Cat Scratch Disease Bacteria Transmitted by Fleas

August 01, 1996

Cat scratch disease
CONTACT: Jim Sliwa
(202) 942-9297


SAN FRANCISCO--August 1, 1996--While a cat scratch may be the most common vehicle for transmitting cat scratch disease (CSD) from felines to humans, the common cat flea is probably the most frequent way of transmitting the infection between cats, say researchers at the University of California, San Francisco and UC, Davis.

Jane Koehler of the UCSF, Bruno Chomel from UC, Davis, and their colleagues report on a controlled experiment which demonstrates that the cat flea is a competent vector for Bartonella henselae (the cause of CSD) and that direct cat-to-cat transmission could not be demonstrated in the absence of fleas. The results appear in the August 1996 issue of the Journal of Clinical Microbiology.

"These data suggest that control of flea infestation may reduce the number of insects capable of transmitting B. henselae from cat to cat, and that in turn will reduce the feline reservoir from which humans can become infected via a cat scratch," says Dr. Koehler.

CSD has been recognized since the 1950s but the bacterium that causes it has only recently been identified. The most common mode of transmission to humans is a scratch from an infected cat. Infections apparently cause no harm to cats and they display no symptoms. Symptoms in humans begin with an inflammation around the scratch site, which progresses to swelling of the lymph nodes and a low-grade fever.

"Our real concern is infections in AIDS patients," says Dr. Koehler. B. henselae infection in people with weakened immune systems can lead to a potentially fatal disease called bacillary angiomatosis (BA), which looks like Kaposi's sarcoma. "But, because so few HIV-infected people acquire this infection from their cats and cat ownership is so beneficial, we do not recommend giving up pet cats. Instead, HIV-infected individuals should make their care-givers aware that they have a cat, control flea infestation as much as possible, avoid getting scratched and if they do get scratched, wash the scratch immediately."

In a study of BA patients published in the Journal of the American Medical Association in February 1994, Dr. Koehler and her colleagues identified the cat as the reservoir of B. henselae, and went on to determine the number of cat that were bacteremic (had the bacteria in their bloodstream). The study estimated that 41% of the cat population in the greater San Francisco Bay area could be bacteremic.

"Additional studies in other regions of the United States have demonstrated that feline B. henselae infection is very common. This prompted the question `How did this tremendous number of cats get infected?'" says Koehler. "The identification of the factors contributing to this large reservoir is essential for developing strategies to prevent human and feline infection."

Drs. Koehler and Chomel discourage the mass screening and treating of infected cats as an effective control method for several reasons. "First, although there appears to be a huge number of cats infected the number of people who actually get CSD is relatively small." There are over 50 million cats in the United States and approximately 22,000 cases of CSD each year, she says, and CSD in people with intact immune systems is almost always a benign infection that resolves without antibiotic treatment. Secondly, cats who are treated for B. henselae bacteremia often relapse.

In the first experiment, Drs. Koehler and Chomel and their colleagues evaluated the ability of the cat flea to transmit B. henselae by removing fleas from infected cats and transferring them to two uninfected kittens in a controlled flea-free environment. Both became infected with B. henselae. The experiment was repeated 10 months later with three more uninfected kittens, with the same results. Additionally, in both experiments fleas were collected from the infected cats and tested for the presence of B. henselae DNA. The researchers estimated that B. henselae was present in 80% of the fleas used in the first experiment and 45% of the fleas used in the second.

The second experiment involved putting infected and uninfected kittens together in a flea-free environment for 21 days. In contrast to flea-borne transmission, direct cat-to-cat transmission among kittens could not be demonstrated in the absence of fleas, says Dr. Koehler. Individuals shouldn't worry about acquiring CSD infection directly from a flea, though.

"Although the flea-borne transmission of B. henselae to cats appears to be very efficient, epidemiological data do not support efficient transmission from cat to human via the cat flea," say Koehler. "Most case series of CSD patients report that CSD symptoms developed after the individual received a scratch, and in 93% of cases in one large series, the inflamed cat scratch was still visible at the time of the initial diagnosis of CSD."

Dr. Koehler does not rule out the potential for flea-borne cat-to-human transmission but says it requires further investigation.

Dr. Koehler can be reached directly in San Francisco at (415) 476-9364. The Journal of Clinical Microbiology is a publication of the American Society for Microbiology (ASM). With over 40,000 members worldwide, the ASM is the oldest and largest single biological membership organization in the world.

American Society for Microbiology

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