Cat scratch disease

Cat scratch disease (CSD) is a typically self-limited infectious disease that predominantly affects children and adolescents. It is caused by Bartonella henselae, a gram-negative, aerobic bacillus, which is transmitted via animal (most commonly cat) bites, scratches, or saliva exposure. CSD is typically localized and manifests with ≥ 1 erythematous nontender papule and/or pustule at the cutaneous site of inoculation, followed by painful proximal lymphadenopathy. Extranodal CSD (e.g., ocular involvement, encephalitis, culture-negative endocarditis) is rare, but immunocompromised patients are at increased risk for extranodal involvement and severe infection (e.g., bacillary angiomatosis or bacillary peliosis). Diagnosis is typically clinical but can be confirmed with laboratory studies (e.g., serology). Localized CSD in immunocompetent patients typically resolves within 6 months without treatment, although azithromycin may be used to reduce lymphadenopathy and disease duration. Specialist consultation is recommended for immunocompromised patients and those with extranodal disease. Prevention involves regular cat flea treatment, avoiding scratches and bites, and immediately washing any injuries with soap and water.

Bacillary angiomatosis is covered in more detail in a separate article.

Predominantly affects children and adolescents

Epidemiological data refers to the US, unless otherwise specified.

• Pathogen: : B. henselae (gram-negative, aerobic bacillus)
• Transmission
  • Infection is spread from animal to animal via flea bites. ^[1]
  • Human inoculation
    • Primarily occurs via animal (most commonly cat) scratches, bites, and saliva exposure
    • Rarely transmitted via flea bites ^[2]
  • Kittens are more likely to infect humans than older cats. ^[1]

Localized CSD ^[1][2][3]

Uncomplicated localized disease occurs in 85–90% of patients with CSD and includes the following:

• ≥ 1 papule and/or pustule at the cutaneous site of inoculation
  • 5–10 mm, erythematous, nontender
  • Typically manifests 3–12 days after exposure
• Regional lymphadenopathy
  • Swollen, tender lymph nodes 7–60 days after exposure
  • Usually unilateral
  • Most commonly involves lymph nodes proximal to the site of exposure in the axilla, neck, or groin
  • Occasionally suppurate
• Constitutional symptoms: fever (may be prolonged), malaise, myalgias, loss of appetite ^[2][4]

Extranodal CSD ^[1][2][3]

Up to 15% of patients develop extranodal disease due to autoinoculation (e.g., in ocular involvement) or hematogenous spread. ^[1][2][3]

• Ocular involvement (occurs in up to 10% of patients) ^[1]
  • Parinaud oculoglandular syndrome: unilateral follicular conjunctivitis and tender ipsilateral preauricular lymphadenopathy
  • Neuroretinitis: sudden, unilateral, painless vision loss; optic nerve edema; star-shaped macular exudates
• Bacillary angiomatosis ^[2]
  • Multiple red-purple papules that bleed easily
  • See “Symptoms of bacillary angiomatosis” for more information.
• Bacillary peliosis ^[2][5]
  • A rare vascular condition characterized by multiple blood-filled cysts and vascular sinuses in the liver
  • Often detected incidentally on imaging
  • Symptoms (if present) include abdominal pain, fever, weight loss, jaundice, hepatomegaly, and, rarely, hemorrhagic shock.
• Additional manifestations include: ^[1][6]
  • Culture-negative endocarditis ^[1][5]
  • Encephalitis ^[2]
  • Osteomyelitis
  • Pneumonia
  • Glomerulonephritis

Bacillary angiomatosis and bacillary peliosis predominantly affect immunocompromised patients (e.g., patients with HIV and CD4⁺ count < 100 cells/mm³). ^[2][4]

General principles ^[1][2][3]

• Localized cat scratch disease can be diagnosed clinically if the patient has:
  • Characteristic features of localized CSD
  • Relevant history of exposure to cats
• Extranodal disease or diagnostic uncertainty: Confirm the diagnosis with serology and/or advanced studies.

Serology ^[1][2]

• Test: Bartonella spp. IgG and IgM
• Limitations
  • False negatives in immunocompromised patients, e.g., advanced HIV ^[7]
  • False positives from: ^[1]
    • Previous exposure
    • Infection with other Bartonella spp., Chlamydia pneumoniae, or Coxiella burnetii
• Interpretation of results ^[1]
  • Positive: IgM positive or IgG titer ≥ 1:256 ^[4]
  • Inconclusive: IgG titer between 1:64 and 1:256
    • Repeat the test in 2–4 weeks.
    • A subsequent IgG titer ≥ 1:256 or a fourfold increase from initial testing confirms the diagnosis. ^[1]
  • Negative: IgG titer < 1:64 ^[2]

Advanced testing

The following studies are performed in diagnostic uncertainty, persistent symptoms, extranodal disease, and immunocompromised patients. ^[7][8]

• Blood cultures ^[2][3]
• Biopsy of lymph node or affected tissue and further studies (e.g. histopathology and/or PCR)

Diagnostic testing may help confirm the diagnosis but cannot rule out B. henselae infection. ^[2][4]

• Warthin-Starry staining of the involved lymph node may show clusters of rod-shaped bacteria.
• H&E staining of cutaneous lesions may show necrotizing granuloma formation and neutrophilic infiltrate.

Kaposi sarcoma has lymphocytic infiltrate as opposed to bacillary angiomatosis.

Consider other conditions associated with the following:

• Prolonged fever: fever of unknown origin
• Skin lesions
  • Kaposi sarcoma
  • Pyogenic granuloma ^[7]
  • See also “Differential diagnoses of bacillary angiomatosis.”
• Regional lymphadenopathy: See “Localized lymphadenopathy

The differential diagnoses listed here are not exhaustive.

Localized CSD ^[1][3][7]

• Immunocompetent patients
  • Treatment is not usually required as the infection is self-limited (resolves within 6 months) ^[1][3]
  • Consider azithromycin for 5 days to reduce lymphadenopathy and illness duration. ^[3]
• Immunocompromised patients
  • Treatment is recommended for all immunocompromised patients.
  • Erythromycin OR doxycycline (dosages are the same as for treatment of bacillary angiomatosis) ^[7]
• Painful suppurative lymph nodes: Consider aspiration for symptom relief. ^[1]

Avoid incision and drainage of suppurative lymph nodes due to the risk of fistula formation. ^[1]

Extranodal CSD

• Initiate management in consultation with appropriate specialists.
• Antibiotics are usually recommended. ^[9]
• Commonly used regimes include: ^[1][7]
  • Erythromycin OR doxycycline
  • PLUS rifampin in patients with endocarditis or CNS involvement ^[5]
• See also “Treatment of bacillary angiomatosis.”

The treatment course for extranodal disease is usually prolonged, and some immunocompromised patients may require long-term suppressive antibiotic therapy. ^[7]

• Treat all kittens and cats regularly for fleas.
• Avoid cat scratches and bites where possible; immediately wash any injuries with soap and water.
• Individuals with injuries should not let cats lick their wounds.
• Asymptomatic cats do not need to be tested, treated, or removed from the household.
• Immunocompromised individuals should avoid contact with:
  • Cats < 1 year of age
  • Stray cats
  • Cats with a history of scratching or biting