What is the recommended treatment for rectal cellulitis?

Treatment of Rectal Cellulitis

For rectal cellulitis, initiate prompt empiric broad-spectrum antibiotic therapy covering Gram-positive, Gram-negative, and anaerobic bacteria, combined with urgent surgical evaluation for source control, as perirectal infections typically originate from obstructed anal crypt glands and require drainage in addition to antibiotics. 1

Initial Assessment and Risk Stratification

• Examine for systemic signs of infection including fever, tachycardia, hypotension, or altered mental status, as these mandate hospitalization and intravenous therapy 1, 2
• Assess for crepitus, skin necrosis, severe pain out of proportion to examination, or rapid progression—these are warning signs of necrotizing fasciitis requiring emergency surgical debridement 2, 3
• Obtain blood cultures in patients with systemic toxicity, immunocompromise, or severe systemic features 2
• Consider imaging (CT or MRI) to define the extent of infection and identify abscess formation, particularly in the intersphincteric plane, ischiorectal space, or supralevator space 1

Antibiotic Selection Algorithm

For perirectal cellulitis without abscess or with incomplete source control:

• First-line empiric therapy: Vancomycin 15-20 mg/kg IV every 8-12 hours PLUS piperacillin-tazobactam 3.375-4.5 g IV every 6 hours to cover Gram-positive (including MRSA), Gram-negative, and anaerobic organisms 1, 2
• Alternative combination: Vancomycin or linezolid 600 mg IV twice daily PLUS a carbapenem (meropenem 1 g IV every 8 hours) 2
• Another alternative: Vancomycin PLUS ceftriaxone 2 g IV daily and metronidazole 500 mg IV every 8 hours 2

The rationale for broad-spectrum coverage: Perirectal abscesses originate from obstructed anal crypt glands, creating a polymicrobial infection with mixed aerobic and anaerobic flora from the gastrointestinal tract 1

Surgical Management

• Anorectal abscesses must be promptly drained surgically once diagnosed—antibiotics alone are insufficient 1
• An undrained anorectal abscess will continue to expand into adjacent tissue planes regardless of antibiotic therapy 1
• Surgical options include incision and drainage, Seton placement for complex fistulae, or fistulectomy depending on anatomy 1

Treatment Duration

• Continue antibiotics for 7-14 days for complicated perirectal infections, guided by clinical response and adequacy of source control 2
• For simple cellulitis without abscess that responds promptly to therapy, 5 days may be sufficient if clinical improvement occurs 2
• Extend treatment beyond the initial timeframe only if infection has not improved or source control remains incomplete 1, 2

Transition to Oral Therapy

Once clinical improvement is demonstrated (typically after 4+ days of IV treatment):

• Transition to oral clindamycin 300-450 mg every 6 hours, which provides coverage for streptococci, staphylococci including MRSA, and anaerobes 2
• Alternative: Amoxicillin-clavulanate 875/125 mg twice daily for continued Gram-positive and anaerobic coverage, though this lacks MRSA activity 2

Critical Pitfalls to Avoid

• Never delay surgical consultation when perirectal abscess is suspected—antibiotics without drainage will fail 1
• Do not use beta-lactam monotherapy for perirectal cellulitis, as anaerobic coverage is essential 1
• Do not assume simple cellulitis when the infection is perirectal—always consider underlying abscess requiring drainage 1
• Recognize that suppository use can cause anorectal ulcerations predisposing to infection, particularly when corticosteroid-containing 4
• In patients with rectal carcinoma presenting with apparent cellulitis, consider inflammatory carcinoma (cancer cellulitis) if the patient is afebrile with normal white blood cell count—obtain punch biopsy 5

Adjunctive Measures

• Elevate the affected area when possible to promote drainage 2
• Provide nutritional support (enteral or parenteral) if the patient is malnourished 1
• Consider subcutaneous heparin for thromboembolism prophylaxis in hospitalized patients 1

Special Considerations for Inflammatory Bowel Disease

If the patient has underlying Crohn's disease with perianal involvement:

• First-line antibiotics: Metronidazole 400 mg three times daily and/or ciprofloxacin 500 mg twice daily for simple perianal fistulae 1
• Add azathioprine 1.5-2.5 mg/kg/day or mercaptopurine 0.75-1.5 mg/kg/day for maintenance after acute infection is controlled 1
• Reserve infliximab (three infusions of 5 mg/kg at 0,2, and 6 weeks) for refractory cases as part of a combined medical-surgical strategy 1