Cat Scratch Disease (CSD) is a bacterial infection most commonly associated with contact with felines. While the disease is overwhelmingly linked to cats, dogs can, in extremely rare circumstances, carry the causative bacteria. Transmission from a dog to a person is highly uncommon, making the risk profile very different between the two species. CSD is a zoonotic illness, meaning it can pass from animals to humans, and primarily affects the lymph nodes.
The Bacteria Behind Cat Scratch Disease
Cat Scratch Disease (CSD) is caused by the bacterium Bartonella henselae. This bacterium is typically transmitted among animals by arthropods like fleas. Cats, particularly kittens and younger cats, are the primary natural reservoir. Many healthy cats carry Bartonella in their bloodstream without showing signs of illness. They become infected after being bitten by infected cat fleas, and the bacteria can persist in the cat’s blood for months, turning the animal into a carrier.
The bacteria are shed in the feces, or “flea dirt,” of infected fleas. When a cat scratches itself or another animal, this bacteria-laden flea dirt can be deposited under the claws. Cats with active flea infestations are significantly more likely to be positive for Bartonella.
Understanding Transmission and Dog Involvement
Transmission to humans most often occurs when an infected cat scratches, bites, or licks an open wound. The bacteria are transferred via contaminated flea feces present on the cat’s claws or fur. Infection can also occur if a person rubs their eye after handling a cat with flea dirt.
Dogs can become infected with Bartonella henselae and related Bartonella species, often through exposure to ticks and fleas. Unlike cats, infection in dogs is more likely to cause noticeable clinical symptoms. While transmission from dogs to humans is possible, it is extremely rare and poorly documented compared to the risk associated with cats. The vast majority of human cases are directly traceable to a cat scratch or bite.
Recognizing Symptoms and Prevention
The first sign of CSD often appears as a small, raised lesion, such as a papule or pustule, at the site of the scratch or bite. This initial bump develops within a few days to two weeks after the injury. The most common symptom is the swelling and tenderness of lymph nodes closest to the injury site. For example, a scratch on the arm would likely cause swelling in the lymph nodes of the armpit. Patients may also experience mild symptoms such as a low-grade fever, fatigue, or headache.
To prevent contracting CSD, immediate and thorough washing of any scratch or bite with soap and water is recommended. Consistent flea control for all pets is also necessary, as this prevents the spread of bacteria between animals. Avoiding rough play that could lead to bites or scratches, especially with stray or young animals, further reduces the risk of transmission.