What antibiotics (abx) are recommended for a patient presenting with a spider bite?

Antibiotic Management for Spider Bites

Antibiotics are NOT routinely recommended for uncomplicated spider bites, but should be initiated when signs of secondary bacterial infection develop, with empiric coverage targeting methicillin-resistant Staphylococcus aureus (MRSA) as the predominant pathogen. 1, 2, 3

When Antibiotics Are NOT Indicated

• Initial swelling and erythema at the spider bite site do not require antibiotics, as these findings typically result from venom-mediated inflammatory responses rather than bacterial infection 1, 4
• Universal antibiotic prophylaxis is not recommended for bite wounds in general 1
• Most non-venomous spider bites resolve completely within 24-48 hours with supportive care alone 4

When Antibiotics ARE Indicated

Initiate antibiotics when any of the following signs of secondary bacterial infection develop:

• Progressive erythema extending beyond the initial bite site 1, 2
• Purulent discharge from the wound 1, 2
• Systemic signs of infection including fever or elevated white blood cell count 1, 2
• Necrotic wounds or tissue breakdown requiring debridement 5

Empiric Antibiotic Selection

For infected spider bites requiring antibiotics, empiric coverage must target MRSA, as 86.8% of cultured spider bite infections grow methicillin-resistant Staphylococcus aureus 3

Oral Options (Outpatient Management):

• Trimethoprim-sulfamethoxazole (160-800 mg twice daily) - all isolated organisms in spider bite infections were sensitive to this agent 3
• Doxycycline (100 mg twice daily) as an alternative 6, 7

Intravenous Options (Severe Infections):

• Vancomycin for confirmed MRSA or severe infections requiring hospitalization 3
• Adjust antimicrobial therapy based on inoperative wound cultures and sensitivity data 3

Critical Management Pitfalls

Do NOT use penicillin-based antibiotics empirically - 29% of patients with serious spider bite infections failed initial outpatient therapy with penicillin-based oral antibiotics before requiring surgical intervention 3

Avoid these common errors:

• Do not assume all erythema requires antibiotics - initial inflammatory response is venom-mediated 1
• Do not delay surgical evaluation if infection progresses despite antibiotics 3
• Do not use first-generation cephalosporins or standard penicillins as empiric therapy given high MRSA prevalence 3

Adjunctive Wound Management

Beyond antibiotics, proper wound care is essential:

• Thorough irrigation with warm water and application of antibiotic ointment with clean occlusive dressing 2, 4
• Elevation of the affected extremity to accelerate healing if swollen 2
• Infected wounds should NOT be closed - allow healing by secondary intention 2
• Update tetanus prophylaxis if outdated or unknown 2
• Surgical debridement is required for necrotizing infections, with mean excised tissue size of 26 cm² in severe cases 3

Follow-Up Protocol

• 24-hour follow-up (phone or office visit) for all spider bites to monitor for infection development 2
• Immediate surgical evaluation if lesions enlarge rapidly or systemic symptoms develop 8, 3
• Mean time to surgical evaluation in severe cases was 5 days, but earlier intervention may prevent extensive tissue loss 3

Special Considerations for Brown Recluse Bites

For confirmed or suspected brown recluse (Loxosceles) envenomation with dermonecrosis:

• Systemic antibiotics to prevent secondary infection in necrotic wounds 6, 7, 5
• Consider dapsone for reducing polymorphonuclear leukocyte activity in necrotic ulcerations, though evidence is limited 7, 5
• Delay surgical debridement for several weeks to allow full demarcation of necrotic tissue 6
• Osteomyelitis can occur as a rare complication requiring prolonged antibiotic therapy 5