenosynovitis concern.
- Severe crush injury.
- Rapidly spreading infection.
- Systemic toxicity.
- Necrosis.
- An immunocompromised/asplenic patient with significant infection.
Assessment & Diagnosis
- Identify species/person, time of injury, and provoked/unprovoked animal behaviour.
- Assess depth, contamination, tendon function, joint movement, neurovascular status, and any foreign body/tooth fragment.
- Assess tetanus immunization status and give indicated prophylaxis per the current Canadian Immunization Guide.[2]
- Complete a rabies exposure assessment using current public-health guidance — antibiotics do not replace rabies post-exposure prophylaxis.[3]
- Culture only clinically infected wounds; do not routinely culture fresh uninfected bites.
Treatment Targets
- Prevent or treat polymicrobial infection.
- Protect tendon, joint, bone, and neurovascular structures.
- Prevent tetanus and rabies when exposure warrants prophylaxis.
- Achieve wound healing without trapping infection.
Choose Initial Treatment
When antibiotic prophylaxis or treatment is indicated for a human, cat, or dog bite, choose amoxicillin/clavulanate first line — 875/125 mg PO Q12H, generally 3-5 days for prophylaxis or 5-10 days for established mild infection.
Beta-lactam allergy
Alternative: cefuroxime + metronidazole.
Alternative for severe allergy: doxycycline + metronidazole.
Moderate infection needing parenteral therapy
1st line: ceftriaxone + metronidazole.
Severe, limb-threatening infection
1st line: piperacillin/tazobactam.
Do not use cephalexin, cloxacillin, clindamycin, or macrolide monotherapy for animal bites — they do not reliably cover Pasteurella and the mixed oral flora.
Non-Pharmacologic Treatment
- Copious irrigation is mandatory.
- Debride devitalized tissue and remove foreign material.
- Do not routinely close high-risk puncture wounds or infected wounds primarily.
- Elevate an injured hand/limb.
- Update tetanus immunization as indicated.
- Complete rabies risk assessment and public-health consultation when appropriate.
Special Populations
Pregnancy
Amoxicillin/clavulanate is preferred when susceptible/appropriate. Avoid doxycycline-based alternatives when another effective regimen is available.
Asplenia or severe immunocompromise
Use a lower threshold for prophylaxis, systemic therapy, and urgent assessment — Capnocytophaga infection can become severe rapidly in these patients.
Children
Use weight-based amoxicillin/clavulanate dosing. Any suspected joint/tendon/hand deep injury warrants medical assessment.
Monitoring & Follow-Up
High-risk bite wounds require reassessment within 24-48 hours. Worsening pain, reduced joint range of motion, new swelling, fever, or lymphangitic spread requires urgent review. Treatment duration should be extended for deeper infections such as tenosynovitis, septic arthritis, or osteomyelitis under specialist direction.
Common Errors to Avoid
- Using cephalexin alone for a cat or dog bite.
- Missing a closed-fist human bite because the skin wound is small.
- Forgetting tetanus and rabies assessment.
- Closing a contaminated puncture wound without considering infection risk.
- Treating a deep hand infection with oral antibiotics alone.
Clinical Decision Algorithm
Drug Tables
How to use this table: Doses are adult doses and require patient-specific confirmation (renal/hepatic function, weight, allergy history). Pediatric dosing is weight-based per the current product monograph unless stated. Drug pricing is intentionally omitted. Select any drug name to open its complete Pharmasist Drug Database page.
Oral therapy
| Drug | Dosage | Adverse effects | Comments |
|---|---|---|---|
| Amoxicillin/clavulanate | Adults: 875/125 mg PO Q12H or 500/125 mg PO Q8H Children: 40 mg/kg/day (amoxicillin component) divided Q8H PO (max 1750 mg/day) | GI effects, hypersensitivity | 1st line for prophylaxis and mild infected bites. |
| Cefuroxime + Metronidazole | Combo (adults): cefuroxime 500 mg PO BID + metronidazole 500 mg PO BID-TID Combo (children): cefuroxime 20-30 mg/kg/day divided Q12H PO + metronidazole 15-30 mg/kg/day divided Q8-12H PO | GI effects; metronidazole peripheral neuropathy with prolonged use | Selected penicillin allergy. |
| Doxycycline + Metronidazole | Combo (adults only): doxycycline 100 mg PO BID + metronidazole 500 mg PO BID-TID | Photosensitivity/esophagitis; metronidazole GI effects | Severe penicillin allergy (adult); avoid doxycycline in pregnancy. |
Parenteral therapy
| Drug | Dosage | Adverse effects | Comments |
|---|---|---|---|
| Ceftriaxone + Metronidazole | Combo (adults): ceftriaxone 1-2 g IV once daily + metronidazole 500 mg PO/IV Q8-12H Combo (children): ceftriaxone 50-75 mg/kg IV once daily (max 1000 mg/day) + metronidazole 15-30 mg/kg/day divided Q8-12H | Hypersensitivity, GI effects | Moderate infection needing parenteral therapy. |
| Piperacillin/tazobactam | Adults: 3.375 g IV Q6H Children: 240-300 mg/kg/day (piperacillin component) divided Q6-8H IV (max 16 g/day) | Hypersensitivity, GI effects, cytopenias | Severe limb-threatening polymicrobial infection. |
| Meropenem | Adults: 500 mg IV Q6H (institutional severe-infection dosing may differ) Children: 10 mg/kg Q8H IV (max 500 mg/dose) | GI effects, hypersensitivity | Reserve for resistant polymicrobial pathogens under specialist direction. |
References
- Microbiology of animal bite wound infections — Clinical Microbiology Reviews (Abrahamian FM, Goldstein EJ). Clin Microbiol Rev 2011;24(2):231-246
- Tetanus toxoid: Canadian Immunization Guide — Public Health Agency of Canada. Canadian Immunization Guide
- Rabies vaccine: Canadian Immunization Guide — Public Health Agency of Canada. Canadian Immunization Guide
- Practice guidelines for the diagnosis and management of skin and soft tissue infections: 2014 update by the Infectious Diseases Society of America — Clinical Infectious Diseases (Stevens DL, Bisno AL, Chambers HF, et al.). Clin Infect Dis 2014;59(2):e10-e52
Professional reference only. Apply clinical judgment and verify patient-specific decisions against current authoritative guidance and product information.