What is cat scratch disease, a bacterial infection caused by Bartonella henselae, typically affecting children or cat owners with a history of recent cat scratches or bites?

What is Cat Scratch Disease

Cat scratch disease (CSD) is a bacterial infection caused by Bartonella henselae, typically acquired through scratches or bites from infected cats (especially kittens), presenting most commonly as self-limiting regional lymphadenopathy in children and young adults. 1

Causative Organism and Transmission

• Bartonella henselae is the primary causative agent, a fastidious gram-negative bacillus that is difficult to culture from clinical specimens 2, 3
• The cat serves as the primary reservoir, with the cat flea (not direct flea bites to humans) acting as the vector for transmission among cats 4
• In some U.S. regions, up to 50% of pet cats carry B. henselae bacteremia 4
• Transmission to humans occurs when cat claws become contaminated with feces from infected fleas, then scratch human skin 4
• In approximately 1% of cases, patients develop CSD without any documented scratch or bite 5
• B. quintana (associated with body lice and homelessness) can also cause similar disease but is less common 4

Clinical Presentation

Typical Disease Course

• A papule or pustule develops 3-30 days after the initial scratch or bite 1, 6
• Regional lymphadenopathy appears approximately 3 weeks after inoculation 1, 6
• Lymphadenopathy generally resolves spontaneously within 1-6 months 1, 6
• Suppuration of lymph nodes occurs in approximately 10% of cases 1, 6
• The disease predominantly affects children and young adults 2
• In temperate climates, higher incidence occurs in autumn and winter due to seasonal cat breeding patterns 2

Atypical Presentations

• Extranodal disease develops in ≤2% of immunocompetent patients 1, 6
• Atypical manifestations occur in up to 25% of cases and include: 2, 7
  • Parinaud's oculoglandular syndrome (conjunctivitis with preauricular lymphadenopathy)
  • Neuroretinitis and stellate retinitis
  • Encephalopathy
  • Granulomatous hepatitis and hepatosplenic infection
  • Osteomyelitis (usually caused by B. quintana)
  • Endocarditis
  • Persistent fever without localizing signs

Disease in Immunocompromised Patients

High-Risk Population

• CSD most commonly occurs late in HIV infection, with median CD4+ count <50 cells/µL 4
• In advanced HIV (CD4+ <100 cells/µL), up to 25% of culture-positive patients may never develop antibodies 1, 8
• Bartonella is a major cause of unexplained fever in late-stage AIDS patients with CD4+ <100 cells/µL 1

Severe Manifestations in Immunocompromised Hosts

• Bacillary angiomatosis (BA): vascular proliferative lesions affecting skin and internal organs, clinically indistinguishable from Kaposi's sarcoma 4, 1
• Bacillary peliosis hepatis: caused only by B. henselae, not B. quintana 4
• Relapsing bacteremia with chronic illness lasting months to years 4
• BA lesions can involve nearly every organ system, though cutaneous lesions are most readily identified 4
• Systemic symptoms include fever, night sweats, and weight loss 4

Diagnostic Approach

Clinical Diagnosis

• Diagnosis is primarily clinical in typical cases: history of cat exposure + characteristic papule/pustule + regional lymphadenopathy 1, 6

Laboratory Testing Indications

• Serologic testing is indicated when: 6
  • Clinical presentation is atypical
  • Extranodal disease is suspected
  • Confirmation needed for immunocompromised patients
• Critical pitfall: Serologic testing may not show detectable antibodies until 6 weeks after acute infection 1
• Cross-reactivity can occur between B. henselae and B. quintana 6

Advanced Diagnostic Methods

• PCR or Warthin-Starry silver stain of infected lymph node tissue can confirm diagnosis in difficult cases 1, 6
• Blood or tissue culture is the gold standard but rarely practical due to the fastidious nature of Bartonella 6
• Do not test cats for Bartonella infection—this provides no clinical benefit 1

Treatment Considerations

Immunocompetent Patients

• The majority of CSD cases in normal hosts resolve spontaneously without antibiotic treatment 2, 3
• Azithromycin is first-line when treatment is indicated (for patients >45 kg: 500 mg day 1, then 250 mg for 4 days; for patients <45 kg: 10 mg/kg day 1, then 5 mg/kg for 4 days) 1, 6
• Alternative agents include doxycycline 100 mg twice daily or erythromycin 500 mg four times daily 1

Immunocompromised Patients

• Treatment is mandatory for immunocompromised patients 1
• Erythromycin or doxycycline for more than 3 months is recommended for HIV-infected patients with BA, peliosis hepatis, or CNS involvement 1, 8
• Long-term suppression with erythromycin or doxycycline should continue until CD4+ >200 cells/µL for >6 months 8

Special Populations

• Pregnant women: Erythromycin is the only safe option; tetracyclines (doxycycline) are contraindicated 1, 8
• Children: Weight-based dosing of azithromycin applies 1

Prevention Strategies

• Control cat flea infestation on pets 4, 1
• Avoid rough play with cats and situations where scratches are likely 1
• Choose cats >1 year old in good health (kittens have higher transmission rates) 1
• Promptly wash any cat-associated wounds 1
• Do not allow cats to lick open wounds 1
• Severely immunosuppressed persons should carefully consider risks of cat ownership 1
• To avoid B. quintana exposure, avoid and treat body lice infestation 4