I have a recurrent crusted lesion on my forehead between the eyebrows that improves with mupirocin but returns; I practice good hygiene and have recent cat exposure—could this be impetigo or a dermatophyte infection, and how should it be diagnosed and treated?

Recurrent Forehead Lesion with Cat Exposure: Diagnosis and Management

This is most likely recurrent impetigo caused by Staphylococcus aureus, and you should obtain a bacterial culture from an active lesion to confirm the pathogen and guide definitive therapy, while simultaneously considering dermatophyte infection (ringworm) from cat exposure as an important differential diagnosis. 1

Why This is Probably Bacterial (Impetigo), Not Fungal

The clinical pattern strongly favors bacterial infection over dermatophytosis:

• Mupirocin responsiveness indicates bacterial etiology: The fact that mupirocin (an anti-staphylococcal/streptococcal topical antibiotic) temporarily clears the lesion is diagnostic evidence that this is a bacterial infection, not a fungal one. 1, 2

• Dermatophytes don't respond to mupirocin: Mupirocin has zero antifungal activity—it only works against bacteria, particularly S. aureus and streptococci. If this were ringworm from the cat, mupirocin would do nothing. 2, 3

• Location is atypical for cat-transmitted ringworm: While Microsporum canis (the cat dermatophyte) can infect humans, it typically causes circular patches with raised erythematous borders and central clearing ("ringworm"), most commonly on exposed areas like arms and hands where contact occurs—not specifically the glabella (between eyebrows). 3, 4

Diagnostic Approach

Obtain cultures during active infection:

• Bacterial culture: Gram stain and culture of pus or exudate from the lesion when active to identify whether S. aureus (including MRSA) or streptococci are present. 1

• Consider fungal culture if bacterial culture negative: If bacterial culture is negative and the lesion has the classic ringworm appearance (circular with raised border and central clearing), collect scales and hairs from the active edge of the lesion for fungal culture on Sabouraud agar. 3, 5

• Wood's lamp examination: Can be performed immediately—approximately 50% of M. canis infections fluoresce apple-green under Wood's lamp, but negative results don't rule out dermatophytosis. 3, 4

Why Mupirocin Alone is Failing

Mupirocin monotherapy is insufficient for recurrent staphylococcal infections:

• Topical therapy doesn't address colonization reservoirs: The patient likely has persistent S. aureus colonization in the anterior nares or other body sites that continuously reseeds the facial skin. 1, 2, 6

• The forehead location suggests autoinoculation: The glabellar area is frequently touched by hands, and nasal carriers of S. aureus can transfer bacteria from nose to hands to face. 1, 6

Definitive Treatment Strategy

For confirmed recurrent bacterial impetigo:

1. Acute treatment of active lesion:

   • Continue topical mupirocin 2% twice daily for 5 days to the active lesion. 1, 2
   • If extensive or not responding, use oral therapy: dicloxacillin or cephalexin for 7 days (if methicillin-susceptible), or doxycycline/clindamycin/TMP-SMX if MRSA is suspected or confirmed. 1

2. Decolonization protocol to prevent recurrence (this is the critical missing piece):

   • Intranasal mupirocin: Apply mupirocin 2% ointment to anterior nares twice daily for 5-10 days to eliminate nasal carriage. 2, 6
   • Chlorhexidine body washes: Daily chlorhexidine washes for 5-14 days, OR dilute bleach baths (¼-½ cup bleach per full bathtub). 2, 6
   • For persistent recurrences: Consider monthly intranasal mupirocin (twice daily for first 5 days of each month), which reduces recurrences by approximately 50%. 1, 2

3. Environmental and hygiene measures:

   • Daily washing of towels, sheets, and pillowcases during treatment. 1
   • Avoid touching face, especially after touching nose. 6
   • Clean high-touch surfaces with commercial cleaners. 6

If Dermatophytosis is Confirmed Instead

Treatment for M. canis ringworm from cats:

• Systemic antifungal therapy is required: Itraconazole is the drug of choice; terbinafine is an alternative. Treatment must continue for several weeks. 3

• Topical therapy as adjunct: Repeated body rinse with enilconazole solution or miconazole with/without chlorhexidine. 3

• The cat must be evaluated and treated: Cats are often asymptomatic carriers, and reinfection will occur unless the cat is treated. Many adult cats carry M. canis without visible lesions. 3, 4, 7

Critical Pitfall to Avoid

Don't assume cat exposure equals ringworm: The mupirocin responsiveness is the key clinical clue that this is bacterial. The cat exposure is likely a red herring in this case. However, if bacterial cultures are negative and the patient has ongoing cat contact, fungal culture becomes essential. 1, 3

Resistance concern: Prolonged or indiscriminate mupirocin use can lead to resistance development, which is why addressing the underlying colonization (not just treating active lesions repeatedly) is critical. 2, 6