Bartonella is a genus of stealth bacteria that infect red blood cells and the cells lining blood vessels, causing diseases that range from the familiar swollen lymph nodes of cat scratch disease to life-threatening heart valve infections and, in one species found in the Andes, severe anemia that killed a medical student who deliberately infected himself in 1885. At least forty species have been identified, and roughly a dozen are known to cause illness in people. The bacteria circulate silently in animal populations worldwide, with infection rates in some cat, rodent, and cattle groups running above 50 percent, creating a vast reservoir from which humans get infected through flea bites, louse infestations, and animal scratches.
The Major Species That Infect People
Three Bartonella species account for the bulk of recognized human disease. Bartonella henselae is the agent behind cat scratch disease, transmitted mainly through flea-infested cats. Bartonella quintana causes trench fever, historically associated with soldiers in World War I and now linked to body-louse infestations among people experiencing homelessness. Bartonella bacilliformis, found in the Andean valleys of South America, causes Carrión’s disease, a two-phase illness that can destroy red blood cells in its acute form and produce wart-like skin growths in its chronic form.
Beyond these three, species originally thought to be purely veterinary pathogens keep turning up in human infections. Research on febrile patients in Thailand found that more than half the Bartonella strains identified in human blood were closely related to species normally associated with rodents, including B. elizabethae, B. rattimassiliensis, and B. tribocorum. Among those patients, 71 percent reported recent rat exposure.1PubMed Central. Identification of Bartonella infections in febrile human patients from Thailand and their potential animal reservoirs Surveys across the Eastern Mediterranean have documented at least 18 different Bartonella species in the region, though only B. henselae and B. quintana had been confirmed in human clinical cases there.2PubMed. Geographical distribution of Bartonella spp in the countries of the WHO Eastern Mediterranean Region (WHO-EMRO) The picture that emerges is one of a much broader zoonotic threat than the classic “cat scratch” framing suggests.
How People Get Infected
Bartonella species live inside red blood cells of their animal hosts, which means any blood-feeding arthropod that bites an infected animal has the potential to pick up the bacteria. Five species have well-established transmission routes through lice, fleas, or sandflies.3PubMed. Vector transmission of Bartonella species with emphasis on the potential for tick transmission Among these, fleas are the dominant vectors. Cat fleas carry B. henselae between cats and create the conditions for human infection, since flea feces deposited on a cat’s claws or fur get introduced through scratches or bites. In one study of a cattery where 89 percent of cats were bacteremic, cat fleas experimentally transmitted B. henselae to kittens, while kittens housed with infected cats in the absence of fleas stayed negative.4PubMed Central. Experimental transmission of Bartonella henselae by the cat flea
Rodent fleas, particularly the tropical rat flea, play a similar role in maintaining Bartonella circulation among wild rodents. Field studies in Germany found zoonotic species like B. grahamii and B. elizabethae in both rodents and their fleas, confirming the flea-rodent-human pathway.5PubMed. Bartonella, Rodents, Fleas and Ticks: a Molecular Field Study on Host-Vector-Pathogen Associations in Saxony, Eastern Germany Whether ticks play a meaningful role in transmitting Bartonella to people remains debated. Bartonella DNA has been found in various tick species, and one systematic review noted the absence of convincing evidence for tick-borne transmission of Bartonella to humans.6PubMed Central. Chronic coinfections in patients diagnosed with chronic lyme disease: a systematic review That said, some researchers continue to consider ticks as potential secondary vectors, and the question remains unresolved.
For B. quintana, the vector is the human body louse. Outbreaks of trench fever have been documented in urban homeless populations, including studies in Marseilles linking B. quintana bacteremia to body-louse infestation7PubMed. Chronic Bartonella quintana bacteremia in homeless patients and research in Denver identifying B. quintana as an underrecognized source of serious infection among people experiencing homelessness.8PubMed. Bartonella quintana Infection in People Experiencing Homelessness in the Denver Metropolitan Area
What Happens Inside the Body
Once Bartonella bacteria enter the bloodstream, they employ an unusual survival strategy: they colonize red blood cells. B. bacilliformis does this aggressively, attaching to multiple proteins on the red cell surface, including spectrin, band 3 protein, and glycophorins, using carbohydrate-dependent interactions.9PubMed. Interaction of Bartonella bacilliformis with human erythrocyte membrane proteins Both B. bacilliformis and B. henselae produce proteins that deform red cell membranes, creating inward-folding pockets and internal vacuoles, though only B. bacilliformis is known to bind and enter circulating red cells in large numbers.10FEMS Microbiology Letters. Comparison of the abilities of proteins from Bartonella bacilliformis and Bartonella henselae to deform red cell membranes and to bind to red cell ghost proteins Real-time imaging has shown B. henselae invading mature red blood cells over a 60-hour window in laboratory conditions.11PubMed Central. Bartonella henselae initial infection of mature human erythrocytes observed in real time using bacterial endogenous fluorescence
Living inside red blood cells is a remarkably effective hiding place. Red cells lack the usual immune-recognition machinery found on nucleated cells, and the bacteria resist being broken down even when immune cells try to engulf them. Beyond this red-cell niche, Bartonella species deploy a toolkit of immune-evasion tactics: they alter their surface proteins through antigenic variation, produce unusual forms of the lipopolysaccharide molecule that sidestep the body’s pattern-recognition receptors, form biofilms, and suppress the programmed death of infected endothelial cells.12PubMed Central. Sneaky tactics: Ingenious immune evasion mechanisms of Bartonella The combined effect is persistent, low-level infection that can circulate in the bloodstream for weeks to months.
One of the more striking things Bartonella does is trigger the growth of new blood vessels. Both B. henselae and B. bacilliformis produce a protein called BafA that mimics vascular endothelial growth factor, the body’s main signal for growing new capillaries. BafA binds to the same receptor and activates the same downstream signaling cascade, essentially tricking the body into building extra blood vessels around the site of infection.13Nature Communications. The Bartonella autotransporter BafA activates the host VEGF pathway to drive angiogenesis This vascular-growth trick is what produces the distinctive lesions of bacillary angiomatosis and the verruga peruana warts of Carrión’s disease.
Common Clinical Presentations
The typical cat scratch disease scenario involves a child or young adult who develops a papule at the site of a scratch, followed days to weeks later by enlarged, sometimes painful lymph nodes near the scratch site. In most healthy people, the lymph node swelling resolves on its own over weeks to months, and antibiotics do not significantly speed recovery from this straightforward form of the disease.14PubMed. Pathogenicity and treatment of Bartonella infections
Trench fever from B. quintana presents differently. Classic cases involve recurring bouts of high fever every five days or so, along with headache, shin pain, and general misery. But modern urban trench fever in homeless populations often produces nonspecific symptoms, or even no symptoms at all, while the bacteria quietly persist in the bloodstream for months.15PubMed. Chronic Bartonella quintana bacteremia in homeless patients
Carrión’s disease, caused by B. bacilliformis and transmitted by sandflies in the Andes, is the most dangerous primary Bartonella infection. The acute phase, Oroya fever, involves massive destruction of red blood cells, causing severe anemia, jaundice, and high mortality if untreated. Survivors may later develop verruga peruana, the nodular skin lesions driven by that BafA-triggered blood vessel growth.
Serious Complications
When Bartonella escapes the simple lymph-node pattern, the complications can affect almost any organ system. Heart valve infection, or endocarditis, is one of the most dangerous. Bartonella species are now recognized as an important cause of blood-culture-negative endocarditis, meaning standard blood cultures come back clean even while bacteria are destroying a valve.16PubMed. Diagnosis of 22 new cases of Bartonella endocarditis Cases have been documented even in children, including a four-year-old girl whose aortic valve infection with B. henselae was only identified through serology and molecular testing of the surgically removed valve tissue.17PubMed. Culture-negative endocarditis caused by Bartonella henselae Bartonella endocarditis frequently requires valve surgery, though some cases have been managed with prolonged antibiotics alone.18Emerging Infectious Diseases. Management of Bartonella Prosthetic Valve Endocarditis without Cardiac Surgery
In people with weakened immune systems, particularly those with advanced HIV, Bartonella causes distinctive vascular growths. Bacillary angiomatosis produces reddish-purple skin nodules that can resemble Kaposi sarcoma, while bacillary peliosis hepatis creates blood-filled cysts in the liver.19PubMed. Clinical and pathological features of bacillary peliosis hepatis in association with human immunodeficiency virus infection Both conditions are treatable with antibiotics, and erythromycin is considered first-line therapy for these vascular growths.20PubMed. Pathogenicity and treatment of Bartonella infections
The nervous system is another target. A comprehensive review of neurological Bartonella infections found a wide spectrum of conditions, including inflammation of the brain, inflammation of the optic nerve, Guillain-Barré syndrome, peripheral nerve damage, and various neuropsychiatric symptoms.21PubMed Central. Neurobartonelloses: emerging from obscurity! Bone involvement, while rare, also occurs. In a literature review of cat scratch disease complicated by bone infection, fever was present in about 84 percent of cases, and bone or joint pain in about 88 percent, often with a prolonged course.22PubMed Central. Osteomyelitis in Cat-Scratch Disease: A Never-Ending Dilemma-A Case Report and Literature Review
Why Diagnosis Is So Difficult
Bartonella is one of the most challenging bacteria to detect in the clinical laboratory, and this difficulty is central to why cases get missed. Standard blood cultures are designed to grow bacteria within a few days, but Bartonella species grow extremely slowly and need special conditions. Traditional plating methods are notoriously insensitive. A combined approach using a pre-enrichment liquid culture medium followed by molecular testing dramatically improved detection in dogs, and similar strategies have been adapted for human diagnostics.23PubMed. A combined approach for the enhanced detection and isolation of Bartonella species in dog blood samples: pre-enrichment liquid culture followed by PCR and subculture onto agar plates
Adding sheep blood to liquid culture media has been shown to boost B. henselae growth and improve the sensitivity of molecular detection methods.24PubMed Central. Blood Supplementation Enhances Bartonella henselae Growth and Molecular Detection of Bacterial DNA in Liquid Culture Comparative studies of molecular methods have found that culturing blood samples in liquid media before running molecular tests is significantly more effective at catching Bartonella DNA than testing whole blood directly.25PLoS Neglected Tropical Diseases. Comparison of molecular methods for Bartonella henselae detection in data donors Serology (testing for antibodies) is available and widely used, but antibody levels can be low in chronic infection and cross-react with other bacteria. There is no single test that reliably rules out Bartonella, which is why clinicians dealing with suspected cases often need to combine several methods.
Treatment Approaches
Treatment varies sharply depending on which form of the disease you have. Uncomplicated cat scratch disease with just swollen lymph nodes does not clearly benefit from antibiotic treatment, and many cases resolve without medication. The picture changes when the infection spreads beyond the lymph nodes.
For bacteremia, the recommended combination is gentamicin with doxycycline. For endocarditis, that same pair forms the backbone of therapy, often given for six weeks or longer. For the vascular proliferative lesions of bacillary angiomatosis and peliosis hepatis, erythromycin is first-line. Carrión’s disease has its own protocol: chloramphenicol is used for the acute bacteremic phase, while rifampicin or streptomycin may be used for the chronic verruga peruana phase.26PubMed. Pathogenicity and treatment of Bartonella infections One ongoing frustration for clinicians is that antibiotic susceptibility testing in the lab does not always predict what works in the patient, partly because the bacteria’s intracellular location shields them from drugs that perform well in a test tube.
The Bartonella and Lyme Disease Overlap
Patients diagnosed with chronic Lyme disease sometimes test positive for Bartonella as well, and this overlap has generated both genuine clinical questions and considerable controversy. Bartonella infection can produce symptoms that closely mimic Lyme disease, including joint pain, fatigue, and neurological complaints, making clinical distinction difficult.27PubMed Central. Chronic Lyme Disease and Co-infections: Differential Diagnosis In a population of patients referred to a rheumatologist, antibodies against Bartonella species were found in about 62 percent and active bacteremia in about 41 percent, alongside prior diagnoses of Lyme disease, arthritis, chronic fatigue, and fibromyalgia. The researchers stressed that a causal link between Bartonella and these symptoms had not been established.28Emerging Infectious Diseases. Bartonella spp. Bacteremia and Rheumatic Symptoms in Patients from Lyme Disease–endemic Region
Case reports have documented concurrent detection of both Borrelia burgdorferi (the Lyme agent) and B. henselae DNA in cerebrospinal fluid, suggesting true co-infection of the central nervous system. In those cases, treatment aimed only at Lyme disease had failed, and symptoms improved when Bartonella-directed therapy was added.29JAMA Neurology. Concurrent Infection of the Central Nervous System by Borrelia burgdorferi and Bartonella henselae: Evidence for a Novel Tick-borne Disease Complex A systematic review, however, found the overall evidence for simultaneous Lyme and Bartonella infection unconvincing, noting that tick-borne transmission of Bartonella has not been firmly established.30PubMed Central. Chronic coinfections in patients diagnosed with chronic lyme disease: a systematic review The tension between these findings captures the broader state of Bartonella research: individual cases are compelling, but large-scale, controlled evidence is sparse.
Prevention and Flea Control
Because flea transmission is the primary pathway for the most common human-infecting species, flea control on pets is the single most effective preventive measure for cat scratch disease. Keeping cats indoors helps, as outdoor cats are far more likely to carry the bacteria. A study testing a collar containing imidacloprid and flumethrin on cats found it prevented flea transmission of B. henselae for eight months.31PubMed Central. A flea and tick collar containing 10% imidacloprid and 4.5% flumethrin prevents flea transmission of Bartonella henselae in cats Regular topical or oral flea preventives achieve the same goal. For people with compromised immune systems, some physicians go further and recommend avoiding kittens entirely, since young cats are more likely than adults to be bacteremic.
For B. quintana, prevention centers on louse control. In homeless shelters, delousing programs, access to clean clothing, and regular laundry services are the most practical interventions. There is no vaccine for any Bartonella species.
Blood Transfusion Risks
Bartonella bacteremia in apparently healthy people raises a quiet concern for blood safety. A study of blood donors in southeastern Brazil found that contact with cats and a history of tick bites were both independent risk factors for Bartonella bloodstream infection, and the authors recommended that these risk factors be considered during donor screening because standard laboratory methods do not reliably detect the bacteria.32PubMed Central. Risk Factors for Bartonella species Infection in Blood Donors from Southeast Brazil Animal reservoirs sustain background bacteremia rates that are startlingly high, with prevalence reaching 50 to 95 percent in selected cat, rodent, deer, and cattle populations.33PubMed Central. Bartonella infection in animals: carriership, reservoir potential, pathogenicity, and zoonotic potential for human infection Whether transfusion-transmitted Bartonella poses a practical clinical risk to recipients has not been conclusively answered, but the theoretical basis for concern is solid.
How Bartonella Evolved Its Virulence Toolkit
The evolutionary story of Bartonella is one of parallel innovation. At the heart of the genus’s ability to cause disease is a molecular injection system called the VirB type IV secretion system, which acts like a tiny syringe that pumps bacterial proteins directly into host cells. Genomic analysis has revealed that this system was acquired by horizontal gene transfer not once but at least three separate times in different Bartonella lineages.34Genome Biology and Evolution. Evolutionary Dynamics of Pathoadaptation Revealed by Three Independent Acquisitions of the VirB/D4 Type IV Secretion System in Bartonella Two of those lineages went on to diversify into species-rich clades, each generating its own expanded set of effector proteins through repeated gene duplication and functional diversification.35PLOS Genetics. Parallel Evolution of a Type IV Secretion System in Radiating Lineages of the Host-Restricted Bacterial Pathogen Bartonella
A separate secretion system, the Trw system, appears to have co-evolved with host specificity. The genes encoding the surface-exposed parts of this system show signs of diversifying selection and concerted evolution through gene conversion, a process where gene copies within a single genome homogenize while diverging from copies in other species. The pattern suggests that Trw components evolve rapidly to match the surface molecules of each species’ preferred host.36Molecular Biology and Evolution. Diversifying Selection and Concerted Evolution of a Type IV Secretion System in Bartonella This host-specificity machinery helps explain why most Bartonella species stay locked into narrow host ranges and only occasionally spill over into humans.
Daniel Carrión and the Origins of Bartonella Research
The history of Bartonella research is anchored by an act of medical self-experimentation that remains both celebrated and disturbing. In 1885, Daniel Alcides Carrión, a 28-year-old Peruvian medical student, wanted to prove that Oroya fever and verruga peruana were two phases of the same disease. On August 27, he had material scraped from a verruga lesion on a 14-year-old patient inoculated into his own arms.37Mayo Clinic Proceedings. Daniel Alcides Carrión: Martyr of Peruvian Medicine and Subject of Self-experimentation Within three weeks he developed fevers, chills, severe anemia consistent with brisk hemolysis, and jaundice. He died on October 5, 1885, but his self-experiment provided the first direct evidence linking the two conditions. Peru later declared him a national hero, and the acute disease was renamed Carrión’s disease in his honor.38PubMed. Daniel Alcides Carrion (1857-1885) and a history of medical martyrdom The causative organism itself, Bartonella bacilliformis, would not be identified until decades later, but Carrión’s sacrifice framed the field’s earliest understanding that a single bacterial species could produce wildly different clinical pictures depending on the phase of infection.