For most healthy people, a Bartonella infection (commonly known as cat scratch disease) resolves on its own within a few weeks to months. But when the infection doesn’t clear or goes unrecognized, it can spread beyond the lymph nodes to affect the heart, brain, liver, eyes, and bones. The risks climb sharply for anyone with a weakened immune system, where untreated Bartonella can cause serious, even life-threatening complications.
There are also several Bartonella species beyond the one that causes cat scratch disease, and some carry far graver consequences without treatment. What happens depends heavily on which species is involved and how well your immune system is functioning.
Cat Scratch Disease in Healthy Adults
The most common Bartonella infection, caused by B. henselae, typically starts with a small bump at the site of a cat scratch or bite, followed by swollen, tender lymph nodes near the wound. In people with healthy immune systems, this usually runs its course without antibiotics. The swollen lymph nodes can persist for weeks or even months, which is uncomfortable but not dangerous in most cases.
The real concern is the small percentage of cases where the bacteria spread to other parts of the body. Even in otherwise healthy people, the infection can occasionally reach the eyes, liver, spleen, brain, bones, or heart valves. These complications are uncommon, but they’re the reason “untreated Bartonella” is worth taking seriously.
Heart Valve Infection
The most dangerous complication of untreated Bartonella is endocarditis, an infection of the heart valves. B. henselae and B. quintana together account for roughly 95% of Bartonella-related endocarditis cases. This is particularly treacherous because Bartonella is notoriously hard to detect in standard blood cultures. Most labs discard bacterial cultures after five to seven days, but Bartonella can take up to 21 days to grow. That means the infection is frequently missed on initial testing.
When Bartonella endocarditis goes undiagnosed, the bacteria slowly damage the heart valves. In about half of aortic valve cases, the valve develops holes or becomes floppy enough to fail. Delayed diagnosis and treatment correlate with worse outcomes, and some patients ultimately need heart valve surgery. This is one of the leading causes of “culture-negative” endocarditis, where doctors know the heart is infected but standard tests can’t identify the culprit.
Neurological Complications
Bartonella can cross into the nervous system, producing a range of problems from mild to severe. Documented neurological complications include encephalitis (brain inflammation), seizures, stroke caused by inflamed blood vessels in the brain, and inflammation of the spinal cord. In rare cases, patients have developed status epilepticus (prolonged seizures), coma, or fatal brain swelling.
One case in the medical literature describes a 3-year-old boy who developed progressive difficulty walking six weeks after a typical cat scratch disease episode. Over the following eight weeks, he lost the ability to run or climb stairs and began falling frequently. He was diagnosed with a chronic inflammatory nerve condition triggered by the infection. While neurological involvement is uncommon, it illustrates how Bartonella can quietly progress if the initial infection isn’t fully cleared.
Eye Problems
Ocular complications show up in roughly 2% to 8% of cat scratch disease cases. The most recognizable is Parinaud oculoglandular syndrome, where the eye itself becomes the entry point for infection, causing redness, irritation, and swollen lymph nodes near the ear. Other eye-related problems include inflammation of the optic nerve, inflammation of the retina, and in uncommon cases, retinal detachment that can threaten vision. These complications are more frequently seen in children.
Bone and Joint Involvement
Bartonella occasionally reaches the bones, causing osteomyelitis (bone infection). This is rare, occurring in an estimated 0.2% to 0.3% of cases, and it predominantly affects children. The spine is the most common site, accounting for about 42% of bone infection cases. About one in four patients with bone involvement develop infections in multiple locations. Joint pain (arthralgia) has been reported in roughly 20% of children with musculoskeletal complications.
Severe Risks for Immunocompromised People
The stakes change dramatically for people with weakened immune systems, particularly those with advanced HIV/AIDS. In these patients, untreated Bartonella doesn’t just linger. It actively reshapes blood vessels.
The bacteria colonize the cells lining blood vessels and trigger a cascade that blocks normal cell death while stimulating abnormal blood vessel growth. This process produces vascular tumors throughout the body, a condition called bacillary angiomatosis. First recognized in the 1980s in people with AIDS, these lesions appear on the skin as red or purple nodules that can resemble Kaposi sarcoma. The tumors depend on the continuous presence of bacteria to keep growing, meaning they persist as long as the infection is active.
In the liver and spleen, this same process creates blood-filled cavities, a condition called peliosis. These fragile, blood-filled spaces can rupture and cause internal bleeding. B. henselae has a particular tendency to cause peliosis in the liver and spleen, while B. quintana more often causes bone lesions. Both species can also cause persistent bloodstream infections in immunocompromised patients.
Trench Fever and Persistent Bloodstream Infection
B. quintana, spread by body lice rather than cats, causes trench fever. This infection follows several patterns: a relapsing form with recurring fevers, shin pain, headaches, and dizziness; a typhoid-like form with prolonged fever, enlarged spleen, and rash; or a milder, shorter course. Without treatment, B. quintana can establish long-term residence inside red blood cells, producing an asymptomatic bloodstream infection that persists indefinitely. This silent bacteremia allows infected individuals to serve as ongoing reservoirs, spreading the bacteria to lice and then to other people.
Oroya Fever: The Most Lethal Form
The most dangerous Bartonella species, B. bacilliformis, is found only in parts of South America and causes Carrión’s disease. The acute phase, called Oroya fever, produces severe anemia by destroying red blood cells. Without treatment, the mortality rate ranges from 44% to 88%, and during outbreaks it can exceed 80%. This makes it one of the deadliest bacterial infections known when left untreated. For anyone traveling to endemic areas in the Andes, this species is the primary concern.
Potential Risks During Pregnancy
Evidence on Bartonella in pregnancy is limited but concerning enough to warrant attention. In animal studies, cats experimentally infected with B. henselae experienced abortions and fetal death, and similar outcomes have been documented in horses and mice. Infected mice delivered smaller pups with higher rates of fetal death.
In humans, the picture is less clear. One case report describes a woman with chronic difficulty conceiving who was found to have B. henselae infection. Her twin sons, conceived through IVF, were also infected, and one died nine days after birth with a congenital heart defect. Another report documented a child born with anemia, jaundice, and enlarged liver and spleen since birth, with evidence pointing to vertical transmission from an asymptomatic mother. Vertical transmission of B. bacilliformis has also been reported.
A larger surveillance study of eight pregnant women with cat scratch disease in Israel found that seven had uncomplicated pregnancies and healthy newborns, while one experienced a miscarriage with no confirmed link to the infection. The evidence is too thin to draw firm conclusions, but researchers have called for more investigation into Bartonella’s role in recurrent miscarriage and whether transplacental transmission occurs more often than currently recognized.
Why Diagnosis Is Often Delayed
One of the biggest practical problems with Bartonella is that it’s easy to miss. Standard blood cultures are designed to catch common bacteria within a week. Bartonella grows so slowly that by the time it would become detectable, the culture has already been thrown away. Specialized tests like antibody panels and PCR (which detects bacterial DNA directly) are more reliable but aren’t always ordered early in the diagnostic process, especially when symptoms are vague.
Cat scratch disease accounts for up to one-third of children evaluated for a fever with no obvious cause. In adults, the symptoms of chronic Bartonella infection, including fatigue, intermittent fevers, and joint pain, overlap with many other conditions. This means weeks or months can pass before testing is directed at Bartonella specifically, and during that window, the bacteria may be silently spreading to organs where they cause more significant damage.