Cat scratch disease (CSD) was first defined as a human disease in 1931, and the term 'cat scratch disease' was adopted in 1950. It is a bacterial infection that occurs throughout the world and is most common in young people (more than 80 per cent of cases occur in people under 21 years of age, with the highest prevalence in children aged between three and 12 years). The disease is somewhat more common in men (60 per cent) than women, and although occasionally clusters of disease have been reported, more commonly only one individual in a household is affected. In the USA, the disease has an estimated annual incidence of approximately two to 10 cases per 100,000 of the population.
In most cases of CSD, the patient has recent history of being scratched or bitten by a cat, or having close contact with a cat, hence the name - CSD. The disease in humans is typically benign and self-limiting. It may start as a small skin papule or pustule (at the site of inoculation of the organism – most commonly a cat scratch), which may rupture before healing after around three weeks. A few weeks later, painful or non-painful local lymph node swelling develops, and in the majority of cases (85 per cent) this is limited to a single lymph node (often in the axilla - arm pit - or neck). The lymph node swelling may be marked, and may persist for several weeks or months. In most cases no further signs develop and the swelling eventually resolves. Sometimes the affected lymph node(s) form an abscess and rupture. Other signs develop in some patients – most commonly mild fever and malaise. Muscle pain (myalgia), headaches, inappetence and nausea are seen in some.
In up to 10 to 15 per cent of cases, atypical signs of disease develop including chronic conjunctivitis (thought to be due to local inoculation of the organism close to the eye) and occasionally involvement of the lungs, bones, liver and other organs. Central nervous system involvement is reported in around 2 per cent of cases and signs include delirium, headache, seizures and depression. These signs (along with the majority of atypical manifestations of the infection) usually resolve rapidly without causing any permanent damage. More serious disease complications are more likely to occur in immunosuppressed individuals. Conversely, there is also evidence that a number of humans may be infected with the CSD organism, but clear the infection without ever developing any clinical sings of disease.
An unusual form of disease caused by the same organism responsible for cat scratch disease is called ‘bacillary angiomatosis’. This disease is seen most commonly in immuncompromised people (eg, AIDS patients) and is characterised by lesions associated with proliferation (excessive growth) of blood vessels in the skin and/or elsewhere in the body.
For many years the organism responsible for CSD proved elusive. Small bacteria could be seen in lymph node biopsies from patients with CSD, but no organism could be grown in the laboratory. Although a number of different organisms were suggested as possible causes of CSD, it is now known that the vast majority are caused by infection with a bacterium called Bartonella henselae. This organism is thought to account for around 90 per cent of cases of CSD, with the remainder being caused by closely related bacteria in the Bartonella genus. The reason it took so long to identify this as the cause of CSD, is that the bacteria have very specific requirements for growth in the laboratory and are slow to grow.
Considerable research has been undertaken to study the association between B henselae and cats. Early studies demonstrated a high proportion of pet cats (25 to 30 per cent) in the USA had antibodies to this bacterium in their blood indicating that they had been previously, or were currently, infected with the bacterium. It was further shown that climate affected the number of cats with exposure to the bacterium with up to 60 per cent of cats in warm, humid climates having antibodies.
Later it was found that not only were antibodies common in blood samples from cats, but that the organism could also commonly be isolated from cat blood samples. It is now known that in cats, the organism actually infects and lives inside red blood cells (and possibly some other cells in the body), but it is a very well adapted bacteria, very rarely causing any significant illeffects in the infected cats.
One study from the UK looked at blood samples from 360 different cats, and demonstrated the presence of B henselaein just over 9 per cent of the samples. Again, infection rates vary from country to country, but it is not uncommon to find up to 20 per cent of healthy cats to be infected with the bacterium.
At present, it is thought that the organism is mainly transmitted between cats via fleas. Because fleas feed on blood, they ingest the bacteria during feeding and may then inject it into another cat at a later time. Flea faeces ('flea dirt') from a flea that has ingested infected blood will also contain B henselae and the organism may remain viable in flea dirt for more than a week. Exposure to fleas and flea dirts are therefore thought to be the most important ways the organism is transmitted between cats. This explains why infection is more common in geographic areas and climates where fleas are more prevalent, and in cats that are allowed free access outdoors. Ticks and other biting arthropods may be another potential (but less important) source of transmission of the bacterium.
The majority of humans that develop CSD have a history of a cat scratch (often being scratched by a kitten or young cat). It is thought that the cats probably have B henselaeorganisms on their claws - most likely the organism is present in flea dirts on the cat and the organism is spread to the claws by the cat during grooming and licking. The bacteria can then become inoculated under the skin of a human when he or she is scratched. Some humans do not have a history of a scratch or a bite, and in these cases it is possible that if a skin wound is exposed to the bacteria
(via close contact with cats, or infected flea faeces in the environment) infection could be established, or perhaps some humans are infected by being bitten by an infected flea.
Despite infection with B henselaebeing extremely common in cats in many parts of the world, CSD in humans remains a rare disease, and so clearly the spread of infection to humans is inefficient. The fact that it is rare to get multiple cases of CSD in the same household also demonstrates the relative difficulty of establishing disease in humans.
Treatment and eradication of infection with B henselaeis surprisingly difficult. Cats that are infected with the bacterium often remain infected for prolonged periods (several years in some cases) before they eventually eliminate the organism. Although some antibiotics (eg, doxycycline and enrofloxacin) may reduce the number of bacteria present in the blood, these treatments are not always successful in eliminating the infection completely. At present, the best antibiotic(s), and the most appropriate duration of treatment to try to eliminate B henselaeinfection from cats has not been established. However, as in the vast majority of cases the infection does not cause clinical disease in the cat, there is probably little or no reason to try to treat healthy cats that are infected cats. If a Bartonella infection is suspected to be causing clinical disease in a cat, therapy with antibitotics known to have some activity against the organism (eg, doxycycline, or potentially fluoroquinolones or azithromycin) would be appropriate.
Treatment in humans with CSD is also difficult. Results of different antibiotic regimes are quite variable, and no therapy produces consistently rapid responses. However as the disease is usually relatively benign and self-limiting this is not considered a major problem. In general, the more severe forms of disease are likely to respond much better to antibiotic therapy.
Although B henselaeinfection is generally common in cats, its transmission to man is highly inefficient. Current evidence strongly implicates fleas to be directly or indirectly involved in the
transmission in the vast majority of cases (without infected flea dirts, the organism is highly unlikely to be present in the mouth or on the claws of cats). The key to helping prevent human infection is therefore control of fleas on the cat and in the environment.
Good control of fleas generally requires the use of two or more products - an insecticide to kill adult fleas on the cat, and a product to prevent development of the immature forms of the flea that live in the environment. There are many highly effective, safe, and easy-to-use products that make good flea control very practical now, and this will be the most effective way of preventing CSD occurring. The most effective products for controlling fleas, and the best advice on flea control, can be obtained from your veterinary surgeon. Some flea control products available in pet shops and supermarkets have poorer efficacy and great care must be taken not to use dog flea control products on cats as this can be fatal. The FAB also has a detailed information sheet available on fleas and flea control, click here for more info...
Currently, testing cats to see if they are infected with B henselae is not recommended - eradication of infection cannot be reliably achieved in cats, and the widespread non-discriminate use of antibiotics would be likely to cause more harm than good. Good, year-round flea control is a much better and effective alternative for controlling the risk of disease in humans.