What is the most common organism causing infection from a cat scratch?

Most Common Organism from Cat Scratch

Bartonella henselae is the most common organism causing infection from a cat scratch worldwide. 1

Epidemiology and Transmission

• Bartonella henselae is the primary causative agent of cat scratch disease (CSD)
• Cats serve as the main reservoir for this bacterium 1
• Transmission occurs through:
  • Cat scratches (most common route to humans) 1
  • Cat bites
  • Potentially through cat flea (Ctenocephalides felis felis) feces 1
  • Blood transfusion (rare but possible) 1

Clinical Presentation

• Regional lymphadenopathy develops approximately 3 weeks after inoculation
• A papule or pustule appears at the site of scratch/bite 3-30 days after exposure
• Extranodal manifestations occur in approximately 2% of cases
• Suppurative nodes develop in about 10% of cases 2

Complications

While typically self-limiting in immunocompetent hosts, B. henselae can cause several serious conditions:

1. In immunocompetent individuals:

   • Parinaud oculoglandular syndrome
   • Stellate neuroretinitis
   • Persistent fever
   • Hepatosplenic infection
   • Encephalopathy
   • Osteomyelitis
   • Endocarditis 3

2. In immunocompromised individuals:

   • Bacillary angiomatosis
   • Bacillary peliosis hepatitis and splenitis
   • Relapsing bacteremia with fever syndrome 3, 4

Diagnosis

• Serology is the primary diagnostic method, though antibodies may not be detectable for up to 6 weeks after infection
• Histopathology using Warthin-Starry silver stain can reveal characteristic bacilli
• PCR is available for identification but not widely accessible
• Culture is difficult due to the fastidious nature of the organism 2
• Diagnosis is often made clinically in patients with lymphadenitis and history of cat exposure 5

Treatment

For uncomplicated cat scratch disease:

• Often self-limiting and may not require antibiotic therapy 3, 6

For more severe or persistent cases, the Infectious Diseases Society of America recommends:

• First-line treatment: Azithromycin

  • For patients >45 kg: 500 mg on day 1, followed by 250 mg for 4 additional days
  • For patients <45 kg: 10 mg/kg on day 1, followed by 5 mg/kg for 4 more days 2

• Alternative treatments:

  • Erythromycin 500 mg four times daily
  • Doxycycline 100 mg twice daily
  • Treatment duration: 2 weeks to 2 months (minimum 3 months recommended for some cases) 2

Prevention

To minimize infection risk:

• Avoid rough play with cats
• Keep cats' nails trimmed
• Implement proper flea control
• Do not allow cats to lick open wounds
• Wash hands after handling cats 2
• Immunocompromised individuals should take extra precautions when interacting with cats 2

Clinical Pearls

• Most cats naturally infected with B. henselae do not show clinical signs 1
• Young cats are more likely to carry B. henselae 6
• Cat scratch disease should be included in the differential diagnosis of neurologic disease when regional lymphadenopathy develops suddenly in a patient who owns a cat 6
• Lymphadenopathy may persist for 1-6 months despite appropriate treatment 2