Most CSD begins with a scratch from the claw or tooth of a kitten younger than six months of age. It can also be inflicted by an adult cat, or from contact of the animal's saliva with broken skin or the eye. Previous investigations into the responsible organism identified a family of a-proteobacteria based on 16S ribosomal RNA gene sequences (6). Currently it is believed that Bartonella henselae, a Gram-negative bacterium, is the causative organism in CSD. In California, about 40% of cats carry Bartonella (9). Fleas are the vector transmitting the infection between cats, with bacteria subsequently found in the animal's saliva. In cats, the carrier state is generally asymptomatic (although experimental inoculations have produced a mild illness with fever, anemia, and transient neurological dysfunction) and an animal may carry the bacteria for months. The disease seems to rarely occur following a dog scratch or even from porcupine quills, cactus spines, or rosebush thorns. Most cases of CSD occur in children between the ages of 2 and 14, and in veterinarians. For reasons yet to be determined, most cases occur in the fall or winter months (75% of cases occur between September and March). CSD is not transmitted between human hosts.
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