Antibiotic of Choice for Spider Bite
For severe spider bites, amoxicillin-clavulanate (875/125 mg twice daily) is the antibiotic of choice due to its broad coverage against common skin and soft tissue pathogens, including Staphylococcus aureus. 1
Initial Assessment
When evaluating a spider bite, consider:
- Severity of the bite (local vs. systemic symptoms)
- Time since bite occurred
- Signs of secondary infection
- Patient's immune status
Antibiotic Selection Algorithm
First-line therapy:
- Amoxicillin-clavulanate (875/125 mg twice daily orally) 1, 2
- Provides excellent coverage against both gram-positive and gram-negative organisms
- Effective against beta-lactamase producing bacteria
- Can be taken with food to minimize GI side effects
For penicillin-allergic patients:
- Clindamycin (300-400 mg three times daily) 1
- Good activity against staphylococci, streptococci, and anaerobes
- May miss some gram-negative organisms
For suspected MRSA infection:
- Trimethoprim-sulfamethoxazole (1-2 double-strength tablets twice daily) 1, 3
- All MRSA isolates in spider bite infections were sensitive to TMP-SMX in a key study 3
For severe infections requiring IV therapy:
- Vancomycin (15 mg/kg IV every 12 hours) for MRSA coverage 1
- Piperacillin-tazobactam (3.37 g IV every 6-8 hours) for broad spectrum coverage 1
Important Considerations
Signs requiring antibiotic therapy:
- Increasing erythema and warmth beyond 24-48 hours
- Purulent drainage
- Systemic symptoms (fever, chills)
- Rapid spread of erythema or necrosis
Special considerations:
- Brown recluse spider bites may cause significant tissue necrosis requiring surgical debridement in addition to antibiotics 4, 5
- Secondary infection with MRSA is increasingly common in spider bites (86.8% in one study) 3
- Patients who have failed initial outpatient therapy with penicillin-based oral antibiotics should be evaluated for MRSA infection 3
Pitfalls to avoid:
- Assuming all spider bites require antibiotics - many do not if no signs of infection
- Failing to consider MRSA as a common pathogen in infected spider bites
- Delaying treatment when necrosis or systemic symptoms are present
- Not obtaining cultures when incision and drainage is performed
Duration of Therapy
- Uncomplicated infections: 5-7 days
- Complicated infections with tissue necrosis: 10-14 days
- Osteomyelitis or deep tissue involvement: 4-6 weeks
Adjunctive Measures
- Wound care with soap and water cleaning
- Elevation of affected limb if significant swelling is present
- Cold compresses for pain relief
- Consider tetanus prophylaxis if not current
- Surgical consultation for debridement if significant necrosis develops
Remember that early aggressive treatment is essential when dealing with infected spider bites, particularly given the high prevalence of MRSA in these infections 3.