Cat Scratch Disease

A generally self limiting regional lymphadenitis caused by Bartonella henselae, a fastidious Gram negative bacterium. It is transmitted through the scratch, bite, or, less commonly, licking of broken skin or mucous membranes by an infected cat, particularly kittens, which serve as the principal reservoir.

The condition typically follows a benign, self resolving course in immunocompetent individuals. It is capable, though, of producing more significant disseminated or atypical manifestations, particularly in immunocompromised patients, in whom the related but clinically distinct entity of bacillary angiomatosis can occur.

Pathophysiology

• The organism is transmitted between cats via the cat flea (Ctenocephalides felis). Cats typically remain asymptomatic reservoirs of infection.

• Human infection follows direct inoculation through a scratch or bite (or, in some cases, simple licking of already broken skin). A papule typically develops at the inoculation site, followed by regional lymph node involvement as the organism spreads via draining lymphatics.

• The resulting lymphadenitis reflects a granulomatous inflammatory response to the organism. Histology characteristically shows stellate (star shaped) necrotising granulomata.

• A small number of other specific infective causes of lymphadenitis share this pattern, so it is useful but not entirely pathognomonic on biopsy where this is performed.

• A small, erythematous papule or pustule develops at the site of inoculation within three to ten days of the causative scratch or bite. It is often unnoticed or dismissed by the patient, given its typically mild and self limiting local appearance.
• Regional lymphadenopathy, developing one to three weeks after the inoculating injury, is the characteristic and most prominent clinical feature. The nodes drain the site of inoculation (commonly axillary or epitrochlear nodes following an upper limb scratch, or cervical or submandibular nodes following a facial or neck scratch).
• The affected node is typically tender, and in a proportion of cases progresses to suppuration.
• Mild constitutional symptoms, including low grade fever, malaise and headache, accompany the lymphadenopathy in a proportion of cases. The overall illness typically resolves spontaneously over two to four months even without specific treatment in the great majority of immunocompetent patients.
• Atypical manifestations, occurring in a minority, include Parinaud oculoglandular syndrome (unilateral conjunctivitis with associated preauricular lymphadenopathy, following inoculation of the conjunctiva, typically from touching the eye after handling an infected cat).
• Other atypical manifestations include neuroretinitis (with characteristic macular star exudate on fundoscopy) and encephalopathy.
• In immunocompromised patients specifically, more severe disseminated disease can occur, including hepatosplenic involvement.

• Diagnosis is predominantly clinical. It rests on the characteristic history of cat exposure (particularly a scratch or bite from a kitten), a compatible inoculation site lesion, and regional lymphadenopathy in the anatomically expected draining distribution.
• Bartonella henselae serology (indirect immunofluorescence assay) supports the diagnosis. Interpretation should account for the possibility of a false negative result early in the illness course, before an adequate antibody response has developed. Cross reactivity with other Bartonella species and, less commonly, other organisms, can occasionally complicate interpretation.
• Polymerase chain reaction testing on lymph node aspirate or biopsy material offers more specific confirmation where available. It is particularly useful in atypical presentations or where diagnostic uncertainty persists despite serological testing.
• Lymph node biopsy or fine needle aspiration is generally reserved for cases with diagnostic uncertainty, particularly where malignancy remains a genuine differential diagnostic concern given the persistent or enlarging nature of the lymphadenopathy.
• Characteristic stellate necrotising granulomatous histology, where present, supports the diagnosis alongside specific microbiological confirmation.

• Uncomplicated cat scratch disease in an immunocompetent patient is generally managed supportively, given the typically self limiting natural history. Treatment is simple analgesia for discomfort and reassurance regarding the expected gradual spontaneous resolution over a period of weeks to a few months.
• Azithromycin 500 mg orally on day one followed by 250 mg once daily for a further four days is considered for patients with significant, particularly large or painful, lymphadenopathy.
• There is modest evidence that this specific regimen gives a somewhat more rapid reduction in node volume. It does not appear to change the overall time to complete resolution of the underlying process.
• Needle aspiration of a suppurative, fluctuant lymph node can provide useful symptomatic relief where there is significant associated pain from a tense, suppurating node.
• Incision and drainage is generally avoided given the risk of subsequent chronic sinus tract formation at the drainage site.
• More significant or disseminated disease, including neuroretinitis, encephalopathy, hepatosplenic involvement, or disease in an immunocompromised host, warrants a more prolonged combination antibiotic course. This typically incorporates a macrolide or doxycycline with rifampicin, guided by specialist infectious disease input given the greater complexity and potential severity of disease in this context.
• Parinaud oculoglandular syndrome and neuroretinitis warrant ophthalmological assessment and input alongside the systemic antibiotic considerations, given the potential, albeit generally favourable with appropriate management, for visual complications.
• Prevention advice is particularly relevant for immunocompromised individuals, given the risk of more significant disease including bacillary angiomatosis. It includes avoidance of rough play with cats (particularly kittens) that might provoke scratching or biting, prompt washing of any scratch or bite wound, and flea control for household cats.
• The cat flea maintains the reservoir of infection through feline to feline transmission.