What is the appropriate management, including antibiotic choice, tetanus prophylaxis, and follow‑up, for an otherwise healthy 5‑year‑old boy with a suspected spider bite that has progressed to cellulitis?

Treatment of Suspected Spider Bite with Cellulitis in a 5-Year-Old

For an otherwise healthy 5-year-old with suspected spider bite and cellulitis, initiate oral cephalexin 25 mg/kg/day divided into four doses (or 500 mg four times daily if weight >40 kg) for 5 days, ensure tetanus immunization is current (administer 0.5 mL DTaP intramuscularly if last dose was >5 years ago or status unknown), elevate the affected extremity, and reassess within 24-48 hours for clinical improvement. 1, 2

First-Line Antibiotic Selection

Beta-lactam monotherapy is the standard of care for typical pediatric cellulitis, with a 96% success rate. 2 The preferred oral agents include:

• Cephalexin 25 mg/kg/day divided every 6 hours (maximum 500 mg per dose) provides excellent coverage against beta-hemolytic streptococci and methicillin-sensitive Staphylococcus aureus, the primary pathogens in typical cellulitis 1, 2, 3
• Alternative options include dicloxacillin or amoxicillin-clavulanate if cephalexin is unavailable 1, 2

MRSA coverage is NOT routinely necessary for typical pediatric cellulitis. 2 However, spider bites that progress to severe soft tissue infections may harbor MRSA, particularly if there is purulent drainage, rapid progression, or failure of initial beta-lactam therapy. 4

When to Add MRSA Coverage

Add MRSA-active antibiotics only if the following specific risk factors are present:

• Purulent drainage or exudate visible at the bite site 1, 2, 4
• Severe pain out of proportion to examination findings (suggests necrotizing infection) 1, 4
• Failure to improve after 48-72 hours of beta-lactam therapy 2, 4
• Systemic toxicity (fever >38°C, tachycardia, altered mental status) 1, 2

If MRSA coverage is needed, use:

• Clindamycin 10-13 mg/kg/dose orally every 6-8 hours (provides single-agent coverage for both streptococci and MRSA, but only if local clindamycin resistance <10%) 2, 5
• Trimethoprim-sulfamethoxazole 4-6 mg/kg/dose twice daily PLUS cephalexin (combination therapy required because TMP-SMX lacks streptococcal coverage) 2

Critical Evidence from Spider Bite Infections

A retrospective study of 38 patients with serious soft tissue infections from spider bites found that 86.8% grew methicillin-resistant Staphylococcus aureus, and 29% had failed initial penicillin-based oral antibiotics. 4 This underscores the importance of:

• Close monitoring within 24-48 hours to detect treatment failure early 1, 2, 4
• Aggressive surgical debridement if the infection progresses despite antibiotics 4
• Switching to MRSA-active therapy immediately if erythema expands, pain worsens, or purulent drainage develops 4

Treatment Duration

Treat for exactly 5 days if clinical improvement occurs (reduction in warmth, tenderness, and erythema). 1, 2 Extend treatment beyond 5 days only if symptoms have not improved within this timeframe. 1, 2 Traditional 7-14 day courses are no longer necessary for uncomplicated cellulitis. 2

Tetanus Prophylaxis

Verify tetanus immunization status immediately. 1

• If the child has completed the primary DTaP series and the last dose was <5 years ago: no additional tetanus prophylaxis needed 1
• If the last dose was >5 years ago OR status is unknown: administer DTaP 0.5 mL intramuscularly (for children <7 years) or Tdap (for children ≥7 years) 1
• Spider bites are considered contaminated wounds requiring tetanus prophylaxis if immunization is not current 1

Essential Adjunctive Measures

• Elevate the affected extremity above heart level for at least 30 minutes three times daily to promote gravity drainage of edema and inflammatory substances 1, 2
• Cleanse the wound with sterile normal saline (no need for iodine- or antibiotic-containing solutions) and remove superficial debris 1
• Do NOT close or suture the wound if there is any sign of infection 1
• Mark the borders of erythema with a pen to objectively track progression or improvement 1, 6

Hospitalization Criteria

Admit the child if any of the following are present:

• Age <6 months with moderate-to-severe disease 2
• Systemic inflammatory response syndrome (SIRS): fever >38°C, heart rate >90 bpm (age-adjusted), respiratory rate >24 breaths/min (age-adjusted) 1, 2
• Hypotension, altered mental status, or confusion 1, 2
• Severe pain out of proportion to examination, skin anesthesia, rapid progression, or "wooden-hard" subcutaneous tissues (suggests necrotizing fasciitis requiring emergent surgical consultation) 1, 4
• Failure of outpatient therapy after 24-48 hours 1, 2

For hospitalized children requiring IV therapy:

• Vancomycin 15 mg/kg IV every 6 hours is first-line for complicated cellulitis with MRSA risk factors 2
• Clindamycin 10-13 mg/kg/dose IV every 6-8 hours is an alternative if the child is stable without bacteremia and local resistance is <10% 2

Follow-Up Protocol

Reassess within 24-48 hours (either by phone or office visit) to verify clinical response. 1, 2, 3 If the infection progresses despite appropriate antibiotics:

• Switch to MRSA-active therapy immediately (clindamycin or TMP-SMX plus beta-lactam) 2, 4
• Consider hospitalization for IV antibiotics if oral therapy fails 2, 4
• Obtain surgical consultation urgently if necrotizing infection is suspected 1, 4

Common Pitfalls to Avoid

• Do NOT routinely add MRSA coverage for typical cellulitis without specific risk factors—this represents overtreatment and increases antibiotic resistance 1, 2
• Do NOT use doxycycline in children <8 years due to risk of permanent tooth discoloration and impaired bone growth 2
• Do NOT use TMP-SMX as monotherapy for typical cellulitis—it lacks reliable streptococcal coverage and must be combined with a beta-lactam 2
• Do NOT delay surgical consultation if any warning signs of necrotizing infection are present (severe pain, rapid progression, skin anesthesia, bullae) 1, 4
• Do NOT extend antibiotics to 7-10 days automatically—only extend if clinical improvement has not occurred by day 5 1, 2