MORBIDITY: ++

MORTALITY: +

ETIOLOGY: BACTERIAL

CSD is caused by Bartonella henselae, a small, gram-negative rod. The disease is seen worldwide and is most commonly diagnosed during the fall and winter months, possibly because this is when cats spend more time indoors.

HOSTS

Cats are the primary reservoir of B. henselae for human infection.

TRANSMISSION

B. henselae is transmitted from cats, primarily kittens less than 1 year old, to humans by a bite, lick, or scratch. There are a number of theories about how cats are originally infected. One theory suggests that the cat is a mechanical vector for the organism that is spread from cat to cat by fleas. The organism in the flea feces is picked up in the cat’s mouth when it bathes and is distributed to the cat’s feet during subsequent baths. The incidence of CSD decreases when the presence of fleas is controlled. CSD is seen more commonly in warmer climates, where the number of fleas is greater than in cooler climates.

Another theory states that B. henselae may be part of the normal flora in a cat’s mouth that is spread to the cat’s feet when it bathes.

B. henselae is found in an infected kitten’s bloodstream on a recurring, cyclical basis that may be short-lived or can last for months or years. B. henselae can be transmitted from one cat to another through blood transfusion, but not transplacentally from mother to kittens, and not sexually.

CSD is not contagious from human to human or from human to cat. Fleas have not been implicated in the spread of CSD directly to humans. A person with CSD does not have to be isolated from the rest of the family.

CAT SCRATCH DISEASE IN CATS

Cats rarely get clinically ill from B. henselae infection. Many adult cats have antibodies to B. henselae in their blood, indicating a previous exposure to the organism.

CAT SCRATCH DISEASE IN HUMANS

CSD in humans is usually self-limiting, without any permanent damage. Most cases are seen in people under 21 and those in the veterinary profession. There is usually, but not always, a history of a cat scratch or bite. Letting a cat lick an open wound can also result in CSD. A small pustule resembling an insect bite develops at the site within 3 to 10 days (Figure 5). In 1 to 2 weeks, lymph nodes near the area will become swollen and painful (lymphadenopathy). Nearly half of the people who develop the pustule will also develop a headache, mild fever, and lethargy. The lymph nodes of the neck and extremities are most often affected because most cat bites and scratches occur on the arms and legs. The lymphadenopathy is unilateral, on the side where the infection occurred. Nearly a quarter of infected patients will develop very painful, pus-filled lymph nodes (lymphadenitis). Even though CSD is self-limiting, it can take up to 3 weeks for the lesion at the site of infection to heal and several months for the lymph nodes to return to their normal size. In mild cases, treatment is usually not required.

FIGURE 5 Enlarged lymph nodes in a person with cat scratch disease

(From Greene CE: Infectious diseases of the dog and cat, ed 3, St Louis, 2006, Saunders.)

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