What is the treatment for a pruritic (itchy) circular rash that develops after exposure to a cat?

Treatment of Pruritic Circular Rash After Cat Exposure

For a pruritic circular rash developing after cat exposure, the most likely diagnosis is cat scratch disease (CSD), and azithromycin 500 mg on day 1 followed by 250 mg daily for 4 additional days is the recommended treatment for patients >45 kg (or 10 mg/kg on day 1, then 5 mg/kg for 4 days if <45 kg). 1

Diagnostic Considerations

The presentation of a circular, pruritic rash after cat exposure most strongly suggests cat scratch disease caused by Bartonella henselae. 1 Key clinical features to confirm this diagnosis include:

• A papule or pustule develops 3-30 days following a scratch or bite from a cat 1
• Regional lymphadenopathy typically appears about 3 weeks after inoculation and may persist for 1-6 months 1
• The circular/target appearance can occur with a blue ring and peripheral red halo 1
• Pruritic rash has been documented as a presenting symptom of cat scratch disease, though less commonly emphasized 2

Important Differential Diagnoses to Rule Out

While cat scratch disease is most likely, consider:

• Allergic contact dermatitis from cat allergen (Fel d 1) found in cat dander, saliva, and urine 1
• Erysipeloid if there was handling of animals, which presents as a red maculopapular lesion with centrifugal spread and central clearing, creating a target appearance 1
• Dermatophyte infection (ringworm) transmitted from cats, though this is typically less acutely pruritic

Treatment Algorithm

First-Line Treatment: Azithromycin

For patients ≥45 kg: Azithromycin 500 mg orally on day 1, then 250 mg once daily for 4 additional days 1

For patients <45 kg: Azithromycin 10 mg/kg orally on day 1, then 5 mg/kg once daily on days 2-5 1

This recommendation is based on a double-blind, placebo-controlled study showing that lymph node size regressed by 80% at 30 days more frequently in azithromycin-treated patients (P = 0.02). 1

Symptomatic Management of Pruritus

While treating the underlying infection, address the pruritus directly:

• Topical corticosteroids: Apply mild-to-moderate potency topical corticosteroid (such as hydrocortisone or clobetasone butyrate) to affected areas 3-4 times daily for symptomatic relief 3, 4
• Emollients: Regular application to maintain skin hydration 4
• Topical menthol 0.5%: Can provide additional symptomatic relief 4
• Oral antihistamines: Non-sedating antihistamines (fexofenadine 180 mg or loratadine 10 mg) may be used as adjunctive therapy if pruritus is severe 4

Important Caveats

• Limit topical steroid application to affected areas only and use for short durations (1-2 weeks) to avoid skin atrophy 4
• Avoid sedating antihistamines as they have limited efficacy and may cause cognitive impairment, particularly in elderly patients 4
• Treatment results may be variable: While azithromycin is recommended, antimicrobial treatment of cat scratch disease has shown "variable, but rarely dramatic, results" 1

Prevention and Follow-Up

Preventing Recurrence and Transmission

• Wash any cat-associated wounds promptly 1
• Avoid rough play with cats and situations where scratches are likely 1
• Do not allow cats to lick open wounds or cuts 1
• Implement flea control for the cat, as fleas may play a role in transmission 1

When to Escalate Care

• If lymph nodes suppurate (occurs in ~10% of cases), consider drainage 1
• If extranodal disease develops (CNS, liver, spleen, bone, lung involvement in ≤2% of cases), refer for specialist evaluation 1
• If pruritus persists despite treatment, consider referral to dermatology for alternative diagnoses 4

Diagnostic Confirmation if Needed

• Serological testing for Bartonella henselae supports the diagnosis, though cross-reactivity with B. quintana can occur 1
• PCR testing is a diagnostic option 1
• Warthin-Starry silver stain of lymph node tissue can confirm diagnosis 1
• Note that Bartonella is fastidious and difficult to culture 1

Special Populations

For immunocompromised patients (especially HIV/AIDS): Be alert for bacillary angiomatosis, which requires longer treatment with erythromycin 500 mg four times daily or doxycycline 100 mg twice daily for 2 weeks to 2 months. 1