Ticks Have Potential To Transmit Cat Scratch Disease

"We cannot say for certain that ticks are vectors of these diseases, but at the least we can say they carry Bartonella DNA and could be potential vectors," says Dr. Bruno Chomel, Professor of Zoonoses at UC Davis and one of the investigators on the study.

Working on the hypothesis that ticks may serve as a transmission vector for Bartonella bacteria, Dr. Chomel and his associates tested Ixodes pacificus ticks from the coastal range area of California (which includes the San Francisco area) for the presence of Bartonella DNA. "We found that almost 20% of the ticks were positive, a percentage that's even higher than for known tick-borne diseases like Borrelia burgdorferi (the cause of Lyme disease) or ehrlichia," says Dr. Chomel.

One species of the bacteria, Bartonella quintana, has long been known to cause trench fever in humans, but it has only been in the last decade or so that B. henselae was identified as the cause of cat scratch disease. More recently, two other species, B. washoensis and B. vinsonii subspecies berkhoffii have been found to cause disease in humans. All four were among the Bartonella bacteria found in the ticks.

Cat scratch disease is a relatively mild disease in humans, generally consisting of a low-grade fever and swollen lymph nodes, but in patients with compromised immune systems it can lead to a potentially fatal disease known as bacillary angiomatosis.

Several years ago, it was discovered that fleas could transmit B. henselae and in fact they were the primary mode of transmission between cats, says Dr. Chomel. But dogs also appeared to carry the infection, and studies suggested that fleas were not a common transmission method for canines. Additional findings of B. henselae in cows, where fleas are not common, further supported the theory of a different vector, such as ticks.

"Some earlier work in Europe had tested ticks for a variety of bacteria and did find evidence of Bartonella, " says Dr. Chomel. "But they looked at ticks that had already fed on blood. We were looking to see if the DNA of Bartonella was present in the ticks before they fed on large mammals."

And it was, meaning the ticks could not have acquired the infection from cats, dogs or cows. There are two possible hypotheses on the ultimate source of the bacteria, says Dr. Chomel. The first is that the tick acquired the infection during an earlier stage in its life cycle, as a larva or nymph, by feeding on a smaller mammal (such as a rat) or even a lizard. The second is the possibility of vertical transmission. An infected female could have had the bacteria in her ovaries and passed it on to her offspring.

More research is necessary to understand the origin of the infection in these ticks. One method might be to test those animals that are common food sources for larvae and nymphs. "Can we find them in lizards? Can we find them in rodents?" asks Dr. Chomel.

As to whether ticks can transmit these diseases to humans, Dr. Chomel is cautious in his conclusions. "We have no clear experimental demonstration of ticks being able to transmit the infection to humans," he says. The next step is to see if transmission is possible in animal models.

He does point to evidence, though, that suggests it's possible. A paper published in the early 1990s found two cases of cat scratch disease where the only known risk factor was a tick bite. Reports on cat scratch disease before the identification of Bartonella henselae as the cause describe cases that did not appear to be caused by a cat scratch. It is estimated that these non-traditional modes of infection may account for up to 5 percent of all cases of the disease.

The Journal of Clinical Microbiology is a publication of the American Society for Microbiology. The American Society for Microbiology is the largest single life science society, composed of over 42,000 scientists, teachers, physicians, and health professionals. Its mission is to promote research and training in the microbiological sciences and to assist communication between scientists, policymakers, and the public to improve health, economic well being, and the environment.