Antibiotic Treatment for Infected Brown Spider Bites
For infected brown spider bites, start trimethoprim-sulfamethoxazole (TMP-SMZ) 160-800 mg twice daily or doxycycline 100 mg twice daily as first-line therapy, as these provide optimal coverage against methicillin-resistant Staphylococcus aureus (MRSA), which is the predominant pathogen in these infections. 1, 2
Why MRSA Coverage is Critical
- Brown spider bite infections are overwhelmingly caused by MRSA. In a surgical series of 38 patients requiring debridement for infected spider bites, 86.8% grew methicillin-resistant S. aureus, and 100% grew S. aureus of some type. 2
- All isolated organisms in this series were sensitive to trimethoprim-sulfamethoxazole, making it the empiric antibiotic of choice. 2
- Standard penicillin-based antibiotics fail frequently—29% of patients in this series had already failed outpatient penicillin therapy before requiring surgical intervention. 2
First-Line Oral Antibiotic Options
For mild to moderate infections:
- TMP-SMZ 1-2 double-strength tablets (160-800 mg) twice daily is the preferred first-line agent due to proven MRSA coverage and bactericidal activity. 1, 3
- Doxycycline 100 mg twice daily is an excellent alternative, providing coverage against staphylococci including many resistant strains. 1, 3
- Treatment duration should be 7-10 days for established infections. 1
When to Add Anaerobic Coverage
Add metronidazole or switch to broader coverage if:
- Signs of necrosis are present (brown recluse bites characteristically cause necrotic ulcerations). 3, 4
- Purulent drainage or foul odor develops. 1
- Metronidazole 250-500 mg four times daily can be added to TMP-SMZ for anaerobic coverage. 1
- Clindamycin 300-450 mg three times daily covers staphylococci, streptococci, and anaerobes, but may miss some MRSA strains due to inducible resistance. 1, 3
Severe Infections Requiring Hospitalization
For severe infections with systemic symptoms:
- Vancomycin 30 mg/kg/day in 2 divided doses IV is the parenteral drug of choice for confirmed or suspected MRSA. 1, 3
- Linezolid 600 mg every 12 hours IV or PO or daptomycin 4 mg/kg every 24 hours IV are alternatives for MRSA coverage. 1, 3
- Aggressive surgical debridement is mandatory for necrotizing infections—all 38 patients in the surgical series required wide debridement with mean excised tissue size of 26 cm². 2, 1
Essential Adjunctive Wound Care
Beyond antibiotics, proper wound management is critical:
- Thorough wound irrigation with sterile normal saline (avoid iodine or antibiotic solutions). 1
- Do not close infected wounds primarily, except facial wounds managed by specialists with copious irrigation and preemptive antibiotics. 1
- Elevate the affected extremity if swollen to accelerate healing. 1
- Administer tetanus toxoid if not current within 10 years (Tdap preferred over Td if not previously given). 1
Red Flags Requiring Emergency Care
Seek immediate medical attention if:
- Difficulty breathing, muscle rigidity, dizziness, or confusion develops. 3, 1
- Pain extends beyond the bite site and becomes severe despite over-the-counter medications. 3, 1
- Rapid progression of erythema or development of bullae/necrosis occurs. 1
- Signs of systemic toxicity such as fever, hypotension, or altered mental status appear. 1
Common Pitfalls to Avoid
- Do not use penicillin-based antibiotics alone (amoxicillin, dicloxacillin, cephalexin) as empiric therapy—they will fail in the majority of cases due to MRSA prevalence. 2
- Do not delay surgical evaluation if the infection appears severe—mean time to surgical evaluation in the series was 5 days, but earlier intervention may prevent extensive tissue loss. 2
- Do not assume the patient is afebrile means no serious infection—mean temperature in the surgical series was only 98.8°F despite requiring debridement. 2
- Brown recluse bites often go unnoticed for 4-6 hours, making early diagnosis difficult, but once infection is evident, aggressive treatment is warranted. 5