Bartonella henselae infection or cat scratch disease (CSD)
People become infected with Bartonella henselae from the scratch of domestic or feral cats, particularly kittens. Cats can have fleas that carry B. henselae bacteria. These bacteria can be transmitted from a cat to a person during a scratch that is contaminated with flea stool. Infected cats that lick a person’s open wound or bite can also spread the bacteria. Some evidence suggests that these bacteria may spread directly to people by the bite of infected cat fleas, but this has not been proven.
B. henselae infection (CSD) occurs most often in children under the age of 15. Though more common in the southeast, CSD occurs throughout the United States. Stray cats are more likely than pets to be infected with B. henselae. In the United States, most cases of CSD occur in the fall and winter.
Signs and symptoms
- Low-grade fever
- Enlarged, tender lymph nodes that develop 1–3 weeks after exposure to a cat
- A papule or pustule at the site of the scratch
Rarely, infections of the eye, liver, spleen, brain, bones, or heart valves can occur. Some of these infections occur primarily in people with weakened immune systems, such as those with advanced HIV infection.
As with many Bartonella species, B. henselae can sometimes cause infection of the heart valves, called endocarditis. In many cases, blood cultures might be negative (culture-negative endocarditis), which can make the diagnosis more challenging.
- Avoid cat scratches, bites, and licks, especially from kittens or stray cats. This is especially important for people who have weakened immune systems.
- Wash hands promptly after handling cats.
- Talk to your veterinarian about flea prevention products for your cat. (Never use products that contain permethrin on cats.)
- Keep cats indoors and away from stray cats. People who have weakened immune systems should avoid owning cats less than one-year-old.
Diagnosis and testing
- B. henselae infection may be diagnosed clinically in patients with typical signs and symptoms and a compatible exposure history.
- B. henselae is a fastidious, slow-growing bacterium. Cultures should be held for a minimum of 21 days. It is often helpful for providers to alert the microbiology laboratory that B. henselae is suspected to optimize conditions for growth.
- Serology can aid the diagnosis of B. henselae, although cross-reactivity with other Bartonella species may limit interpretation. Providers should be aware that serological tests do not reliably differentiate among Bartonella species and positive results may persist for years even after effective treatment.
- B. henselae DNA may be detected by molecular assay of lymph node aspirates or blood, though sensitivity of these methods is not optimal for blood samples. However, lymph node aspiration is not generally recommended except to relieve severe pain and swelling or in cases where the diagnosis is unclear.
- Patients with infectious endocarditis sometimes have damaged heart valves that need to be surgically replaced. Excised heart valve tissue can be tested by molecular assay to confirm infection with B. henselae.
The use of antibiotics to shorten the course of CSD is debated. Most cases resolve without treatment, although some patients may develop complications from disseminated disease. Azithromycin has been shown to decrease lymph node volume more rapidly compared to no treatment. The suggested dose of azithromycin for CSD is:
- Adults and children > 45.5 kg: 500 mg on day 1, followed by 250 mg for 4 days
- Children ≤ 45.5 kg: 10 mg/kg on day 1, followed by 5 mg/kg for 4 days