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By Maureen C. Carroll DVM, DACVIM

As veterinarians, we are expected to be experts in the field of infectious disease. As a result, we must now become learned in more and more infectious organisms that not only affect our veterinary patients, but that can also infect the human counterparts with whom these animals are associated. Flea and tick-borne diseases are at the center of our attention as veterinarians, and our knowledge of Bartonella is evolving rapidly. Although much has been learned about this organism in the past two decades, years of research are likely necessary to fully understand the spectrum of its pathogenicity.

Bartonella was originally isolated from an HIV infected person, and was first diagnosed in a dog (1993) diagnosed with endocarditis. In general  this organism is now known to be associated with cases of endo/myocarditis, granulomatous disease, chronic intravascular infection, and vasoproliferative disorders in humans and animals.

Today there are 32 named, and many other unnamed species of Bartonella. Among the many species, we do know that B. henselae, B. Clarridgeae can be pathogenic in cats; B. Vinsonii ( berkhoffii) and B. Clarridgeae in dogs. Exposure to these serovars can be found throughout most of the United States and the world.

Bartonella is a very fastidious gram negative rod that remains in a mammalian host as an intraerythrocytic and intraendothelial bacteria, and can remain so for a prolonged period of time. Other cell targets for this organism include: microglial cells, macrophages, and CD34 bone marrow progenitor cells, which may play a role in cytopenias we can observe with Bartonella (humans and dogs).


Bartonella spp is transmitted by more vectors than any other infectious agent: ticks, sandfly, human lice, cat fleas, rodent flea, and horn fly. In cats Bartonella is found most frequently in flea- infested animals; in dogs the tick is the usual vector of transmission, although fleas are also a vector. In flea endemic areas seroprevalence in cats can be as high as 90%; bacteremia rates 50% and greater.  As this organism can live in flea dirt for > 9 days, year-round flea control is very important. Transmission between animals and humans can occur via scratch with a claw contaminated with flea dirt, or via open wounds in contact with flea dirt, or from biting or licking from infected animals.

Bartonella in Cats:
Although controlled studies in cats are limited, our knowledge continues to evolve. Bartonella Henselae lives in the cat flea. Cats infested with fleas are 40% more likely to be Bartonella positive on serology or PCR, than cats without fleas. Alternatively, there is a high prevalence rate in non-clinical carriers in the general cat population, thereby hindering our ability to establish disease causation, regardless of flea status.

Although we often associate Bartonella with endocarditis and myocarditis, these manifestations are usually observed in research settings in cats. Cats who present with clinical signs associated with naturally acquired Bartonella usually suffer from fever, and lymphadenopathy, lethargy, uveitis, gingivitis, and sometimes neurological symptoms.  If one examines a patient with these symptoms that is flea-infested, Bartonella should be on the list of differentials.

Interestingly, Bartonella in cats usually does not result in any biochemical or hematological abnormalities.

Bartonella in Dogs
Although the full spectrum of disease manifestations in dogs is unclear, we do know that human bartonellosis mimics what we see in dogs.  B. vinsonii in dogs can cause chronic intraerythrocytic and intraendothelial infections that can result in vasoproliferative disease, in addition to endocarditis (Boxers overrepresented), myocarditis, peliosis hepatis, granulomatous disease, granulomatous hepatitis, lymphadenitis, rhinitis-epistaxis, encephalitis; skin lesions: vasculitis, panniculitis, polyarthritis, uveitis, meningitis among others. Sometimes dogs can present with only fever of unknown origin and epistaxis.

Hemotologically, monocytosis and eosinophilia can be observed in ~ 1/3 of dogs; thrombocytopenia in ~ 50%, and anemia is also common. As in cats, biochemical abnormalities are uncommon.

As a rule of thumb for the diagnosis for any disease: A ‘positive’ result on a blood test or a culture does not necessarily prove causation for disease.  We can observe false positive and false negative results in both bacteremic and non- bacteremic patients.

Dogs: Serology for antibodies to B. Vinsonii (berkoffii) is a very specific test as < 4% of sick dogs are seropositive. Therefore a positive is indication of exposure and/or active disease and should be treated. Because the sensitivity of antibody detection is so low, combined testing is recommended for dogs. The tests of choice are a combination of 1. and 2. below:

1.    PCR (blood)
2.    BAPGM culture (insect medium) on blood

Cats: Testing should be reserved for cats with suspected clinical disease due to prevalence of subclinical carrier state in this species.

Serology for antibody detection is cats can be useful for determining exposure, however due to the carrier state, its usefulness as a sole test is questionable.  PCR on whole blood for DNA detection, however, is often positive before seroconversion occurs, so using this diagnostic in addition to serology will be of better diagnostic utility in sick cats. Culture on blood can also be performed in lieu of PCR.

Therefore testing of choice in cats is a combination of 1. and 2. below:

1. PCR or BAPGM Culture on blood
2. Serology for Antibody detection (blood)

Sample submissions for all Bartonella testing can go to Galaxy Laboratories ( or Colorado State University (

Azithromycin is no longer recommended as Bartonella develops rapid resistance to this drug.
The recommended therapy includes Doxycycline 10 mg/ kg po / 12-24 combined with a Flouroquinolone for 4- 6 weeks.
Doxycycline as a sole therapeutic will not clear most Bartonella infections.

Bartonella spp can be transmitted to dogs, cats and humans via flea or tick bites, or via the bite, lick or scratch from a bacteremic animal. Infected flea dirt in the mouths, fur, or nails of animals are also risk factors to humans. Flea and tick control is therefore imperative in preventing transmission of this organism to humans. As mentioned earlier, the human disease manifestations are very similar to the canine, and therefore can result in significant morbidity to the human population, veterinarians most importantly! Any fluid: blood, body cavity, joint, cerebrospinal, etc. from an infected animal can harbor live organisms. Proper and safe specimen handling at all times is of paramount importance as a veterinary community.

For more information, please contact Angell’s Internal Medicine service at 617-541-5186 or