Bartonella henselae is a slow-growing bacterium best known for causing cat scratch disease (CSD), one of the most common infections transmitted from animals to humans. It lives in the bloodstream of domestic and feral cats, spreads between cats through flea bites, and reaches humans primarily through cat scratches contaminated with infected flea feces. In the United States alone, roughly 4.7 out of every 100,000 people under 65 develop cat scratch disease each year, and about 500 are hospitalized annually.
How Cats Carry and Spread It
Cats pick up B. henselae from flea bites. Once infected, the bacteria circulate in a cat’s blood, often without causing any visible illness. Fleas feeding on that cat ingest the bacteria, then deposit infected feces (commonly called “flea dirt”) on the cat’s fur and claws. When a cat scratches you, flea feces contaminating its claws can enter the wound and deliver the bacteria directly into your skin. Cats can also transmit the infection by licking an open wound or biting.
Kittens are more likely to carry the bacterium and more likely to scratch during play, which is why young cats are the most common source of human infections. Some researchers suspect that flea bites themselves could transmit B. henselae directly to humans, though this hasn’t been confirmed.
Infection rates in cats vary widely by region. About 24% of domestic cats in the United States test positive for Bartonella species. Rates in other countries range from as low as 1% in parts of Spain to over 90% in some areas of Brazil, depending on climate, flea prevalence, and whether cats live indoors or outdoors.
Symptoms of Cat Scratch Disease
The hallmark of cat scratch disease is a self-limited infection, meaning it typically resolves on its own. It begins at the scratch or bite site, where a small raised bump or blister often appears within a few days. Over the following one to three weeks, nearby lymph nodes swell and become tender. This swollen-node response (lymphadenitis) is the most recognizable feature of the illness. Many people also develop low-grade fever, headache, fatigue, and nausea.
For most healthy people, these symptoms fade within a few weeks to a couple of months without treatment. The swollen lymph nodes can linger longer and occasionally drain pus, but serious complications in otherwise healthy adults and children are uncommon.
When the Infection Spreads Beyond Lymph Nodes
In a small percentage of cases, B. henselae doesn’t stay confined to the local lymph nodes. The eye is the most common site of dissemination. Between 5% and 10% of people with cat scratch disease develop ocular symptoms. The most frequent eye-related complication is Parinaud oculoglandular syndrome, seen in about 4% of patients, which causes redness and irritation of the eye along with swelling of a nearby lymph node. A less common but more concerning complication is neuroretinitis, an inflammation of the optic nerve and retina that can cause a central blind spot in your vision. Cat scratch disease is actually the leading infectious cause of neuroretinitis. On examination, doctors typically see swelling of the optic disc and a characteristic star-shaped pattern of deposits in the retina.
Serious Complications in Weakened Immune Systems
B. henselae poses a much greater threat to people with compromised immune systems, including those living with HIV, organ transplant recipients on immunosuppressive drugs, and people undergoing chemotherapy. In these groups, the bacterium can trigger two distinctive conditions.
The first is bacillary angiomatosis, where the bacteria stimulate abnormal blood vessel growth in the skin and sometimes in bone. This produces reddish-purple raised lesions that can look similar to other vascular skin conditions. B. henselae is one of the only known bacteria that can directly trigger new blood vessel formation. It does this by prompting cells to produce growth signals that drive the creation of new, fragile blood vessels.
The second condition is peliosis, where blood-filled cysts form in the liver or spleen. This can cause fever, enlarged organs, anemia, and abnormal liver tests. These internal lesions are often asymptomatic but in rare cases can rupture and bleed. People with either condition commonly experience systemic symptoms like fever, chills, weight loss, and loss of appetite. Bacillary angiomatosis can also present as an unexplained fever with no obvious source, which sometimes delays diagnosis.
How B. Henselae Is Diagnosed
Diagnosing cat scratch disease can be tricky because there is no single definitive test. Doctors typically rely on a combination of your history (particularly recent contact with cats), physical exam findings, and lab work.
The two main testing approaches are blood antibody tests and PCR (a method that detects the bacterium’s genetic material directly). Antibody testing using an immunofluorescence assay is the more widely available option. A positive result has a sensitivity of about 70%, meaning it catches roughly seven out of ten true infections, with a specificity of 95%, so false positives are rare. The tradeoff is that about 30% of people with cat scratch disease will test negative on antibodies alone, particularly early in the illness before the immune response fully develops.
PCR testing, usually performed on tissue from a swollen lymph node, is more definitive. Studies have found it to be 100% specific, meaning a positive result essentially confirms the diagnosis. Its sensitivity runs around 76%, so it can still miss some cases, but a positive PCR result is considered the strongest single piece of diagnostic evidence available. In practice, doctors often combine clinical history, antibody results, and sometimes PCR to build a confident diagnosis.
Treatment and Recovery
Most cases of cat scratch disease in healthy people resolve without antibiotics. The lymph node swelling can take weeks or even a few months to fully subside, but the infection clears on its own. For cases where symptoms are more severe, persistent, or involve organ systems beyond the lymph nodes, antibiotics can shorten the course of illness. Treatment decisions depend on which organs are affected and the patient’s immune status.
For immunocompromised patients with bacillary angiomatosis or peliosis, antibiotic treatment is essential. These conditions do not resolve on their own and can become life-threatening without proper therapy. Treatment courses for complicated infections are typically longer than for routine bacterial infections.
Reducing Your Risk
Prevention centers on flea control and basic wound hygiene. Keeping your cats on a regular flea prevention regimen is the single most effective step, since fleas are how cats become infected in the first place. Indoor cats with consistent flea treatment carry significantly lower risk.
If a cat scratches or bites you, wash the area thoroughly with soap and running water right away. Avoid rough play with cats, especially kittens, that makes scratches more likely. Don’t let cats lick any open wounds or broken skin. If you’re immunocompromised, these precautions are especially important, and adopting an older cat rather than a kitten can lower your exposure since kittens are more commonly infected and more prone to scratching.