What is the diagnostic guidance for cat scratch disease?

Diagnostic Guidance for Cat Scratch Disease

The diagnosis of cat scratch disease relies primarily on serological testing with high titers (greater than 1:256) of immunoglobulin G antibody to Bartonella henselae, along with a history of cat exposure, as the organism is fastidious and difficult to grow in culture. 1

Clinical Presentation

• Classic cat scratch disease presents with a papule or pustule developing 3-30 days after a cat scratch or bite, followed by regional lymphadenopathy (typically draining the affected area) about 3 weeks after inoculation 1
• Lymphadenopathy generally resolves within 1-6 months, with suppuration occurring in approximately 10% of cases 1
• Extranodal disease (involving central nervous system, liver, spleen, bone, and lung) develops in ≤2% of cases 1
• Fever of unknown origin may be the presenting symptom in some cases, especially in disseminated disease 2, 3

Diagnostic Algorithm

Step 1: Clinical Assessment

• Evaluate for history of cat exposure (present in approximately 70% of cases) 2
• Look for characteristic lymphadenopathy, most commonly axillary (61.5% of cases) 2
• Note that lymphadenopathy may be unresponsive to standard antibiotics 2

Step 2: Laboratory Testing

• Serological testing is the cornerstone of diagnosis 4
  • Look for IgG antibody titers >1:256 to B. henselae 5
  • Be aware of potential cross-reactivity between B. henselae and B. quintana 1

Step 3: Additional Diagnostic Methods

• PCR testing of tissue specimens can be valuable, especially in atypical presentations 1, 3
• Warthin-Starry silver stain of infected lymph node tissue can confirm diagnosis, though it cannot differentiate Bartonella species 1
• Imaging findings:
  • Ultrasound typically shows conglomerated, lobulated lymph nodes with cortical thickening 2
  • Approximately one-third of cases show cystic suppurative components 2
  • CT/MRI may reveal low-density lesions in spleen, liver, and lymph nodes in disseminated disease 3

Special Considerations

• Consider cat scratch disease in cases of:
  • Regional lymphadenopathy unresponsive to standard antibiotics 2
  • Fever of unknown origin 5, 2
  • Intraabdominal lymphadenopathy with abdominal pain 3
• Differential diagnosis should include:
  • Tuberculosis and other mycobacterioses 3
  • Epstein-Barr virus infection 3
  • Malignant diseases 3

Common Pitfalls and Caveats

• Absence of documented cat scratch does not rule out the diagnosis 3
• The organism is fastidious and difficult to culture, making serological testing more reliable 1
• Early serological testing for B. henselae may prevent unnecessary invasive diagnostic procedures 3
• Atypical presentations may lead to delayed diagnosis 3
• Cat scratch disease can occasionally present without the classic triad of cat contact, primary lesion, and peripheral lymphadenopathy 3

Treatment Considerations

• Most cases of cat scratch disease are self-limited and do not require antibiotic treatment 5
• When treatment is indicated, azithromycin has been shown to speed recovery 5, 4
  • For patients >45 kg: 500 mg on day 1 followed by 250 mg for 4 additional days 1
  • For patients <45 kg: 10 mg/kg on day 1 and 5 mg/kg for 4 more days 1