What is the appropriate treatment and management for a working-age adult with no significant medical history, presenting with worsening symptoms of spreading redness, swelling, pain, burning, nausea, headaches, and feeling unwell over four days, following a suspected spider or bug bite?

Management of Suspected Cellulitis from Insect Bite with Systemic Symptoms

This patient requires immediate empiric antibiotic therapy for presumed secondary bacterial cellulitis, along with supportive care for systemic symptoms, and should be evaluated urgently for potential tick-borne rickettsial disease given the constellation of spreading erythema, systemic symptoms, and four-day progression.

Immediate Treatment Algorithm

Step 1: Rule Out Life-Threatening Conditions First

Assess for tick-borne rickettsial disease (TBRD) given the combination of:

• Spreading redness over four days 1
• Systemic symptoms (nausea, headaches, feeling unwell) 1
• Up to 40% of TBRD patients don't recall a tick bite 2

Critical action: If TBRD is suspected based on geographic location, season, and symptom pattern, start doxycycline 100 mg twice daily immediately without waiting for confirmatory testing, as delay increases mortality from 5% to 20% 2. Fever typically resolves within 24-48 hours if TBRD is the diagnosis 2.

Step 2: Treat Presumed Secondary Bacterial Cellulitis

For spreading erythema, swelling, and pain over four days, initiate antibiotics immediately 1:

• First-line: Oral cephalexin 500 mg four times daily OR dicloxacillin 500 mg four times daily for 5-10 days
• If penicillin-allergic: Clindamycin 300-450 mg three times daily
• If MRSA risk factors present (injection drug use, prior MRSA, severe infection): Add trimethoprim-sulfamethoxazole or use clindamycin alone

The spreading redness and four-day progression strongly suggest bacterial superinfection, not just allergic inflammation 1. Unlike simple insect stings where swelling is purely inflammatory 3, this patient's worsening course over days indicates secondary infection requiring antibiotics.

Step 3: Symptomatic Management

For pain and inflammation:

• Oral ibuprofen 400-600 mg every 6-8 hours OR acetaminophen 650-1000 mg every 6 hours 1, 3
• Cold compresses to affected area 1, 3

For nausea:

• Ondansetron 4-8 mg as needed
• Ensure adequate hydration given systemic symptoms 1

Avoid oral antihistamines and topical corticosteroids at this stage—these are appropriate for simple local reactions 1, 3, but this patient has progressed beyond that with spreading infection and systemic symptoms.

Critical Red Flags Requiring Emergency Evaluation

Send to emergency department immediately if any of the following develop:

• Fever >101°F (38.3°C) 1, 2
• Rapidly expanding erythema (>2 cm/hour)
• Severe pain out of proportion to examination 4
• Bullae, skin necrosis, or crepitus
• Altered mental status, confusion, or seizures 1
• Hypotension or tachycardia
• Difficulty breathing or tongue swelling (anaphylaxis) 1, 3

Geographic and Epidemiologic Considerations

If patient lives in or recently traveled to tick-endemic regions (northeastern, mid-Atlantic, or north-central United States), TBRD becomes more likely 1:

• Rocky Mountain Spotted Fever: headache, fever, rash typically appears days 2-4 but may be absent initially 1
• Ehrlichiosis/Anaplasmosis: fever, headache, malaise, nausea—rash less common 1, 2
• Lyme disease: erythema migrans typically expands over days to weeks 1

Key distinction: Simple insect sting reactions improve within 24-48 hours with symptomatic care 3. This patient's four-day worsening course with systemic symptoms demands broader differential consideration.

Follow-Up Plan

Reassess in 24-48 hours:

• Mark the border of erythema with a pen to track progression
• If no improvement or worsening despite antibiotics, consider hospitalization for IV antibiotics
• If fever develops or systemic symptoms worsen, obtain CBC, comprehensive metabolic panel, and blood cultures 2

Medical certificate: Provide work excuse for at least 2-3 days given systemic symptoms and need for close monitoring of infection progression 5.

Common Pitfalls to Avoid

Never assume spreading redness is "just allergic inflammation" after four days—this timeline strongly suggests bacterial superinfection requiring antibiotics 1, 3.

Don't delay antibiotics waiting to "see if it gets better"—spreading cellulitis can rapidly progress to abscess, necrotizing fasciitis, or sepsis 1, 4.

Don't dismiss systemic symptoms (nausea, headache, malaise) as unrelated—these may indicate TBRD, which requires immediate doxycycline to prevent severe complications or death 1, 2.

Don't prescribe antibiotics for simple insect stings with localized swelling—but this patient has crossed that threshold with four-day progression and systemic symptoms 3.