I Fixed the Fundamentals and Still Had Brain Fog. Was It My Cat? What the Bartonella Evidence Supports.

Bartonella henselae is a bacterium that lives in cats and spreads between them through fleas. Flea dirt carries it onto claws, and a scratch or bite passes it to people. In most healthy people it causes cat scratch disease: a bump where the scratch was, then a tender swollen lymph node nearby one to three weeks later, sometimes with fever and tiredness. It usually clears up by itself over weeks. The CDC page in the references covers this well.

Serious complications include eye inflammation, infected heart valves, problems in the liver or spleen and, rarely, encephalitis (brain inflammation). A blood culture can miss Bartonella even when the bacterium has infected a heart valve.

I latched onto a different claim: that Bartonella can stay at low levels for months or years, hide inside blood cells and blood vessel lining, and slowly cause brain fog, agitation, anxiety, insomnia and odd pains. Researchers have published papers on that claim. It helps to be honest about what kind of papers they are.

A 2019 paper describes one boy whose sudden psychiatric symptoms improved after tests found Bartonella in his blood and doctors treated it. A 2024 review gathers the reported neurological and psychiatric effects, and most of its sources are case reports and small case series (write-ups of one or a few patients). A 2024 study found Bartonella DNA more often in people with psychosis than in a comparison group, using research-lab methods your doctor can't order.

Serious researchers did this work, and I'm not dismissing it. But it's evidence that something can happen in selected people, when researchers look with specialized methods. Journals publish case reports because the cases are unusual. Studies recruit people who are already unwell in specific ways. And chronic bartonellosis, as a diagnosis covering many body systems, has no agreed, validated criteria. Infectious disease guidance treats it as contested.

I read all of this the first time as if the possible were the probable. Reading it back, the leap was mine, not the papers'.

It's still worth something. A flea-covered rescue kitten and a handful of scratches belong in a doctor's notes. That history changes what a doctor considers if new symptoms show up: fever, a swollen lymph node, night sweats, new nerve or eye symptoms, skin changes, a heart murmur. It also helps the doctor decide whether to test for infection, and in what order.

On its own, though, that history can't make the diagnosis. Most cat owners have been scratched. Many healthy cats carry the bacterium with no sign of illness. If exposure alone made this likely, it would be common, and the honest reading of the evidence is that lasting brain and mood symptoms from Bartonella are, at most, uncommon and hard to prove.

I asked my GP for Bartonella testing. She ran the standard antibody test. It came back negative. At the time I read that as a failure of the test. That reading was half right.

Routine antibody testing for Bartonella is imperfect. It misses some infections, especially early on or in someone with a weak immune system, so one negative result can't rule it out. If your doctor thinks Bartonella is a real possibility, other options include repeat serology (antibody blood tests) and PCR (a DNA test). The cause guide explains when to ask about them.

But here's the part I skipped. A negative routine test plus brain fog that won't go away isn't evidence of a hidden infection. Many more common things can cause that, and I hadn't finished checking them: sleep that looks fine on paper but not on a sleep study, mood and anxiety problems, medication effects, slow recovery after a virus, thyroid problems and the other things a broad blood panel catches. The hard but useful step was to look beyond the one dramatic possibility.

I did go on to see a specialist, and the earlier version of this article told that story in detail: a positive result from a specialised test, then months of antibiotics and gradual improvement. I won't repeat it here, for two reasons.

First, it proves nothing. I'd changed a dozen things over the same months, my symptoms came and went anyway, and one person's timeline can't separate cause from coincidence. Second, readers used that story to push for months of combination antibiotics based on a negative routine test and a symptom list.

Most cases of cat scratch disease clear without antibiotics. People with weakened immune systems need antibiotic treatment. Doctors use longer courses, often with more than one antibiotic, for cat scratch disease that affects the eyes, liver or heart valves. The antibiotics carry real risks of side effects and drug interactions.

Brain fog, a cat and a negative test can't justify months of combination antibiotics, and I don't want this page to be the reason anyone thinks they can.

And see a doctor promptly, though not as an emergency, if a scratch site becomes hot, red and spreading, or if a tender swollen lymph node shows up near a scratch along with fever. That's what acute cat scratch disease looks like, and it's easy to check.

I'm not a vet, but sources agree on the basics. Many healthy cats carry Bartonella without any sign of illness, kittens more often than adults, and a kitten with fleas carries the highest risk. A vet can test your cat, though a negative result today says little about six months ago, because cats can clear the bacterium by themselves.

Fleas are the part you can control. Use flea prevention all year, not only in summer. Beyond that, keep claws trimmed, wash scratches straight away, keep cats from licking open wounds, and keep your hands out of rough play with kittens.

Keep your cat. Giving it up was never the message, and it still isn't. Keep it flea-free, handle scratches properly, and if you get unexplained symptoms, make sure your doctor knows you've been around cats.

If you've fixed the fundamentals and one piece still won't budge, the honest next step is a broader medical workup. Bartonella can sit on that list, particularly if your history and your symptoms point that way. It shouldn't be the whole list, and persistent brain fog alone doesn't put it at the top.

1. CDC. Cat scratch disease: symptoms, prevention and treatment. About cat scratch disease; Clinical overview.
2. Breitschwerdt EB, Greenberg R, Maggi RG, Mozayeni BR, Lewis A, Bradley JM. Bartonella henselae bloodstream infection in a boy with pediatric acute-onset neuropsychiatric syndrome. J Central Nervous System Disease. 2019. Single case report. DOI: 10.1177/1179573519832014
3. Bush JC, Robveille C, Maggi RG, Breitschwerdt EB. Neurobartonelloses: emerging from obscurity! Parasites & Vectors. 2024. Narrative review, largely of case reports and case series. PMID: 39369199
4. Delaney S, et al. Bartonella species bacteremia in association with adult psychosis. Frontiers in Psychiatry. 2024. Research cohort using specialised detection methods. DOI: 10.3389/fpsyt.2024.1388442