Outpatient Treatment for Suspected Infected Spider Bite
Start trimethoprim-sulfamethoxazole (TMP-SMZ) 160-800 mg twice daily as first-line antibiotic therapy for suspected infected spider bites, as MRSA is the predominant pathogen in these infections. 1
Initial Assessment and Red Flags
Before initiating outpatient treatment, you must rule out conditions requiring emergency care:
- Call emergency services immediately if the patient develops difficulty breathing, muscle rigidity, dizziness, confusion, or systemic symptoms 2
- Refer urgently to the emergency department if pain extends beyond the bite site and is severe despite over-the-counter medications, if an open wound develops, or if there are signs of necrotizing infection (rapid progression of erythema, bullae, or necrosis) 2, 1
- Watch for systemic toxicity including fever, hypotension, altered mental status, or signs of hemolysis/rhabdomyolysis that warrant immediate hospitalization 2, 1
First-Line Antibiotic Therapy
TMP-SMZ is the preferred outpatient antibiotic because 87% of infected spider bites grow methicillin-resistant Staphylococcus aureus (MRSA), and all isolated organisms in published series were sensitive to TMP-SMZ 1, 3. This is critical because 29% of patients fail initial penicillin-based therapy 3.
Alternative antibiotic options:
- Doxycycline 100 mg twice daily provides excellent coverage against staphylococci and resistant strains 1
- Add metronidazole 250-500 mg four times daily to TMP-SMZ if there are signs of necrosis, purulent drainage, or foul odor suggesting anaerobic involvement 1
- Clindamycin 300 mg three times daily covers staphylococci, streptococci, and anaerobes but may miss some MRSA strains, making it less ideal than TMP-SMZ 1
Treatment duration: 7-10 days for established infections 1
Essential Wound Care Measures
Beyond antibiotics, proper wound management is mandatory:
- Irrigate thoroughly with sterile normal saline (avoid iodine or antibiotic solutions in the irrigation) 1
- Do not close infected wounds primarily except for 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
Pain Management
While treating the infection, address pain with:
- Over-the-counter acetaminophen or NSAIDs for local pain control 2
- Topical lidocaine (if skin is intact) can provide additional relief 2
- Ice application may help with pain and swelling 2
Critical Pitfalls to Avoid
Do not use penicillin-based antibiotics (amoxicillin, amoxicillin-clavulanate, or first-generation cephalosporins) as empiric therapy for infected spider bites, as they will fail in the majority of cases due to MRSA prevalence 3. The bite wound guidelines for animal bites 2 do not apply to spider bites because the microbiology is fundamentally different.
Do not delay surgical consultation if the infection appears to be progressing despite antibiotics—necrotizing infections require aggressive surgical debridement, and delays worsen outcomes 1, 3.
Recognize that brown recluse bites can cause dermonecrosis within 72-96 hours and rarely systemic loxoscelism with massive hemolysis 4. While most heal with supportive care and serious complications are rare (only 3% require grafting) 5, close monitoring is essential during the first week.
When to Escalate Care
Patients should return immediately or be referred if: