Cat Scratch Disease

Most cat owners never hear about cat scratch disease until someone in the household develops a swollen lymph node and a doctor asks about the family pet. The name sounds alarming, but the reality is more straightforward: it is a bacterial infection that cats carry without getting sick themselves, and it only rarely causes problems for the people who live with them.

The bacterium responsible is called Bartonella henselae. It lives inside a cat's red blood cells, where it causes no visible illness in the vast majority of cases. Cats pick it up from fleas: an infected flea sheds the bacteria in its faeces, and when a cat grooms itself, that flea dirt gets trapped under its claws. The next scratch, bite, or lick that breaks human skin can then introduce the bacteria into a person's body. This is also why the infection does not pass directly from one cat to another without fleas in the picture. It is the flea, not the cat, that drives transmission.

That distinction matters because it shapes how owners should think about risk. The CDC estimates that 30 to 40 percent of cats, particularly kittens under two years old and those from shelters or outdoor environments, carry Bartonella at some point. They show no signs. They eat normally, play, and purr. Because a healthy-looking cat can still be a carrier, testing healthy cats is not recommended by either the Advisory Board on Cat Diseases or the CDC. A positive antibody test simply confirms past exposure, not active infection. Research in Southern Germany found that while antibodies were present in 15 to 46 percent of cats, actual bacteria circulating in the blood appeared in only 2.5 percent. Treating a symptom-free cat with antibiotics is discouraged: long courses rarely clear the bacteria completely and contribute to antibiotic resistance.

For most people who do become infected, cat scratch disease is mild and self-limiting. A small red bump may appear at the scratch site within three to ten days, followed by tender swelling of nearby lymph nodes one to three weeks later, often in the armpit, neck, or groin. A low fever, tiredness, and a headache are common. Children under fifteen get it more often than adults, likely because they play more closely with kittens and are scratched more frequently. The lymph node swelling can linger for two to four months but almost always resolves on its own. In rare cases, especially for people with weakened immune systems, the infection can spread further and cause serious complications affecting the eyes, internal organs, or blood vessels. That group needs prompt medical attention rather than watchful waiting.

Prevention is about managing the flea, not the cat. Year-round flea control is the single most effective measure, because it interrupts the flea life cycle and keeps infected flea dirt off the cat's coat. Keeping cats indoors reduces hunting and fighting, both of which increase flea exposure. Trimming claws regularly and washing any scratch immediately with soap and water add a second layer of protection. If someone in the house does develop cat scratch disease, there is no reason to rehome or euthanize the cat. The bacteria do not make the cat ill, and sensible flea management protects everyone in the home.