Cat-Scratch Disease

Dr. Greene's Answer

The uncovering of the cause of cat-scratch fever (now called cat-scratch disease) has been one of the great mystery stories of modern medicine. The hallmarks of cat-scratch disease are enlarged, tender lymph nodes (especially in the armpit) found under toughened, warm, red skin. For centuries children have often had these symptoms with no known cause. For centuries children have played with cats and been scratched, but no connection was made.

In 1946, a Dr. Hanger in New York, the owner of a “ferocious tiger,” got what appeared to be an infected hangnail and then a swollen, tender lymph node full of pus under his collarbone. His friend, Dr. Rose, drained the pus from the lymph node — but to their surprise, the pus was found to be sterile (containing no germs). They re-injected Dr. Hanger with some of this material, under the skin of his arm, and he had a strong skin reaction (sort of like a TB skin test, but this was a tiger-claw skin test).

Then, across the Atlantic in 1950 Paris, a 6-year-old French boy was observed to have a swollen, tender lymph node near the site of a scratch from a house cat. His physician, Dr. Debre, obtained some of the Hanger/Rose material (which he actually got from the University of Cincinnati, where microbiologists were now trying to figure out this stuff). He injected some of the material under the Parisian boy’s skin, and the boy developed a positive skin-test reaction!

This was the first documented case of what they called “La maladie des griffes de chat.” Over the next several years the international collaboration continued, and cat-scratch fever was defined as a specific disease. A positive skin test in someone with swollen lymph nodes, who had been exposed to a cat, was considered to make the diagnosis.

When I entered medical school, cat-scratch disease was still a mystery. We knew that it happened after cat scratches, and it seemed that it must be caused by some type of bacteria, but no bacteria had ever been found in those swollen lymph nodes. Then, while I was in my second year of medical school, very tiny proteobacteria were seen in early cat-scratch lymph nodes that had been stained with silver. An organism had been found!

But what was it? Many possible identities were proposed and then disproved. When I was a pediatric intern, someone was finally able to grow the bacteria. The definitive pediatric infectious disease textbook (Feigin and Cherry — a wonderful book) announced in the 1992 edition: “The bacterium that causes cat-scratch disease has been conclusively identified over the past decade…Afipia felis.”

I received my copy of the next edition of Feigin and Cherry in today’s mail. It says, “further study has now discredited Afipia felis as the cause of cat-scratch disease!” Even as the previous edition was going to press, new DNA diagnostic techniques applied to cat-scratch disease in AIDS patients were providing a wealth of new information. Another bacterium — Bartonella — is now known to be the cause: “That Bartonella is the cause of cat-scratch disease now has been established beyond doubt.”

Times change.

Most cat-scratch disease begins with a scratch from the claw or tooth of a kitten younger than one year of age. It can also be caught from an adult cat. In California, about 40% of the cats carry Bartonella. The disease seems to rarely occur following a dog scratch or even from porcupine quills or cactus spines. This is different, though, from an infected paper cut. Most cases of cat-scratch disease occur in children between the ages of 2 and 14, and in veterinarians (those most likely to be scratched by a cat). For reasons yet to be determined, most cases occur in the fall or winter months.

Some time between 3 and 30 days following the scratch (usually 7 to 12 days), the child breaks out with one or more pimples at the site of the scratch. These last for 1 to 4 weeks, until nearby lymph nodes begin to swell and become tender. Sometimes these will drain pus. Only about a third of kids will feel sick, with a fever, fatigue, decreased appetite, or headache. This is not a serious disease.

Most of the time, the swollen lymph nodes last from 4 to 6 weeks, and then the child recovers fully. Occasionally, the swollen nodes can last for a year. Only rarely have complications been reported, mostly in immunocompromised individuals. Treatment is usually unnecessary, since people recover well on their own, but several antibiotics (such as Zithromax and Biaxin) have been shown to shorten the course of the illness.

Diagnosis is now made by a blood test (or by biopsy, if other more serious conditions are suspected). The skin test is no longer performed, since it is less accurate and has now been shown to pass other known (and perhaps unknown?) diseases.

Infected cats carry the bacteria in their blood. Research so far has failed to show the growth of Bartonella henselae in cat saliva. It is thought that cats spread the infection to humans by contaminating their own saliva or nails with blood. The infected blood then enters the human body through a cat scratch, bite, or even a lick if there is open skin.

Fleas carry the bacteria and are the main source of infection from one cat to another. While fleas do not directly pass the infection to humans, controlling fleas in cats may decrease the risk of infection to humans. One reason is because it would result in less cats being infected. The other reason is because infected cats with fleas are more likely to scratch themselves than those without fleas. Scratching increases a cat’s risk of contaminating its nails with infected blood and passing the disease onto humans.

The unfolding story of cat-scratch disease is a tale of mystery followed by mistaken certainty, followed again by mystery and then certainty. This twisting tale helps us to put medical knowledge in perspective. Our understanding of disease is truly amazing and is growing rapidly, BUT there is much that we do not know, and much that we now consider certain will one day prove to be in error. Acting based on the best of our knowledge will usually produce good results, but let’s also act in humility before all that we don’t understand!