Cat scratch disease (CSD) is transmitted by the bite or scratch of a cat infected with the microorganism Bartonella henselae.It is a fairly common bacterial infection with a high prevalence in warm and humid climates.
The incidence is slightly higher in males according to some studies and children as well as adolescents are more likely to have a higher risk of exposure.
Inoculation with the microorganism presents with mild infection at the site within 1 – 2 weeks followed by lymphadenopathy and possible systemic symptoms such as fever, malaise, and anorexia.
Diagnosing CSD can be done on grounds of clinical history (e.g., cat ownership and/ or trauma) and symptoms in addition to blood tests. There are a number of differential diagnoses, which depend on exposure risk and/or travel to endemic areas. This list includes all possible causes of lymphadenopathy, such as brucellosis, toxoplasmosis, infectious mononucleosis, syphilis, and tick-borne diseases among many others.
Where applicable, samples may be taken from an enlarged lymph node (i.e., biopsies) to ascertain the diagnosis of CSD. The microorganism responsible for CSD, Bartonella henselae, is difficult to culture and as such, culture is not routinely done. Serology is considered the best initial test when diagnosing CSD.
Other investigations include hematology, cerebrospinal fluid (CSF) analysis, electroencephalogram (EEG), and imaging methods such as ultrasound, CT, or MRI.
Generally considered the best first line diagnostic tool, serology can be performed by enzyme-linked immunosorbent assay or indirect fluorescent assay. Immunoglobulin G (IgG) titers will be elevated during infection. A titer greater than 1:64 is considered to be supportive of a CSD diagnosis and one greater than 1:256 is strongly suggestive of recent or active infection.
Titers lower than 1:64 suggest that there is no current infection with Bartonella. While serological tests are more sensitive than culture, they lack specificity. This is because there are many persons who have no CSD symptoms, but have a positive serology due to a previous and frequently asymptomatic exposure. Moreover, there is a higher prevalence of positive serological tests among cat owners in comparison to the general population.
Lymph Node Biopsy
Biopsy of the lymph node involved is not usually recommended in the majority of cases. Indications for biopsy include investigating nodes that do not involute and in cases where the diagnosis is unclear. A typical CSD node shows stellate granulomas and hyperplasia of the lymphoid tissue. In the severe form of CSD, bacillary angiomatosis, seen in immunocompromised patients, there is lobular proliferation of the small blood vessels.
Hematology may reveal non-specific findings such as elevated erythrocyte sedimentation rate (ESR) and mild leukocytosis. However, these are of very little value in actual confirmation of the disease. Patients who present with encephalopathy, a rare neurological complication of CSD may have diffuse slowing on EEG. This generally tends to improve with clinical recovery from the disease. CSF analysis may be done to rule out other causes of encephalitis while imaging techniques such as CT, MRI, and ultrasound may be necessary to rule out other conditions such as malignancy or tuberculosis.