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Your Cat's Eyes


The Truth about Bartonella



by Arnold Plotnick MS, DVM, ACVIM, ABVP

Bob’s eye looked strange.

Normally a bright-eyed, active young cat, Bob, a 2-year old Manx belonging to Michelle Prager, was feeling out of sorts. His activity level had dropped over the past day or two, and his left eye had an unusually cloudy appearance.

On physical examination, Bob had a high fever (104 degrees). All body systems checked out fine, except for his eyes. Bob’s right iris was its usual green color, but his left iris had a reddish tinge to it. The pupil was slightly smaller in that eye, and the anterior chamber – the space between his iris and his cornea – was a bit murky. Bob’s eye wasn’t clear, but the diagnosis was: anterior uveitis, a type of inflammation involving one of the inner layers of the eye.

Treatment of uveitis consists of topical and oral anti-inflammatory drugs, and most cases respond fairly readily. However, determination of the cause of uveitis in cats can be frustrating. It is important to establish a primary cause of feline uveitis, since this may lead to more specific therapy, may affect long-term prognosis, may identify a contagious disease that could affect other cats in the household, and may even identify a disease that could have public health significance. Common causes of uveitis in cats include viral diseases such as feline leukemia virus (FeLV), feline immunodeficiency virus (FIV), feline infectious peritonitis (FIP), protozoal diseases such as toxoplasmosis, and systemic fungal diseases.

I tested Bob for these disorders, and prescribed topical medication while waiting for the test results, which would be available in a few days. All tests came back negative. This was not surprising, as a definitive cause for feline uveitis is determined in less than 50% of cases. Bob’s eye, however, responded quickly to treatment. The cloudiness disappeared, and both eyes returned to their beautiful green color.

Eight months later, Bob was back. So was his uveitis and fever. I had ruled out most common causes of uveitis in cats. Could this be Bartonella?

In the past decade, the field of veterinary medicine has been abuzz with excitement, and confusion, over the significance of Bartonella as a cause of illness in cats. Unfortunately, what we don’t know about Bartonella still exceeds what we do know.

For many years, the cause of a disease syndrome in humans that was frequently associated with contact with kittens or cats, remained unknown. The syndrome, called Cat Scratch Disease (CSD) was characterized by fever, malaise, and lymph node enlargement. In 1992, the mystery was solved, as a new organism, Rochalimaea henselae, was discovered to be the causative agent for CSD. In 1993, the organism was renamed Bartonella henselae.

“Bartonella is a bacterial organism that can infect people as well as animals. Cats are the main reservoir of infection, and the prevalence of infection is fairly high”, says Dr. Mike Stone, board certified internist at Tufts University. “Of the approximately 60 million pet cats in the United States, at least 20% are infected with Bartonella.” Exposure to fleas or flea feces is the most important factor in transmission of the disease to cats. The odds that a cat will be infected with Bartonella is greatest in young cats, cats that go outdoors, and cats that have fleas.

When people contract a Bartonella-associated illness, a history of close contact with (or being scratched by) a cat or kitten is usually discovered. During grooming, cat claws are often contaminated with infected flea feces or infected cat blood. When a cat scratches, the organism is thus deposited into the skin. There have been, however, a few documented cases of people with Bartonella-associated illness that have not been in contact with cats. It is suspected that these people have picked up the organism through contact with fleas, or flea feces in the environment.

In people, infection with Bartonella can cause a variety of illnesses, depending on the immune status of the person. People with competent immune systems are more likely to develop classical Cat Scratch Disease. Antibiotic therapy is believed to have no significant effect on the course of CSD in people, and most cases resolve on their own. In immunocompromised people, however, Bartonella infection can result in more serious illness.

Bartonella infection in cats is a bit more mysterious. Cats that have been experimentally injected with Bartonella have been reported to develop a variety of symptoms, including fever, loss of appetite, enlarged lymph nodes, aggressive behavior, and generalized tremors. The significance of these findings is debatable because of differences in study design, and because experimental inoculation does not necessarily reflect what happens when cats are infected naturally. Most naturally infected cats show no clinical signs of illness. There are only a few published studies regarding natural infection. In one study from Switzerland, sick seropositive cats (those with antibodies against Bartonella and presumably infected) were more likely to have oral disease and urinary tract disease than sick seronegative cats. However, healthy control cats in that study were just as likely to test positive as the sick cats, so it is impossible to say whether it was the Bartonella that caused the illness in the sick cats, or whether their being Bartonella-positive was just a coincidental finding. In another study from Japan, cats that were co-infected with Bartonella and FIV were at increased risk for developing gingivitis and lymph node enlargement. One study in the United States revealed that sick cats who tested positive for Bartonella were more likely to have blood in their urine than sick cats who tested negative for Bartonella.

The one body system that does seem to be affected by Bartonella is the eyes. Reports of human eye diseases caused by Bartonella prompted others to look for similar diseases in cats. “Bartonella is an emerging infection that is considered responsible for uveitis in human beings, cats, and less commonly in dogs”, says Dr. Stefano Pizzirani, a veterinary ophthalmologist at the Cumming’s School of Veterinary Medicine at Tufts University. A number of feline eye disorders have since been attributed to Bartonella. (See sidebar).

To muddy the waters even further, many of the Bartonella studies are based on serological tests. Serological tests detect antibodies against the organism, confirming that the cat has been exposed to Bartonella. The presence of antibodies does not necessarily mean that the cat has an active Bartonella infection. In fact, it is possible for a cat to be exposed to Bartonella, develop antibodies against it, and then eliminate the infection from its body. Because the antibodies remain in the bloodstream, these cats will test positive on a serological test, despite having defeated the infection. I ran a serologic test on Bob. He tested positive for Bartonella.
Some veterinarians feel that culturing the blood is a better method to prove a cat is infected with Bartonella, because a positive blood culture proves that the cat is infected with the organism. Again, the results should be viewed cautiously because a positive culture proves the cat is infected, but it does not prove that the cat is clinically ill from the infection.

Culturing the blood is a technically difficult process, and it may take weeks before a positive result is obtained because the organism is a slow grower. Another test, called PCR, can be used for diagnosing Bartonella infection, however, this test requires special laboratories and is costly. A positive PCR test suggests that the cat is infected, but again, it does not prove that the cat is clinically ill from the Bartonella infection.

The bottom line on diagnosis is that some tests prove exposure, and some confirm infection, but there is no single test that can definitively prove a cat is sick from a Bartonella infection. If a veterinarian is determined to blame Bartonella for a cat’s particular illness, at minimum the following criteria should be met:

•  The illness is one that has at least been reported to be associated with Bartonella infection
•  Other possible causes of the cat’s illness have been ruled out
•  The cats tests positive for Bartonella (either by serology, culture, or PCR)
• The cat responds to drugs that are known to be effective against Bartonella

Azithromycin has been mentioned as an effective antibiotic in the treatment of Bartonella infections. The fluoroquinolones (for example Baytril®, Orbax®, Zeniquin®)
are also effective, as is doxycycline and rifampin.

Bob satisfied the criteria listed above. He had uveitis, a condition known to be caused by Bartonella. The other common causes of uveitis – FeLV, FIV, FIP, Toxoplasmosis – were ruled out. His serologic test for anti-Bartonella antibodies was positive, and he responded to azithromycin, an antibiotic known to have anti-Bartonella activity. Was Bartonella the cause of Bob’s uveitis? We still can’t be certain. “Among the diseases affecting the eye, uveitis represents a challenging diagnosis because many causes may be responsible, and diagnostic tests are uncertain”, says Dr. Pizzirani. “Many cases are, in fact, classified as idiopathic – the cause cannot be determined.”

Veterinarians and cat owners must always keep in mind that serologic tests do not prove current infection, and the antibiotics used for the treatment of Bartonella infections in cats are broad spectrum antibiotics that may be effective against other organisms that can cause illnesses resembling those caused by Bartonella. In other words, even when the above criteria are satisfied, at the present time it is still not possible to definitively attribute the cause of a cat’s clinical illness to Bartonella.

What about testing healthy cats for Bartonella? “Routine testing of healthy cats, either by serology or culture, is probably not necessary, except in certain circumstances”, says Dr. Stone. “For example, it might be advantageous for a veterinary blood bank to know the Bartonella status of their donors, or a cat breeder to know the status of their breeding stock. People with immunosuppressive disorders may want to know the Bartonella status of a cat before considering adoption, as a negative cat may be a safer pet for them.” But there are disadvantages as well. “A positive antibody test might cause someone to classify the cat as dangerous even though the cat may have eliminated the infection and is now partially immune to it. Another one of my fears is that cat who test positive might be needlessly euthanized. And of course, there’s the expense of testing, which can be considerable, especially in a multi-cat household.” Dr. Stone also cautions that a negative test – either by culture, serology, or PCR – should not give owners a false sense of security. “If preventative measures aren’t taken, cats can easily become infected.” Because exposure to fleas is the most important factor in the transmission of Bartonella to cats, conscientious flea control is of paramount importance. Cats should be kept indoors to minimize the risk of fleas, and adding stray cats or cats from shelters to the household should be done cautiously, with extra attention paid to flea control.

Guidelines for preventing opportunistic infections among HIV-positive people have been published previously. To minimize the occurrence of Bartonella-associated illness in people, the American Association of Feline Practitioners has adapted these recommendations, which include:

• Year-round flea control
• Adopting only healthy cats that are free of fleas and are > 1 year of age
• Regular trimming of cat claws
• Avoiding scratches and bites
• Prompt and thorough washing of cat-associated wounds, followed by professional medical advice

Bob, meanwhile, remains bright-eyed (though not bushy-tailed; he is, after all, a Manx). It is clear that further work needs to be done to determine exactly what medical conditions can be attributed to Bartonella infection in cats.

Sidebar: Feline eye disorders associated with Bartonella

• Uveitis – inflammation of some of the internal structures of the eye, including the iris
•  Retinitis – inflammation of the retina
•  Conjunctivitis – inflammation of the soft tissues surrounding the eye
•  Keratitis – inflammation of the cornea
•  Blepharitis – inflammation of the eyelids



Updated 7/2/07