Cat scratch disease bacteria may be transmitted to cats by fleas

September 1996

SAN FRANCISCO — While a scratch may be the most common method of transmitting cat scratch disease (CSD) from felines to humans, the common cat flea is probably the most frequent way of transmitting the infection among cats, according to researchers.

The cat flea is a competent vector for Bartonella henselae, the organism that causes CSD, according to Jane Koehler, MD, of the University of California at San Francisco.

Koehler and her colleagues at UCSF and the University of California at Davis performed two controlled studies that demonstrated that direct cat-to-cat transmission could not be demonstrated in the absence of fleas.

"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," Koehler said.

Bacterium recently identified

Cat scratch disease has been recognized since the 1950s but the bacterium that causes it, B. henselae 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.

Concerns for the immunocompromised

"Our real concern is infections in AIDS patients," said Koehler. B. henselae infection in people with weakened immune systems can lead to a potentially fatal disease called bacillary angiomatosis, which looks like Kaposi's sarcoma.

"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 bacillary angiomatosis patients published in the Journal of the American Medical Association in February 1994, Koehler and her colleagues identified the cat as the reservoir of B. henselae and determined the number of cats that were bacteremic. The study estimated that 41% of the cat population in the greater San Francisco Bay area could be bacteremic.

How did cats become infected?

"Additional studies in other regions of the United States have demonstrated that feline B. henselae infection is very common," Koehler said.

"The identification of the factors contributing to this large reservoir is essential for developing strategies to prevent human and feline infection," she added.

In the first experiment, Koehler and her 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 environment that was free of fleas. Both became infected with B. henselae. The experiment was repeated 10 months later with the same results — three more uninfected kittens became infected when exposed to fleas.

Eighty percent of fleas infected

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, said Koehler.

Individuals should not worry about acquiring CSD infection directly from a flea, however. Koehler did not rule out the potential for flea-borne cat-to-human transmission, but said it required further investigation.

"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," Koehler said. "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."

For Your Information:

• Chomel BB, Kasten RW, Floyd-Hawkins K, et al. Experimental transmission of Bartonella henselae by the cat flea. J Clin Microbiol. 1996; 34:1952-56.