Max's House

FREQUENTLY ASKED QUESTIONS

ABOUT ZOONOSES


CAT SCRATCH DISEASE


Michael G. Groves,
DVM, MPH, PhD, DACVPM (Epidemiol),
Professor and Department Head, Department of Epidemiology and Community Health,
School of Veterinary Medicine, Louisiana State University;
Director, Louisiana Veterinary Medical Diagnostic Laboratory, Department of Agriculture, Baton Rouge, Louisiana

 

Veterinarians often are consulted by the public, and occasionally by physicians and other veterinarians, for information regarding zoonoses (diseases that are transmitted to humans from animals). When a person is diagnosed with an illness that is somewhat out of the ordinary, many times relatives are worried that the family pet was the source of the trouble. Usually, however, the diseases in question are not zoonoses. There are physicians who, perhaps seeking "zero risk," advise patients to dispose of pets to prevent or alleviate a zoonotic illness. Pregnant women have been advised to get rid of their cats to prevent their contracting toxoplasmosis; HIV-positive people may be told they that should not have animals at all. Although this advice may be well intended, it is often ill informed. Too often, what is missing is some reasonable approximation of the true risk of disease transmission balanced against the benefits of pet ownership. Of the true zoonoses, a few diseases account for most of the questions: cat scratch disease, larva migrans, rabies, and toxoplasmosis.

CAT SCRATCH DISEASE

Cat scratch disease (CSD) is caused by Bartonella henselae, a gram-negative bacterium. As the name suggests, it is carried by cats and is usually associated with catinflicted trauma, e.g., a scratch (in most cases) or a bite. Some 22,000 people reportedly have this disease each year, but its incidence is probably much higher because most cases are, asymptornatic. Clinical CSD in humans typically is characterized by swollen, painful lymph nodes, usually those immediately proximal to the site of inoculation; lowgrade fever, muscle aches, and general malaise are common. In more than 90 per cent of cases the disease is mild and self-limited, and symptoms resolve without treatment within a few weeks to a few months. More severe disease that requires hospitalization does occur. Rarely, B. henselae infection may cause a more serious syndrome called bacillary angiomatosis. This form of the disease affects mainly immunocompromised individuals, particularly AIDS patients. Bacillary angiomatosis causes angioproliferative lesions on the skin and in various organs. When it affects the liver and spleen, it is referred to as bacillary peliosis. Bacillary angiomatosis can be life-threatening. Early diagnosis and prompt treatment with antibiotics are important.

How Common Is B., henselae Infection in Cats?

Prevalence of antibodies to B. henselae in cats varies widely in different geographic regions, with highest rates occurring in warm, humid areas. In the southeastern United States more than 50 per cent of cats are seropositive, whereas in the Rocky Mountains and Great Plains less than 4 per cent have antibodies to B. henselae.'

What Are the Signs of B. henselae Infection in Cats?

B. henselae infection in cats produces a prolonged and often intense bacteremia, but most animals develop few, if any, clinical signs. Occasionally, cats may show a transient lymphadenopathy and low-grade fever when the infection is first acquired, but there seem to be no lasting ill effects associated with infection, despite the long duration of bacteremia.

How Can One Tell if a Cat Is Infected?

Antibodies to B. henselae can be detected by an indirect fluorescent antibody test. When bacteremia is present, the organism can sometimes be cultured directly from the cat's blood.

What Groups of Cats Are Most Likely to Be Infected?

Kittens appear to have the highest prevalence of bacteremia, and epidemiologically, kittens, especially if they have fleas, are most commonly associated with disease transmission. Also, feral cats are more likely to be infected than are pet cats.

How Long Does the Bacteremia Last?

This has not yet been determined, but various studies have shown detectable bacteremia for as long as a year or more. There is evidence that levels of bacteremia- wax and wane, so the organism may not always be present at a culturable level.

If B. henselae Is Only in the Cat's, Blood, How Can It Be Transmitted by a Scratch or Bite?

There are at least two plausible theories about this. One is that the bacteria get into the mouth from the blood, e.g., from teething or gum irritations, and contaminate the saliva. From there the organism could be transmitted to a human by a bite, or it could be transferred to claws when the cat grooms itself, and then transmitted by a scratch. Another possibility is flea-mediated transmission. Flea bites may well be the means of cat-to-cat transmission of B. henselae. Experimental studies have shown that the cat flea readily acquires B. henselae from infected cats and can transmit the infection to uninfected animals. This is not likely to be the primary means of transmission to humans, however. Because viable bacteria are excreted in flea feces, transmission of the organism could easily occur if contaminated "flea dirt" comes into contact with a person's open wound, whether a cat scratch or bite or a non-cat-- inflicted cut or abrasion

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What Can Be Done to Eliminate the Bacteremia?

There is no known effective means of clearing the cat of B. henselae infection. Efforts to completely and permanently eliminate the organism by using antibiotic therapy have met with only limited success.' Thus treatment of cats is not considered a reliable strategy for preventing CSD in humans.

Is Declawing an Effective Means of Preventing Human Infection?

No. Although most cases of clinical CSD in humans can be traced to a cat scratch, a significant number are associated with bites, with wounds not inflicted by cats, or with no known site of inoculation.

Should a Person Who Contracts CSD Get Rid of His or Her Cat?

Although having CSD is not a pleasant experience, permanent sequelae are almost nonexistent. People rarely become reinfected, and more than one case in a household is unusual. Rather than deprive oneself of the pleasures of cat ownership, a better approach is to take some commonsense precautions to minimize the chances of becoming infected.

What Can Be Done to Prevent Human Infection With CSD?

Start with aggressive flea control, involving both pets and the environment. Prevent cat scratches or bites by discouraging aggressive behavior. Do not tease kittens into rough play, and do not allow children to annoy cats or kittens. Immediately wash any bite or scratch thoroughly with soap and warm water and cover the wound to keep it clean. Do not allow cats to lick broken skin. If the offending animal is a kitten, clients should be advised that risk of transmission diminishes greatly as the cat matures. This may be due to at least two factors: levels of bacteremia wane with age, and older cats tend to be more sedate and less likely to scratch.

Should Those Who Are HIV-Positive Even Consider Getting Cats? Or if They Already Have Cats, Should They Get Rid of Them?

HIV-positive individuals and AIDS patients should be able to have cats if they follow the prevention guidelines given earlier. CSD is not a common disease; there are more than 57 million pet cats in the United States and more than one third of all households have at least one cat, yet only 22,000 cases of CSD are reported each year. Bacillary angiomatosis is rare. To be on the safe side, it might be prudent for someone who is HIV-positive to avoid acquiring a kitten or getting a cat from an animal shelter, because kittens and strays tend to have a higher rate of seroprevalence. A combination of serologic testing and blood culture can be used as a screening tool. Although it should be noted that negative results on both tests do not necessarily guarantee that the cat is free from infection, the probability of the cat truly being disease-free is increased. An older, flea-free cat that is kept indoors has less chance of carrying B. henselae. The benefits of a companion animal for some people may outweigh any risks of pet ownership, provided steps are taken to keep the risk at a minimum.' However, anyone who is seriously immunocompromised should tell his or her physician about any animals in the household to increase the chances of early recognition of zoonoses.

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