Cat-Scratch Disease

Cat-scratch disease (CSD), also known as cat-scratch fever, is an infectious disease primarily resulting from a scratch or bite from a cat. The causative agent is the bacterium Bartonella henselae, typically transmitted via the cat's saliva. Young cats pose a greater risk than older cats. Occasionally, dog scratches or bites may also be involved. Diagnosis is generally based on symptoms and can be confirmed through blood tests.

Signs and Symptoms

An enlarged lymph node in the armpit region of a person with cat-scratch disease, and wounds from a cat scratch on the hand.

Cat-scratch disease commonly presents as tender, swollen lymph nodes near the site of the inoculating bite or scratch, usually limited to one side. This regional lymphadenopathy occurs 1–3 weeks after inoculation and most commonly affects the axilla, arms, neck, or jaw. A vesicle or an erythematous papule may form at the initial infection site.

A lesion on the hand of a person with cat-scratch disease.

Systemic symptoms such as malaise, decreased appetite, and aches are common. Other associated complaints include headache, chills, muscular pains, joint pains, arthritis, backache, and abdominal pain. Symptoms may appear as long as two months post-infection and usually resolve spontaneously within a month. Lymphadenopathy, however, may persist for several months.

In rare cases, CSD can lead to serious neurologic or cardiac sequelae such as meningoencephalitis, encephalopathy, seizures, or endocarditis, which has a particularly high mortality rate. Parinaud's oculoglandular syndrome, a granulomatous conjunctivitis with concurrent swelling of the lymph node near the ear, is the most common ocular manifestation. Immunocompromised individuals are at risk of additional conditions such as bacillary angiomatosis or bacillary peliosis.

Cause and Transmission

Bartonella henselae is a fastidious, intracellular, Gramme-negative bacterium. Kittens are more likely to carry the bacteria in their blood and thus more likely to transmit the disease. Fleas serve as a vector for transmission among cats, and viable B. henselae are excreted in the faeces of the cat flea (Ctenocephalides felis). Cats could be infected through intradermal inoculation using flea faeces containing B. henselae. Consequently, transmission to humans may occur via contaminated cat scratches or bites, or through flea faeces inoculation. Ticks and other organisms such as spiders can also transmit Bartonella.

Diagnosis

Micrograph of a lymph node affected by cat scratch disease. H&E stain.

High-magnification micrograph of CSD showing a granuloma (pale cells - right of centre on image) and a microabscess with neutrophils (left of image), H&E stain.

The best diagnostic method is polymerase chain reaction (PCR), with a sensitivity of 43-76% and specificity of 100%. The Warthin–Starry stain can also show B. henselae, but it is often difficult to interpret. B. henselae is difficult to culture and may take 2–6 weeks to incubate. Histologically, CSD is characterised by granulomatous inflammation in the lymph nodes.

Prevention

Prevention primarily involves effective flea control measures. Keeping cats indoors and avoiding handling by young children can reduce the risk. Thoroughly washing hands after handling cats or cat faeces is essential, as cats carrying B. henselae are asymptomatic.

Treatment

Most healthy individuals clear the infection without treatment. However, in 5 to 14% of cases, the infection can disseminate to organs like the liver, spleen, eye, or central nervous system. Azithromycin is the preferred antibiotic for treatment, especially in pregnancy, due to its favourable safety profile. Doxycycline is preferred for B. henselae infections involving optic neuritis due to its ability to penetrate eye and CNS tissues.

Epidemiology

Cat-scratch disease has a worldwide distribution but is nonreportable in humans. The incidence varies with geographical location, season, and cat exposure. In the United States, an analysis from 2005 to 2013 revealed an incidence rate of 4.5/100,000 outpatient cases. Incidence is highest among children aged 5–9 and women aged 60–64. Warmer climates and fall/winter seasons see higher prevalence, likely due to the breeding season of cats.

Outbreaks

Recent increases in CSD cases have been reported in urban and suburban areas, particularly in the northeastern United States. For example, Essex County, New Jersey, saw a rise from 6 cases in 2016 to 263 cases in 2018. Although typically treated with antibiotics, there have been cases of tachycardia persisting more than a year after exposure.

Self-assessment MCQs (single best answer)

What is the causative agent of cat-scratch disease (CSD)?

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Which of the following animals is most commonly associated with transmitting cat-scratch disease?

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What is the most common initial symptom of cat-scratch disease?

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Which diagnostic method is considered the best for cat-scratch disease?

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What is the primary vector for the transmission of Bartonella henselae among cats?

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Which antibiotic is preferred for treating cat-scratch disease, particularly in pregnancy?

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What is Parinaud's oculoglandular syndrome?

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What is the primary prevention method for cat-scratch disease?

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What is the typical duration for cat-scratch disease symptoms to resolve spontaneously?

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Which population group is at the highest risk of contracting cat-scratch disease?

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