Management of Rapidly Progressing Cellulitis on Left Knee
For a patient with a rapidly progressing, hot, red lesion with surrounding edema on the left knee that developed from an insect or animal bite, immediate treatment with oral amoxicillin-clavulanate is recommended as first-line therapy. 1
Clinical Assessment
This presentation is consistent with cellulitis, likely secondary to an animal or insect bite:
- Initial bite on 7/13/25 with golf ball-sized welt
- Progression to softball size with red ring and mild edema
- Hot sensation developing recently
- No systemic symptoms (no fever, no shortness of breath)
- Patient maintains knee mobility
Key Physical Examination Findings to Document:
- Exact size and borders of erythema
- Presence of fluctuance or abscess formation
- Lymphangitic streaking
- Regional lymphadenopathy
- Neurovascular status of the extremity
- Presence of any drainage or purulence
Diagnostic Workup
Laboratory Tests:
- Complete blood count (CBC) to assess for leukocytosis
- C-reactive protein (CRP) and/or erythrocyte sedimentation rate (ESR) to quantify inflammation
- Blood cultures if there are systemic symptoms or significant inflammation
Imaging:
- Plain radiographs are not indicated at this time given the recent onset and lack of trauma
- Ultrasound may be useful if there is concern for abscess formation or foreign body
- MRI is not indicated for uncomplicated cellulitis at initial presentation
Treatment Recommendations
Antimicrobial Therapy:
First-line: Oral amoxicillin-clavulanate (875/125 mg twice daily for 7-10 days) 1
- Provides coverage for common bite-related pathogens including Pasteurella multocida
- Effective against both aerobic and anaerobic organisms
Alternative options if penicillin-allergic:
- Doxycycline (100 mg twice daily)
- Trimethoprim-sulfamethoxazole plus metronidazole for anaerobic coverage
Avoid first-generation cephalosporins, macrolides, and clindamycin as monotherapy for animal bites as they have poor activity against P. multocida 1
Additional Measures:
- Elevation of the affected limb to reduce edema
- Warm compresses to improve circulation
- Mark the borders of erythema to monitor progression
- Close follow-up within 24-48 hours to assess response to therapy
Warning Signs Requiring Immediate Reassessment:
- Expansion of erythema beyond marked borders
- Development of fever or systemic symptoms
- Increasing pain disproportionate to appearance
- Development of bullae or skin sloughing
- Failure to improve within 48 hours of antibiotic initiation
Special Considerations
Risk Factors for Treatment Failure:
- Fever at presentation (temperature >38°C) increases failure risk by 4.3 times 2
- Prior cellulitis in the same area increases failure risk by 2.1 times 2
- Cellulitis at a wound site (such as a bite) increases failure risk by 1.9 times 2
Potential Complications:
- Abscess formation requiring incision and drainage
- Septic arthritis if infection spreads to knee joint
- Bacteremia and systemic infection
If the patient shows signs of treatment failure or has high-risk features, consider switching to intravenous antibiotics or hospitalization for more aggressive management.