Bartonella is a bacterial infection in cats caused by the organism Bartonella henselae. It’s remarkably common: studies have found the bacteria in the blood of up to 1 in 3 healthy cats, with kittens carrying it at even higher rates. Most infected cats show no signs of illness at all, which is part of what makes this infection so easy to overlook. For cat owners, the bigger concern is often that Bartonella can spread to humans, causing what’s known as cat scratch disease.
How Cats Get Infected
Cats pick up Bartonella primarily through fleas. The cat flea is the main vector, and the transmission route is surprisingly indirect. When infected fleas feed on a cat, they leave behind flea feces (often called “flea dirt”) on the cat’s skin and fur. The bacteria in that flea dirt then enter the cat’s body through broken skin, such as flea bite wounds, or through contact with mucous membranes like the eyes or mouth. Cats that scratch at flea bites can drive contaminated flea feces into their own skin.
Fleas begin excreting the bacteria at least 9 days after becoming infected themselves, and the organism can survive in flea feces for up to 3 days outside the flea’s body. This means even brief flea exposure can lead to infection, and environments with flea contamination pose an ongoing risk. Outdoor cats, feral cats, and cats in multi-cat households with poor flea control are most likely to carry the bacteria. Kittens under one year old have especially high infection rates, likely because of their still-developing immune systems and higher flea burdens.
Why Most Cats Never Look Sick
The most important thing to understand about Bartonella in cats is that the vast majority of infected cats are asymptomatic carriers. They carry the bacteria in their bloodstream, sometimes for months, without ever showing obvious signs of illness. Your cat can look perfectly healthy, behave normally, and still have Bartonella circulating in its blood.
When symptoms do appear, they’re typically mild and short-lived. A cat might develop a brief fever lasting about 48 to 72 hours that resolves on its own. In rare cases, more serious signs can develop, including vomiting, lethargy, red or inflamed eyes, swollen lymph nodes, and decreased appetite. These more significant symptoms tend to show up when a cat is already stressed or dealing with another illness. Surgery, trauma, or a concurrent infection can trigger clinical signs in a cat that has been silently carrying the bacteria for weeks or months.
Diagnosing Bartonella in Cats
Testing for Bartonella in cats is tricky precisely because most carriers look healthy. There’s no single perfect test. Veterinarians generally use two approaches: blood antibody tests, which detect whether a cat’s immune system has responded to the bacteria, and PCR testing, which looks for the bacteria’s genetic material directly in a blood sample.
Antibody tests can confirm exposure but don’t necessarily mean the cat is actively carrying the bacteria at that moment. PCR testing is more specific for active infection but can miss cases because the bacteria circulate in the blood intermittently rather than constantly. A negative test doesn’t always rule out infection. Because of these limitations, and because most cats clear the infection without treatment, routine screening of healthy cats isn’t standard practice. Testing is more commonly done when a cat shows symptoms or when a household member is immunocompromised and the risk of human infection matters more.
Treatment and Recovery
Because most infected cats remain healthy carriers, treatment isn’t always necessary or even recommended. Antibiotics can reduce the amount of bacteria in a cat’s blood, but they don’t reliably eliminate the infection entirely. Cats may test positive again after completing a course of antibiotics, which makes the decision to treat a nuanced one.
For cats that do develop clinical signs like fever, eye inflammation, or swollen lymph nodes, antibiotic therapy can help resolve symptoms. Treatment courses typically run several weeks. The mild, self-limiting fever that some cats experience often resolves within a couple of days without any intervention. Your vet will weigh the severity of symptoms, the cat’s overall health, and whether anyone in the household is at higher risk for cat scratch disease before recommending a treatment plan.
The Risk to Humans
Bartonella in cats matters to most people because of cat scratch disease. In the United States, an estimated 12,000 people receive an outpatient diagnosis of cat scratch disease each year, and roughly 500 are hospitalized. The infection spreads to humans when an infected cat scratches or bites and breaks the skin, or when a cat licks an open wound. The bacteria travel from flea feces on the cat’s claws or mouth into the person’s body.
In healthy adults, cat scratch disease typically causes a red bump at the scratch or bite site followed by swollen, painful lymph nodes near the wound. It’s usually self-limiting and resolves within a few weeks to months. Children between ages 5 and 9 are diagnosed most frequently, likely because of how they play with cats and kittens. The infection poses a greater risk to people with weakened immune systems, who can develop more serious complications affecting the heart, brain, or eyes.
Preventing Infection
Flea control is the single most effective way to reduce Bartonella risk for both your cat and your family. Since fleas are the primary transmission route between cats, keeping your cat on year-round flea prevention breaks the cycle. This applies to indoor cats too, since fleas can hitch rides into homes on clothing, shoes, or other pets.
Beyond flea control, a few practical habits lower the risk of human infection. Avoid rough play that leads to scratches or bites, especially with kittens. Wash any cat scratches or bites promptly with soap and water. Don’t let cats lick open wounds or broken skin. If you’re immunocompromised, adopting an adult cat rather than a kitten reduces your exposure risk, since kittens are more likely to carry the bacteria and more likely to scratch during play. Declawing is not recommended as a prevention strategy and doesn’t eliminate the risk, since the bacteria can also be transmitted through bites and saliva contact with wounds.