What is cat scratch disease?

What is Cat Scratch Disease?

Cat scratch disease (CSD) is a bacterial infection caused by Bartonella henselae, typically transmitted to humans through scratches or bites from infected cats (particularly kittens), though 1% of cases occur without documented animal contact. 1, 2

Pathophysiology and Transmission

The cat serves as the primary reservoir, with the cat flea (Ctenocephalides felis) acting as the vector among cats, transmitting B. henselae through flea feces that contaminate cat claws during scratching. 3 When cats scratch humans, the bacteria is inoculated into the skin. 1 In the United States, up to 50% of pet cats in some regions carry B. henselae bacteremia, making flea control and avoidance of scratches critical prevention strategies. 3

• Body lice can transmit B. quintana (a related species), particularly among homeless populations, though this is distinct from typical CSD. 3
• The disease has worldwide distribution with higher rates in autumn and winter in temperate climates, correlating with seasonal cat breeding patterns. 4

Clinical Manifestations

Typical Presentation (Immunocompetent Hosts)

• A papule or pustule develops 3-30 days after inoculation at the scratch or bite site. 1
• Regional lymphadenopathy occurs approximately 3 weeks after inoculation, representing the hallmark of CSD. 1, 2
• The lymphadenopathy typically involves epitrochlear and axillary nodes (for upper extremity scratches) or inguinal nodes (for lower extremity scratches). 5
• Lymph nodes generally resolve spontaneously within 1-6 months, with suppuration occurring in approximately 10% of cases. 1
• Most cases in immunocompetent individuals are self-limiting and do not require antibiotic treatment. 4, 6

Atypical Manifestations (≤2% of cases)

Extranodal disease can involve multiple organ systems: 1

• Parinaud's oculoglandular syndrome (conjunctivitis with preauricular lymphadenopathy) 7
• Neuroretinitis and stellate maculopathy 7
• Encephalopathy (particularly concerning in children) 4, 6
• Hepatosplenic involvement with granulomatous hepatitis 4, 6
• Osteomyelitis (more common with B. quintana) 3, 4
• Endocarditis 4, 6
• Arthritis (rare musculoskeletal manifestation) 8

Severe Disease in Immunocompromised Patients

In HIV-infected patients with CD4+ counts <100 cells/µL, Bartonella infection can cause life-threatening disseminated disease including bacillary angiomatosis (BA), bacillary peliosis hepatis, and relapsing bacteremia. 3, 1

• BA lesions are vascular proliferative lesions that can be clinically indistinguishable from Kaposi's sarcoma and require biopsy for differentiation. 3, 1
• BA represents hematogenously disseminated infection with systemic symptoms including fever, night sweats, and weight loss. 3
• Bartonella is a major cause of unexplained fever in late-stage AIDS patients and should be considered in any HIV patient with fever and CD4+ <100 cells/µL. 3, 1

Diagnosis

Clinical Diagnosis

• Most cases are diagnosed clinically based on history of cat exposure, characteristic papule/pustule at inoculation site, and regional lymphadenopathy developing 3 weeks later. 1, 5

Laboratory Confirmation (When Needed)

Serologic testing is indicated when clinical presentation is atypical, extranodal disease is suspected, or confirmation is needed for immunocompromised patients. 1

• Critical pitfall: Serologic testing may not show detectable antibodies until 6 weeks after acute infection in immunocompetent patients. 1
• In advanced HIV infection (CD4+ <100 cells/µL), up to 25% of culture-positive patients may never develop antibodies, making serology unreliable in this population. 1
• PCR or Warthin-Starry silver stain of biopsied lymph node tissue can demonstrate characteristic bacilli and vascular proliferation in difficult cases. 1
• Blood or tissue culture is the gold standard but rarely performed clinically due to the fastidious nature of Bartonella and should not delay treatment. 1
• Do not test cats for Bartonella infection—this provides no benefit to diagnosis or management. 1

Treatment

Immunocompetent Patients with Typical CSD

Azithromycin is the first-line treatment recommended by the American College of Physicians: 500 mg on day 1 followed by 250 mg daily for 4 additional days (for patients >45 kg), or 10 mg/kg on day 1 and 5 mg/kg daily for 4 more days (for patients <45 kg). 1

• Treatment accelerates lymph node resolution but most cases resolve spontaneously without antibiotics. 1, 6
• Alternative agents if azithromycin is contraindicated: doxycycline 100 mg twice daily or erythromycin 500 mg four times daily. 1

Immunocompromised Patients and Severe Disease

For HIV-infected patients with bacillary angiomatosis, peliosis hepatis, or CNS involvement, erythromycin or doxycycline for more than 3 months is recommended. 1

• Doxycycline with or without rifampin is the treatment of choice for CNS bartonellosis and other severe infections. 1
• Long-term suppression with erythromycin or doxycycline should be continued until CD4+ >200 cells/µL for >6 months. 1
• Gentamicin has shown 73% efficacy with rapid response within 48 hours for severe cases with hepatosplenic involvement. 1

Critical Treatment Pitfalls

• Penicillins and first-generation cephalosporins (like cephalexin) have no in vivo activity against Bartonella and should never be used. 1, 9
• Quinolones and TMP-SMX have variable in vitro activity and inconsistent clinical response and are not recommended. 1, 9

Special Populations

• For pregnant women requiring treatment, erythromycin is the only safe option—tetracyclines (doxycycline) are completely contraindicated. 1
• For children, weight-based dosing of azithromycin applies, with doxycycline (2-4 mg/kg/day, maximum 100-200 mg/day) or erythromycin (30-50 mg/kg/day, maximum 2 g/day) as alternatives. 9

Prevention

The CDC recommends comprehensive prevention strategies for high-risk individuals (particularly those with severe immunosuppression): 1

• Choose cats >1 year old in good health (kittens have higher B. henselae bacteremia rates). 1
• Avoid rough play with cats and situations where scratches are likely. 1
• Implement comprehensive flea control for cats to interrupt transmission. 1
• Promptly wash any cat-associated wounds with sterile saline. 1, 5
• Do not allow cats to lick open wounds. 1
• Update tetanus immunization if not current after cat scratches. 5