What causes cat scratch disease?
Last updated September 6, 2026.
**Cat scratch disease is caused by a bacterium called Bartonella henselae, which cats carry in their blood and pass to people through a scratch, bite, or a lick on broken skin.** Cats become carriers after infected fleas bite them and leave flea dirt in their fur, which the cat then rubs into its own claws and mouth while grooming. Most infected cats look completely healthy, so there is no way to tell by looking which animal is carrying the bacteria. Kittens under one year old and cats with fleas are the most likely sources, and about 1 in 3 people with the illness recall a scratch or bite in the two weeks before symptoms start. Keeping cats on year-round flea control, washing any scratch with soap and running water right away, and avoiding rough play that leads to bites cut the risk substantially. The infection does not pass from person to person.
What to do
- Wash scratches right away: Clean any cat scratch or bite with soap and running water as soon as it happens.
- Treat cats for fleas: Use a year-round flea product from your veterinarian, since fleas are how cats get the bacteria.
- Skip the rough play: Avoid games that end in scratching or biting, especially with kittens and cats you do not know.
- Keep licks off open skin: Do not let a cat lick cuts, scabs, or your eyes, nose, or mouth.
- Watch the lymph nodes: Swollen, tender nodes near the scratch that last more than two weeks are worth a clinician visit.
- Speak up if immunocompromised: Tell your clinician about any cat scratch if you have HIV, cancer treatment, or a transplant, since the infection can spread beyond the lymph nodes.
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