Bartonella
An Overlooked Co-infection
Bartonella is often missed in people with long‑term, unexplained symptoms—not because it’s rare, but because it doesn’t act like a typical infection. It can mimic autoimmune, neurological, or inflammatory conditions and can hide inside cells, making standard testing less reliable.
What is Bartonella?
Bartonella is a group of bacteria that can infect both humans and animals, causing a range of illnesses collectively known as bartonellosis. These bacteria are able to hide inside red blood cells and the lining of blood vessels, which helps them evade the immune system. Because of this, Bartonella infections can be difficult to detect and may persist or relapse even after treatment.
Bartonella is best known for causing cat scratch disease, but this is only one form of the infection. Researchers have identified at least 37 Bartonella species, and several are known to cause illness in humans. Depending on the species and the person’s immune response, infections can range from mild to severe.
How Is Bartonella Transmitted?
Bartonella can spread to humans in several ways. The most familiar route is through scratches or bites from infected cats, which can transmit Bartonella henselae, the bacteria linked to cat scratch disease.
But cats aren’t the only source. Research shows that ticks can carry multiple Bartonella species, and growing evidence suggests they are a likely vector for human infection. Bartonella DNA has been found in several tick species, and laboratory studies have shown that ticks can transmit the bacteria to animals.
Other biting insects, including fleas and lice, are also possible carriers.
Bartonella may also spread through contaminated blood transfusions. In addition, some studies suggest that the bacteria can pass to the unborn child during pregnancy, although more research is needed to understand how often this occurs.
Because different Bartonella species may spread in different ways—and because research is still evolving—scientists continue to study exactly how these bacteria move between animals and people.
Types of Bartonella
Scientists have identified more than 37 species of Bartonella, but only a few are known to cause illness in humans. The three most recognized are Bartonella henselae, Bartonella quintana, and Bartonella bacilliformis. Because each species affects the body in different ways, the treatment approach can vary depending on which bacteria is involved and which organs or systems are impacted.
Bartonella henselae – Cat Scratch Disease
The most common human infection is caused by Bartonella henselae, the bacteria responsible for cat scratch disease. This infection is usually spread through the scratch or bite of a cat, often a kitten, or by exposure to infected fleas.
Bartonella quintana – Trench Fever
Another well-known species is Bartonella quintana, which causes trench fever. Historically, this illness affected soldiers during World War I who were exposed to body lice. Today, it is still seen in populations where body lice infestations are more common, including people experiencing homelessness or living in crowded conditions.
Bartonella bacilliformis – Carrión’s Disease
In certain regions of South America, Bartonella bacilliformis is responsible for Carrión’s disease, a serious and sometimes life-threatening illness spread by sandflies. This disease has two distinct phases: an acute phase, marked by severe fever and anemia as red blood cells are destroyed, and a later phase with skin eruptions known as verruga peruana.
Research suggests that other Bartonella species may also cause human disease. These include Bartonella elizabethae, which has been linked to heart infections such as endocarditis, and Bartonella vinsonii, which has been associated with both heart and neurological problems.
As new information emerges, it is becoming increasingly clear that bartonellosis is more complex than once believed, and that many Bartonella species may be capable of causing illness in people.
Note: The information presented on this page has been reviewed and approved by a member of our Medical Leadership Board.