Bartonella (Cat-Scratch Disease)

Bartonella henselae is the bacterium behind cat-scratch disease — one of the most common reasons a healthy child turns up with a swollen, tender lymph node that lingers for weeks. As the name suggests, people usually catch it from a cat, most often a playful kitten. For the great majority, the illness is a nuisance rather than a danger: a small bump where the scratch was, a swollen gland nearby, maybe a low fever, and then a slow recovery over a couple of months without any specific treatment. But the very same organism has a surprisingly wide range. In people whose immune systems are weakened — and occasionally in healthy people too — it can cause blood-vessel tumors of the skin and organs, a stubborn form of heart-valve infection that ordinary blood cultures miss, inflammation of the retina and optic nerve, and prolonged unexplained fevers. This page focuses on B. henselae and cat-scratch disease, then briefly covers two of its notable relatives — B. quintana (trench fever) and B. bacilliformis (Carrión's disease). The honest bottom line: usually mild, occasionally serious, and worth recognizing early in the cases that matter.

Overview
The Bacterium
Cat-Scratch Disease
Beyond Typical Cat-Scratch: The Wider Spectrum
B. quintana and B. bacilliformis
Who's Most at Risk
Diagnosis
Treatment
Prevention
The Honest Bottom Line
Research Papers
Connections
Featured Videos

Overview

Cat-scratch disease has been recognized for the better part of a century, but the microbe responsible was only pinned down in the early 1990s: a fastidious little bacterium named Bartonella henselae. It turned out to explain not just the classic swollen-gland illness but also several puzzling conditions that had been described separately over the years — a landmark example of one organism wearing many clinical masks.

In the United States, cat-scratch disease leads to roughly 12,000 outpatient diagnoses and about 500 hospitalizations each year. It is most common in children aged 5 to 9, tends to cluster in the warmer southern states, and peaks in the fall and winter — the seasons when kittens are born and flea activity shifts. Most people who get it never need to be hospitalized and never even learn the bacterium's name.

What makes Bartonella worth understanding is the gap between its usual mildness and its occasional severity. The same infection that produces a self-limiting swollen node in a healthy nine-year-old can, in someone with advanced HIV or a transplant, produce disfiguring vascular tumors or seed a heart valve. Knowing when a cat exposure matters — and when a "culture-negative" fever or endocarditis should prompt a doctor to think of Bartonella — is the practical heart of this topic.

The Bacterium

Bartonella henselae is a small, gram-negative bacterium that is notoriously fastidious and slow-growing — it does not grow the way most bacteria do on a standard laboratory plate. In the body it behaves as an intracellular organism, meaning it lives and multiplies inside host cells rather than floating free in the bloodstream. It has two favorite hiding places:

Red blood cells. Bartonella species have an unusual talent for invading and persisting inside red cells, which lets them ride quietly through the bloodstream and evade much of the immune response. This "intra-erythrocytic" lifestyle helps explain why infections can be low-grade, relapsing, and hard to clear.
Endothelial cells — the cells that line the inside of blood vessels. Here the bacterium does something remarkable: it can stimulate the vessel-lining cells to multiply and form new blood vessels. This vessel-growth trick is the reason Bartonella can produce tumor-like vascular lesions (see the spectrum section below), a behavior that sets it apart from most other bacterial infections.

Because it grows so slowly and lives inside cells, Bartonella is easy to miss with routine testing. A blood culture drawn for a fever may sit sterile for the usual few days and be reported as negative even when the organism is present — a fact that becomes very important in the discussion of endocarditis. The genus is named after Alberto Barton, the Peruvian physician who first described a related species in blood cells over a century ago.

Cat-Scratch Disease

This is the flagship illness and the reason most people ever hear of Bartonella.

How it spreads

People typically get cat-scratch disease from a scratch or bite from an infected cat, and kittens are the biggest risk — young cats under a year old are far more likely to be carrying the bacterium in their bloodstream. The cat itself looks perfectly healthy; it is simply a reservoir.

The link that ties it all together is the cat flea (Ctenocephalides felis). Fleas pick up Bartonella from one cat and pass it to another, and the bacteria are shed in flea droppings (flea "dirt"). When a cat grooms itself or scratches, those contaminated droppings get onto its claws and around its teeth. A scratch or bite then inoculates the bacteria into a person's skin. Fleas are what keep the infection circulating from cat to cat; the human is an accidental dead end. Less commonly, a cat licking an open cut can transmit it, and rare cases have been tied to flea exposure directly.

The classic illness

The typical course unfolds in a fairly predictable way:

A papule at the scratch site. Three to ten days after the scratch, a small red bump or blister (a papule or pustule) appears where the bacteria went in. It is easy to dismiss as a minor healing scratch, and it may be gone by the time other symptoms appear.
Tender regional lymphadenopathy. One to three weeks later, the lymph nodes draining the scratched area become swollen and tender — this is the defining feature. A scratch on the hand or arm swells a node in the armpit or at the elbow; a scratch on the face or neck swells a node in the neck. The gland can grow to an inch or more, feel sore, and occasionally soften and drain.
Mild systemic symptoms. Many people also have a low-grade fever, fatigue, headache, or general achiness. Some have no fever at all.

The reassuring part is that in otherwise healthy people, cat-scratch disease is usually self-limiting. The swollen node is the last thing to resolve and can take two to four months to shrink back down, which understandably worries families, but it typically settles on its own without any lasting harm. The main reason to see a doctor is to confirm the diagnosis and rule out other causes of a persistent swollen gland.

Beyond Typical Cat-Scratch: The Wider Spectrum

Most cat-scratch disease is the tidy story above. But Bartonella henselae can also cause a range of less common, more serious conditions — especially, though not only, in people whose immune systems are compromised. Recognizing these forms is what turns a "nuisance bug" into an organism clinicians take seriously.

Bacillary angiomatosis and peliosis

In people with weakened immunity — classically advanced HIV/AIDS, but also organ-transplant recipients and cancer patients — Bartonella can drive the abnormal growth of blood vessels described earlier, producing tumor-like lesions:

Bacillary angiomatosis — raised red or purple vascular bumps on the skin that can resemble Kaposi sarcoma or other tumors. They can also form in bone and internal organs.
Peliosis (especially peliosis hepatis) — blood-filled cystic spaces that develop within the liver or spleen.

These conditions are important because they can look like cancer and because, unlike ordinary cat-scratch disease, they genuinely require antibiotic treatment to resolve. Both B. henselae and its relative B. quintana can cause them.

Culture-negative endocarditis

Bartonella is one of the leading causes of "culture-negative" endocarditis — an infection of the heart valves in which the standard blood cultures keep coming back sterile because the organism is too slow and fastidious to grow in the usual way. A patient may have all the signs of endocarditis — fever, heart murmur, valve damage on echocardiogram — while every blood culture is reported negative. That mismatch is itself the clue: when endocarditis is present but cultures stay blank, Bartonella belongs on the short list, and specific serology or PCR is needed to catch it. It is a diagnosis that is easy to miss and important not to.

Eye disease and neuroretinitis

Bartonella can inflame the eye. One well-known pattern is Parinaud oculoglandular syndrome — a red, granulomatous bump on the surface of the eye paired with a swollen node in front of the ear, essentially cat-scratch disease of the conjunctiva. More dramatically, it can cause neuroretinitis, with swelling of the optic nerve head and a characteristic "macular star" of fluid deposits in the retina, leading to sudden painless blurring or loss of vision in one eye. Vision usually recovers, but this is a reason not to ignore eye symptoms after a cat exposure.

Prolonged fever

In children especially, Bartonella is a recognized cause of fever of unknown origin — weeks of fever with no obvious source. Sometimes imaging reveals tiny abscess-like spots in the liver and spleen (hepatosplenic disease). A quiet history of a kitten scratch weeks earlier can be the missing piece that solves the puzzle.

B. quintana and B. bacilliformis

Bartonella henselae is the species most people encounter, but two relatives are worth a brief mention because they cause distinct and historically important illnesses.

Bartonella quintana — trench fever. This species is spread not by cats or their fleas but by the human body louse, and it flourishes in conditions of crowding and poor hygiene. It earned its name in the trenches of World War I, where it sickened huge numbers of soldiers with a relapsing fever (often recurring every five days — hence quintana) and severe shin and leg pain. Today it is seen mainly in people experiencing homelessness and is sometimes called "urban trench fever." Like B. henselae, it can also cause bacillary angiomatosis and culture-negative endocarditis.
Bartonella bacilliformis — Carrión's disease. Spread by sandflies in the Andean valleys of Peru, Ecuador, and Colombia, this is the most severe member of the family. Its acute phase, Oroya fever, causes profound destruction of red blood cells and dangerous anemia; a later, chronic phase produces reddish, wart-like skin nodules called verruga peruana. The disease is named for Daniel Carrión, a Peruvian medical student who in 1885 inoculated himself to study it and died of the resulting fever, establishing that the two phases were one disease.

Who's Most at Risk

A few groups are more likely either to catch Bartonella or to develop its more serious forms:

Cat owners — especially those with kittens or flea-infested cats. Contact with young cats and with stray or free-roaming cats carries the highest risk, because kittens and flea-burdened cats are the most likely to be infected.
Children. Cat-scratch disease is disproportionately a childhood illness, both because children play more roughly with cats and because they are more likely to be scratched or bitten.
Immunocompromised people. Those with advanced HIV, organ transplants, cancer, or other causes of weakened immunity are the group most likely to develop the dangerous disseminated forms — bacillary angiomatosis, peliosis, and severe systemic infection — rather than a simple swollen node.
People experiencing homelessness are the group most affected by the louse-borne relative B. quintana.

Diagnosis

Bartonella is a genuinely tricky organism to pin down in the lab, and no single test is perfect. Doctors combine the clinical picture (a swollen node plus a cat exposure is often enough to suspect it) with one or more of the following:

Serology — the mainstay. Blood tests that measure antibodies to Bartonella (usually by an immunofluorescence assay) are the most common way the diagnosis is confirmed. They are practical and widely available, though antibody levels can cross-react between Bartonella species and can be hard to interpret in early or immunocompromised cases.
PCR. Molecular testing that detects Bartonella DNA in a lymph node biopsy, heart-valve tissue, or blood is highly specific and especially valuable in serious cases such as endocarditis, where confirming the exact organism guides treatment.
Culture — difficult. Growing Bartonella is slow and demanding; it can take weeks on special media and often fails. This is why culture is rarely the first-line answer.
Tissue examination. A biopsy of a swollen node may show a characteristic pattern of inflammation, and a special silver stain can sometimes reveal the bacteria directly.

The single most useful diagnostic clue in the serious end of the spectrum is "culture-negative" endocarditis: when a person has clear heart-valve infection but every blood culture stays sterile, Bartonella should be actively tested for rather than overlooked.

Treatment

Treatment depends heavily on which form of the illness a person has, and this is a place where honesty matters — the evidence is strong for some scenarios and thinner for others, so real management is individualized in partnership with a doctor.

Typical cat-scratch disease in healthy people

For an otherwise healthy person with the classic swollen-node illness, treatment is often supportive — the infection resolves on its own, and antibiotics are not always necessary. When treatment is used, azithromycin is the usual choice. A small placebo-controlled trial found that azithromycin modestly speeds the shrinking of the swollen node in the first month, though it does not clearly change the ultimate outcome. Many mild cases are simply watched, with pain relief and reassurance.

Serious and disseminated forms

The more serious presentations are a different matter and genuinely require antibiotics, often in combination and for longer courses:

Endocarditis is typically treated with a combination such as doxycycline plus an aminoglycoside like gentamicin (sometimes with rifampin), given for an extended period, and damaged valves occasionally need surgery.
Bacillary angiomatosis and peliosis in immunocompromised patients are treated with prolonged courses of erythromycin or doxycycline — often for months — because stopping too early invites relapse.
Neuroretinitis is commonly treated with doxycycline combined with rifampin.

The reason these regimens look complicated is real: Bartonella hides inside cells, clears slowly, and behaves differently across its various diseases, so treatment is tailored to the specific presentation, the person's immune status, and expert guidance rather than a single one-size-fits-all prescription.

Prevention

Because the infection comes from cats and their fleas, prevention is mostly common-sense pet and hygiene habits:

Avoid rough play that leads to scratches and bites, particularly with kittens. Don't encourage a cat to attack hands or feet.
Wash any scratch or bite promptly with soap and water, and don't let cats lick open wounds or broken skin.
Control fleas on cats with veterinary-recommended products — breaking the flea cycle is the single most effective way to keep cats from carrying and spreading the bacterium.
Practice good hand hygiene after handling cats, especially before touching the face or eyes.
Take extra care around kittens if you are immunocompromised. People with HIV, transplants, or other immune suppression are wise to adopt an older, flea-free cat rather than a kitten or stray, and to be especially careful to avoid scratches. There is no vaccine, so avoiding exposure is the only prevention.

The Honest Bottom Line

For most people, cat-scratch disease is exactly what it sounds like: a swollen, tender gland after a run-in with a kitten, which gets better on its own over a few weeks to a few months. It is common, rarely dangerous, and usually needs nothing more than reassurance and patience. The reason Bartonella earns a full page is the far end of its range — the vascular tumors in people with weakened immunity and the heart-valve infection that ordinary blood cultures fail to catch. Those cases are uncommon, but they are the ones where recognizing the organism early genuinely changes the outcome. If you develop a swollen gland after a cat scratch and it is not settling, or you have unexplained fever, vision changes, or signs of heart infection with a history of cat exposure, it is worth making sure a doctor considers Bartonella.

Research Papers

Florin TA, Zaoutis TE, Zaoutis LB. Beyond cat scratch disease: widening spectrum of Bartonella henselae infection. Pediatrics. 2008;121(5):e1413–e1425. doi:10.1542/peds.2007-1897 — Widely cited pediatric review cataloguing the full range of B. henselae disease beyond the classic swollen node.
Rolain JM, Brouqui P, Koehler JE, Maguina C, Dolan MJ, Raoult D. Recommendations for treatment of human infections caused by Bartonella species. Antimicrobial Agents and Chemotherapy. 2004;48(6):1921–1933. doi:10.1128/AAC.48.6.1921-1933.2004 — The reference treatment guidelines matching each Bartonella syndrome to its recommended antibiotic regimen.
Angelakis E, Raoult D. Pathogenicity and treatment of Bartonella infections. International Journal of Antimicrobial Agents. 2014;44(1):16–25. doi:10.1016/j.ijantimicag.2014.04.006 — Review connecting how the bacterium causes disease to why specific treatments are chosen.
Chomel BB, Boulouis HJ, Maruyama S, Breitschwerdt EB. Bartonella spp. in pets and effect on human health. Emerging Infectious Diseases. 2006;12(3):389–394. doi:10.3201/eid1203.050931 — Explains the cat-and-flea reservoir and the zoonotic route from pets to people.
Nelson CA, Saha S, Mead PS. Cat-scratch disease in the United States, 2005–2013. Emerging Infectious Diseases. 2016;22(10):1741–1746. doi:10.3201/eid2210.160115 — National surveillance data on incidence, hospitalizations, age groups, and geographic and seasonal patterns.
Koehler JE, Sanchez MA, Garrido CS, et al. Molecular epidemiology of Bartonella infections in patients with bacillary angiomatosis–peliosis. New England Journal of Medicine. 1997;337(26):1876–1883. doi:10.1056/NEJM199712253372603 — Landmark study linking B. henselae (cat/flea exposure) and B. quintana (homelessness) to the vascular tumors of immunocompromised patients.
Raoult D, Fournier PE, Drancourt M, et al. Diagnosis of 22 new cases of Bartonella endocarditis. Annals of Internal Medicine. 1996;125(8):646–652. doi:10.7326/0003-4819-125-8-199610150-00004 — Established Bartonella as a major cause of culture-negative endocarditis and how to diagnose it.
Bass JW, Freitas BC, Freitas AD, et al. Prospective randomized double blind placebo-controlled evaluation of azithromycin for treatment of cat-scratch disease. The Pediatric Infectious Disease Journal. 1998;17(6):447–452. doi:10.1097/00006454-199806000-00002 — The controlled trial showing azithromycin modestly speeds shrinking of the swollen node in early cat-scratch disease.
Zangwill KM, Hamilton DH, Perkins BA, et al. Cat scratch disease in Connecticut — epidemiology, risk factors, and evaluation of a new diagnostic test. New England Journal of Medicine. 1993;329(1):8–13. doi:10.1056/NEJM199307013290102 — Population study confirming kitten and flea exposure as key risk factors and validating early serologic testing.
Foucault C, Brouqui P, Raoult D. Bartonella quintana characteristics and clinical management. Emerging Infectious Diseases. 2006;12(2):217–223. doi:10.3201/eid1202.050874 — Focused review of the louse-borne relative behind trench fever and "urban" cases in homelessness.
Maguiña C, Gotuzzo E. Bartonellosis: new and old. Infectious Disease Clinics of North America. 2000;14(1):1–22. doi:10.1016/s0891-5520(05)70215-4 — Authoritative account of B. bacilliformis and Carrión's disease (Oroya fever and verruga peruana) from the Andean region where it is endemic.
Breitschwerdt EB. Bartonellosis: one health perspectives for an emerging infectious disease. ILAR Journal. 2014;55(1):46–58. doi:10.1093/ilar/ilu015 — Overview of Bartonella as a stealthy, animal-associated pathogen spanning veterinary and human medicine.

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