Bartonella sp. and Your Cat (or Dog, Horse, Rat): An Emerging Infectious Disease

If we’ve learned nothing else since February or March of 2020, we’ve learned that emerging infectious diseases are a real threat to human health globally. In a bad news/good news kind of way, Bartonellosis is an emerging infectious disease; however, it’s not spread by airborne droplets or respiration.

But if any of your family pets bring a flea or tick into the house, or if you live in proximity to mice, rats, ground squirrels, rabbits, sheep, horses or cattle–you could be at risk.

Bartonella sp. is a Gram negative, rod-shaped bacteria that has been around since ancient times. It’s the bacteria responsible for cat scratch disease (1) and for Trench fever (2), which affected soldiers during WWs I and II, and affects people living in over-crowded, unsanitary conditions around the world today.

Bartonella henselae bacteria, the causative agent of cat-scratch disease or bartonellosis,. 3D illustration
Bartonella henselae bacteria, the causative agent of cat-scratch disease or bartonellosis

Bartonella sp. are known to be spread by vectors such as fleas, which are part of the transmission cycle for cat scratch disease and the human body louse, the vector for transmission of Trench fever (3).

This animal-to-human transmission of Bartonella sp. classifies it as a zoonosis.

Infection due to Bartonella sp. often appear to be self-limiting, such as swelling in regional lymph nodes due to a cat scratch disease. In such cases, symptoms can subside without intervention. But Bartonella sp. have a nasty habit of hiding in red blood cells and in cells lining blood vessels, where they can remain undetected for a substantial period of time. This hiding place affects a host’s ability to mount an immune response, as well as affecting the ability of antibiotics to attack the bacteria.

An Old Bacteria, Many New Strains

Although Bartonella spp. have been around for a long time, what’s new to the story is the number of additional species and subspecies of Bartonella that have been identified in the past 30 years, along with the number of new animal reservoirs identified. While B. henselae has been known as the causative agent of cat scratch disease, and B. quintana the agent of Trench fever, Dr. Edward Breitschwerdt, DVM and others now list more than 30 species, found in rats, mice, bats, flying squirrels, ground squirrels, cattle, sheep and horses, to name a few animal hosts (4).

What these animals have in common is the seeming ability to harbor Bartonella species without becoming ill. They are a reservoir for their respective Bartonella sp. and subspecies.

But when vectors, such as blood-seeking fleas, sand flies (and potentially arthropods and arachnids) transmit one of these Bartonella sp. to a naïve host, such as a human or dog, strange and serious illnesses can occur in otherwise healthy humans or canines.

In one report, members of a family developed neuropsychiatric illnesses that were ascribed to bites from a woodlouse hunter spider (5). The family members had bites, and Bartonella DNA was amplified by PCR from spiders and wood lice and two family members. The family members’ neurological symptoms subsided after antibiotic treatment.

In another case, a veterinarian and his daughter exhibited neurological symptoms and weight loss (6). Their dogs were found to be infected with B. vinsonii subsp. berkhoffii. The father and daughter’s symptoms disappeared after antibiotics were administered. In several cases a variety of Bartonella species have been identified in dog saliva, but in this case the source and timing of transmission was unknown and a flea or tick vector was suspected.

In addition to their propensity for hiding in red blood cells, in vitro studies have shown that Bartonella sp. can infect a variety of cell types, including dendritic cells, microglia, monocytes and bone marrow progenitor cells (4). It’s believed that in reservoir hosts the bacteria occasionally leave red cells and endothelium and seed the blood, moving to new locations before detection by the immune system. DNA from Bartonella sp. has been identified in endocarditis lesions as well as liver and spleen tissues (4). For this reason researchers believe the bacteria can access many tissues in the body, by moving stealthily through blood vessels.

Known Vectors, Suspected Vectors

One of the many questions still being studied concerning spread of Bartonella sp. is how the bacteria is spread. There is clear evidence of vector transmission for cat scratch disease, where fleas and flea dirt are accepted means of transmission to other cats and to humans (flea dirt on a cats claws can be introduced to broken skin during a scratch (1). And Carrions disease is known to be transmitted by sand fly bites (3).

Ticks, the known mode of transmission for Lyme disease caused by Borrelia burgdorferi, and ehrlichiosis due to Ehrlichia sp. bacteria, are believed to be a transmission agent for a variety of Bartonella sp. But efforts to prove vector competence are on-going (4). PCR-amplified Bartonella DNA has been identified in Ixodes and other tick species, both in laboratory studies where ticks were fed blood containing Bartonella, and in ticks taken from dogs and other animals from which Bartonella sp. were isolated (4).

Earlier mention of woodlouse hunter spiders and wood lice found to contain Bartonella DNA when taken from the home of people sickened by the bacteria, makes a convincing case for a variety of additional vectors that could transmit these bacteria.

Yet the ideal for proof of microbial transmission is to show that ticks become carriers of the bacteria by feeding on the blood of an infected animal, and then transmitting Bartonella to another animal. Breitschwerdt and researchers around the world are working to add evidence to this sort of vector transmisison (4).

Vector to Animal to Tissues

Bartonella sp. has shown the ability to invade hepatic, splenic and dendritic cells, another active are of study. It has been postulated that hemangiosarcoma, a liver, spleen or cardiac tumor of dogs that often goes undetected, and can rupture and cause sudden death in dogs, may be connected to Bartonella infection. Breitschwerdt continues studies in this area as well (4,7).

The Take Away

Serious respect is due to the genus Bartonella for its ability to hide inside animals and people, due to the neurological manifestations some infected persons have shown, because of it’s demonstrated ability to impact multiple organ systems and because of its ability to spread by vector transmission, from many different reservoir animals, both domesticated and wild.

In a 2017 review, veterinarian Dr. Ed Breitschwerdt noted “As cats and dogs with prior exposure to fleas, ticks and other arthropod vectors are at risk for acquiring Bartonella infections, veterinarians should … determine client compliance if acaricides have been used to prevent arthropod infestations. The fact that a product was dispensed on a routine basis, does not mean that the product was administered or used in the manner recommended by the manufacturer”(4).

The availability and use of preventative agents, anti acaricides, leads to the assurance that at least in our lives at home and work, we pet owners and animal workers have some ability to keep ourselves and our families, including our pets, safe from Bartonella sp., both from contracting the bacteria, and from vector transmission, by taking a few simple steps to keep our homes and pets free of common vectors such as fleas and ticks.

Until more is known about vectors for, and transmission of Bartonella sp., an ounce of prevention is worth a pound of cure.


  1. Centers for Disease Control: Cat Scratch Disease [] Accessed May 5, 2021.
  2. Centers for Disease Control: Bartonella Symptoms [] Accessed May 5, 2021.
  3. Centers for Disease Control: Bartonella Transmission [] Accessed May 5, 2021.
  4. Breitschwerdt, E.B. (2017) Bartonellosis, One Health and all creatures great and small. Vet. Dermatol. 28, 96–e21.
  5. Mascarelli, P.E. et. al. Bartonella henselae infection in a family experiencing neurological and neurocognitive abnormalities after woodlouse hunter spider bites. Parasites Vectors 6, 98.
  6. Cheslock, M.A., Embers, M.E. (2019) Human Bartonellosis: An Underappreciated Public Health Problem? Trop. Med. Infect. Dis. 4, 69.
  7. Copeland, Sue. (2020) American Kennel Club Canine Health Foundation. Bartonella: The major health threat you’ve never heard of. Accessed May 04, 2021.

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Kari Kenefick

Kari Kenefick

Kari has been a science writer/editor for Promega since 1996. Prior to that she enjoyed working in veterinary microbiology/immunology, and has an M.S. in Bacteriology, U of WI-Madison. Favorite topics include infectious disease, inflammation, aging, exercise, nutrition and personality traits. When not writing, she enjoys training her dogs in agility and obedience. About the practice of writing, as we say for cell-based assays, "add-mix-measure".

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