Cat Scratch Disease (CSD)

By Malcolm Weir, DVM, MSc, MPH; Krista Williams, BSc, DVM; Ernest Ward, DVM

What is cat scratch disease and what causes it?

Cat scratch disease (CSD), also known as cat scratch fever or bartonellosis, is caused by a bacterial infection. There are at least eight species of Bartonella implicated in causing human disease, while Bartonella henselae is the most common species found in cats. CSD can affect humans, dogs, cats, and other animals.

The disease got its name because it is often associated with a cat scratch; however, transmission of the bacteria occurs through cat fleas and possibly other biting flies or ticks. The flea ingests blood containing Bartonella from an infected cat, then the bacteria reproduce in the flea, and then shed in the feces. Those feces can infect humans through scratches or other abrasions in the skin, as well as through the eye.

What are the signs of cat scratch disease?

The typical signs are mild fever, chills, and tiredness (lethargy), accompanied by enlarged lymph nodes and lesions on the skin or conjunctiva (the membrane that covers the white of the eye and inside of the eyelid). Most symptoms last for a few days, but the enlarged lymph nodes may last for weeks or months.

"Most symptoms last for a few days, but the enlarged lymph nodes may last for weeks or months."

Physicians have traditionally been taught that CSD is a mild, self-limiting infection - one that typically goes away on its own, without the need for medication or other intervention. Although this is true for most cases, B. henselae and several other Bartonella species can occasionally cause chronic, asymptomatic, or intermittently symptomatic illness.

In these cases, a more severe disease can develop, with any combination of the following signs: arthritis, enlarged liver and spleen, high fever, nervousness, pneumonia, eye inflammation, and weight loss. These more serious forms of the disease are often associated with an underlying immunodeficiency (e.g., due to HIV/AIDS infection or chemotherapy treatments). However, we currently do not fully understand why some people contract a more serious form.

Where did human bartonellosis originate?

Before 1990, there were only two known pathogenic or disease-causing Bartonella species, B. quintana (the agent of trench fever in World War I) and B. bacilliformis (the agent of Oroya fever in Peru and other South American countries). These two diseases were known to infect people but were not yet recognized as a disease in pets, domestic animals, or humans in North America. Since 1990, more than 24 Bartonella species have been identified; at least half of these have been implicated or confirmed as animal or human pathogens.

Beginning in the early 1990s, Bartonella organisms have been a focus of considerable scientific and medical research. Veterinary healthcare professionals are considered to be one group at high risk for human bartonellosis. Several studies are examining the prevalence and spread of the disease within this group, with one study showing approximately 28% of veterinary staff having Bartonella species DNA in their bloodstream.

One important fact we do know is that cats are not the only carriers of Bartonella. Currently, 27 species of animals that may harbor the Bartonella organism have been identified. Of course, not all of these animals can infect humans, but research is ongoing to identify those that can. There are still many questions about human bartonellosis to be answered.

How common is cat scratch disease?

It is not possible to give accurate estimates about how common CSD is because not all cases are diagnosed or reported. However, it is thought to be somewhat uncommon, with about 1 in 10,000 persons being affected, and approximately 2,000 patients requiring hospitalization per year in the United States. It is likely that many human Bartonella infections go unnoticed without symptoms and appear to be nothing more than a mild cold.

Once infected, most humans seem to develop some form of immunity against Bartonella. Kittens are more likely than adult cats to be infected and to pass the bacterium to people. Experts believe that about 30% of cats carry B. henselae at some point in their lives. Cats that carry B. henselae do not show any signs of illness; therefore, you cannot tell which cats can spread the disease.

"The term “cat scratch disease” incorrectly implies that cats are the only source of transmission and infection."

The term “cat scratch disease” incorrectly implies that cats are the only source of transmission and infection. Although many cases of CSD follow a scratch from a cat, and cats are a major reservoir for B. henselae and other Bartonella species that can cause human disease, some people infected with Bartonella have no history of a cat scratch or bite wound, and others have had no known contact with cats. In these people, transmission from environmental sources, various biting insects (fleas and ticks), or other animal hosts is likely.

Cats become infected through flea bites. When a flea feeds on an infected cat, which can carry extremely high numbers of circulating B. henselae in its blood, it ingests the organisms, some of which, it is speculated, could make their way into a human if the flea next feeds on a person. However, so far there is no evidence that a bite from an infected flea can give someone CSD. Rather, infection results from exposure to the B. henselae-infected flea excrement (flea dirt).

Cats 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 person being infected more than once. There is still much to be learned about the actual disease transmission and process of CSD. We do know that flea control and prevention are key to preventing CSD in humans.

Is there a vaccine or treatment for cat scratch disease?

There is currently no CSD vaccine available for cats or people. B. henselae is sensitive to several antibiotics. A combination of two different types of antibiotics is most often prescribed to treat infected humans. The disease is typically self-limiting and most mild cases will resolve without the need for antibiotics.

Is there a test for cat scratch disease?

There are tests available, especially for humans thought to have contracted bartonellosis. If your cat is showing signs of the disease, several tests can be done to diagnose this disease, including antibody testing to identify exposure to the bacteria, DNA testing (PCR) to identify the bacteria in your cat’s blood, and blood culture to grow any bacteria that may be in your cat’s bloodstream.

Will declawing my cat help reduce the risk of spreading cat scratch disease?

There is no evidence that declawing cats decreases the risk of transmission of B. henselae to humans. Declawing is not recommended. Neither is treating your cat with a course of antibiotics, just to be safe.

The best way to reduce transmission is to keep your cat flea-free and avoid or prevent situations that may result in cat bites and scratches. Remember, B. henselae is transmitted to humans through contact with infected flea dirt. You can contact infected flea dirt in many ways – from cat scratches, as the flea dirt may lie under the claws; from cat bites, as the organisms may be present in the cat’s saliva due to ingesting the flea dirt while grooming; and from getting flea dirt on your hands (from your cat or the environment) and transferring it into an eye or open wound.

Effective flea treatment and prevention products are recommended and available from your veterinarian.

What steps can I take to reduce my risk of CSD?

  • Keep your cat’s nails trimmed short.
  • Keep all your pets on year-round flea control.
  • Keep your cat indoors.
  • Avoid rough play with your cat.
  • Wash any bites or scratches immediately with soap or disinfectant.
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