What antibiotics are used to treat spider bites?

Antibiotic Treatment for Spider Bites

For spider bites with signs of infection, amoxicillin-clavulanate (875/125 mg twice daily) is the first-line antibiotic treatment, with clindamycin (300-400 mg three times daily) as an alternative for penicillin-allergic patients. 1

Initial Assessment of Spider Bites

When evaluating a spider bite, consider:

• Type of spider (if known)
  • Medically important US spiders: Brown recluse (Loxosceles) and Black widow (Latrodectus) 2, 3
• Presence of infection signs:
  • Increasing redness, warmth, swelling
  • Purulent discharge
  • Worsening pain
  • Fever or systemic symptoms

Antibiotic Treatment Algorithm

1. Uninfected Spider Bites

• Antibiotics are not routinely indicated for early, uninfected spider bites 4
• Focus on wound care:
  • Clean thoroughly with soap and water
  • Apply topical corticosteroids for inflammation
  • Consider antihistamines for itching
  • Use acetaminophen or NSAIDs for pain relief

2. Infected Spider Bites

• First-line oral antibiotics (for mild-moderate infection):

  • Amoxicillin-clavulanate: 875/125 mg twice daily 1
  • Cephalexin: 500 mg four times daily 1
  • Dicloxacillin: 500 mg four times daily 1

• For penicillin-allergic patients:

  • Clindamycin: 300-400 mg three times daily 1
  • Doxycycline: Standard adult dosing 1
  • Sulfamethoxazole-trimethoprim: Standard adult dosing 1

3. Severe or MRSA Infections

• Recent evidence shows high rates (86.8%) of methicillin-resistant Staphylococcus aureus (MRSA) in spider bite infections requiring surgical intervention 5
• For suspected or confirmed MRSA:
  • Vancomycin (IV): Standard dosing 1
  • Linezolid (IV/oral): Standard dosing 1
  • Daptomycin (IV): Standard dosing 1
  • Sulfamethoxazole-trimethoprim (oral): Standard dosing 1, 5

Special Considerations

Brown Recluse Spider Bites

• May cause dermonecrosis within 72-96 hours 2
• Treatment approach:
  • Rest and ice compresses
  • Appropriate antibiotics if infected
  • Consider dapsone in select cases 2
  • Delay surgical debridement for several weeks 2
  • For non-healing wounds, hyperbaric oxygen therapy may be beneficial even months after the bite 6

Black Widow Spider Bites

• Primarily cause neuromuscular symptoms rather than local tissue damage 2, 3
• Treatment focuses on:
  • Pain control
  • Muscle relaxants
  • Calcium gluconate
  • Specific antivenin in severe cases

Important Caveats

• Failure of initial penicillin-based oral antibiotics has been reported in 29% of cases requiring surgical intervention 5
• Early aggressive treatment with appropriate antibiotics and surgical debridement may be necessary for severe infections 5
• Always obtain wound cultures before starting antibiotics when possible, especially in severe cases
• Monitor the bite site for 48-72 hours for signs of infection progression 4
• Consider follow-up within 24-48 hours to assess response to treatment 4

Remember that many conditions are misdiagnosed as spider bites. When in doubt about the diagnosis, treat according to clinical presentation of the wound and local antibiotic resistance patterns.