What percentage of cat scratches become infected?

Cat Scratch Infection Rates

Cat scratches have a high infection rate of 30-50%, making them significantly more likely to become infected compared to dog bites (5-25%) or human bites (20-25%). 1

Epidemiology of Cat Scratch Infections

Cat scratches pose a substantial risk of infection due to several factors:

• The infection rate of 30-50% is significantly higher than other animal bites 1
• Young cats have particularly high carriage rates of Bartonella henselae, with prevalence approaching 50% in some areas of the United States 2
• Cat scratch disease is most commonly caused by Bartonella henselae, transmitted through scratches, cat fleas, and flea feces 2
• Seasonal variations exist, with higher rates reported in temperate climates during autumn and winter, corresponding to domestic cat breeding seasons 3

Risk Factors for Infection

Several factors influence the likelihood of infection following a cat scratch:

• Location of injury: Scratches on hands, feet, areas near joints, face, and genitals have higher infection risk 1
• Timing of treatment: Wounds treated more than 24 hours after injury have increased infection risk 1
• Patient factors: Immunocompromised individuals, especially those with HIV and low CD4+ counts, are at significantly higher risk for severe infections 2
• Depth and severity of wound: Deep wounds have higher infection rates 1

Common Pathogens

Cat scratch infections typically involve:

• Bartonella henselae: Primary causative agent of cat scratch disease 2
• Pasteurella species: Including P. multocida, P. canis, P. dagmatis 1
• Staphylococcus species: Including MRSA 1
• Streptococcus species: Including S. pyogenes 1
• Anaerobes: Including Fusobacterium, Prevotella, Bacteroides, and Porphyromonas species 1

Prevention of Infection

To reduce the risk of infection following a cat scratch:

• Promptly wash the scratch with soap and water 2
• Implement flea control for cats 2
• Avoid rough play with cats and keep their nails trimmed 2
• Do not allow cats to lick open wounds or cuts 2
• Consider the risks of cat ownership if immunocompromised 2

Management of Cat Scratch Wounds

Immediate Care

• Deep irrigation of the wound to remove foreign bodies and pathogens 1
• Avoid irrigation under pressure as it may spread bacteria into deeper tissue layers 1
• Remove any necrotic tissue to reduce pathogen burden 1

Antibiotic Prophylaxis

• Universal prophylaxis with antibiotics is not recommended for all cat scratches 1
• Prophylactic antibiotics are recommended for:
  • Fresh, deep wounds
  • Wounds in critical areas (hands, feet, near joints, face, genitals)
  • Persons at elevated risk of infection
  • Persons with implants such as artificial heart valves 1

Treatment of Established Infections

• First-line treatment: Azithromycin (500 mg on day 1, followed by 250 mg for 4 additional days for patients >45 kg) 2
• Alternative treatments: Erythromycin 500 mg four times daily or doxycycline 100 mg twice daily 2
• Treatment duration ranges from 2 weeks to 2 months, with a minimum of 3 months recommended for severe cases 2
• For immunocompromised patients, longer treatment courses (>3 months) are recommended 2

Clinical Pearls and Pitfalls

• Pitfall: Assuming all cat scratches require antibiotics. In reality, prophylactic antibiotics are only indicated for specific high-risk situations 1
• Pitfall: Using penicillins or first-generation cephalosporins for Bartonella infections, as they have no in vivo activity 2
• Pitfall: Using quinolones or TMP-SMX, which have variable in vitro activity and inconsistent clinical response 2
• Pearl: Be aware that lymphadenopathy from cat scratch disease may persist for 1-6 months despite appropriate treatment 2
• Pearl: Most cases of cat scratch disease in immunocompetent hosts resolve spontaneously without antibiotic therapy 4