Cat Scratch Disease

What is cat scratch disease and what causes it?
Cat Scratch Disease or Cat Scratch Fever is a disease of humans, not of cats. However, a cat scratch is often associated with the disease, although this is not believed to be the means by which infection occurs. A microorganism called Bartonella henselae is the most common cause of this disease. The typical symptoms in people are mild fever, chills, and lethargy accompanied by enlarged lymph nodes and skin or conjunctival lesions. Most symptoms last for a few days, but the enlarged lymph nodes may persist for weeks or months. In a minority of people, a more severe disease can develop with various combinations of high fever, weight loss, arthritis, enlarged liver and spleen, pneumonia and nervous signs. These more serious forms of the disease are often associated with underlying immunodeficiency states, such as in HIV (AIDS), infected people.

How common is cat scratch disease?
It is not possible to give accurate estimates of the prevalence of Cat Scratch Disease since not all cases are diagnosed or reported, but it is probably an uncommon disease in people. Surveys carried out in the USA indicate that about 5% of the population has been exposed to infection, but only a small proportion of these reported having the disease. It is likely that many infections are without symptoms and go unnoticed, or are of a trivial nature, seemingly a mild “cold”.

How do humans become infected?

Although many cases of Cat Scratch Disease follow a scratch from a cat, this is not universally true. A few cases have occurred in people with no apparent contact with cats. Recent evidence suggests that the major route of infection is by a flea bite. Infected cats carry the microorganism in their blood, where it can be present in extremely high numbers. When a flea feeds on an infected cat it ingests large numbers of the Bartonella hensalae organisms, some of which may be inoculated into a human the next time the flea takes a meal. Even cats with extremely high levels of B. hensalae organisms in their blood do not show any signs of disease. It is mainly younger cats and kittens that carry the organism. They remain infectious for a few weeks, after which the organism disappears from the blood. It is not clear whether cats can be re-infected. There are no reported cases of any human being infected more than once.

Is there a vaccine or treatment for Cat Scratch Disease?
There is no vaccine available for Cat Scratch Disease either for cats or for people. Bartonella henselae is sensitive to limited antibiotics; penicillin, ampicillin, erythromycin, tetracycline or some other commonly prescribed antibiotics are not effective. Azithromycin is an antibiotic that is usually effective.