Fleas, which carry B. henselae, inoculate a cat or kitten with the bacteria while biting. The cat, in turn, transmits the bacteria to a person through a bite or scratch. Kittens transmit the disease more often than cats. Nearly 40% of cats carry the bacteria at some point in their lives but show no signs of illness; therefore, it’s impossible to detect whether a cat is a carrier. There is no evidence to suggest that fleas can directly transmit the bacteria to humans. Cat scratch disease is not transmitted from person to person.

Immunocompromised individuals, such as transplant recipients, those undergoing immunosuppressive cancer treatment, and patients with human immunodeficiency virus, are more likely to develop complications. Complications associated with cat scratch disease occur in approximately 14% of diagnosed cases and include bacillary angiomatosis and Parinaud’s oculoglandular syndrome. Encephalitis occurs in less than 5% of infected patients. These complications are rare and typically occur in individuals with compromised immune systems.

The incubation period is typically 3 to 14 days. Signs and symptoms include a mild infection with suppurative papules at the site of injury that commonly occurs 3 to 10 days after injury. One to 2 weeks later, lymphadenopathy develops, especially in the lymph nodes around the head, neck, and upper extremities. (See Lymph node swelling in cat scratch disease.) Nearly half of those affected also develop a rash, general malaise, fever, headache, fatigue, and anorexia.